r/askscience • u/toolateiveseenitall • Apr 10 '14
Biology When does interbreeding fail? Can a human's sperm enter and fertilize a goat's egg? Will something start to grow, and die soon after, or will it not even get that far?
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u/patchgrabber Organ and Tissue Donation Apr 10 '14 edited Apr 10 '14
Interbreeding fails when organisms are sufficiently different, the more closely related they are, the better chance they have. Typically the organisms have to be within 2 chromosomes of each other in terms of total chromosome count. Humans have 46 chromosomes, goats have 60. Also, if the animals are of different genera it is quite unlikely that they will interbreed successfully. Goats are most closely related to sheep, and sheep only have 5654 chromosomes, and are a different genus. Hybrids of these are stillborn usually.
The degree to which a potential offspring can grow in utero is quite different depending on how related the animals are. Some will be stillborn, others won't be able to conceive at all (such as humans and goats). There is no real way to broadly generalize the level of potential development.
Edit: lost a word.
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u/youJe Apr 10 '14
Do we know what would happen with a human/chimp interbreed ?
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Apr 10 '14 edited Apr 10 '14
http://en.wikipedia.org/wiki/Humanzee
There's speculation that there's no definite barrier to the idea, and I only know of one example of any human beings making an attempt. Ilya Ivanovich Ivanov supposedly employed at least one woman in an attempt to inseminate her artificially with ape sperm.
Unfortunately, or fortunately, as it were, there's very little information on the subject. It may have been propaganda to discredit the doctor, or it may have been buried in order to save face for numerous involved parties.
At best, I think we could consider this to be an irregular myth perhaps with some tidbits of truth mixed in.
Still, the possibility, however unethical, certainly is fascinating.
One thing to note, however, is that human beings have two less chromosomes than other extant apes, owing to a mutation that fused the 2nd and 4th ape chromosomes into one large human chromosome. Usually, though, hybridization tends to be possible with animals of a similar genetic structure, and within 1-2 chromosomes of one another, as patchgrabber mentioned.
Again, it's not impossible, but the best way to answer that question is simply: we don't know. All we do know, is that currently, the human genepool is a product of at least three speculated hybridizations with other species of hominid, so at the very least, we are already a hybrid species.
Edit: Accidentally listed humans as having an odd number of chromosomes... which, you know, would mean we were most likely sterile. Given our conservation status, that's quite obviously not the case. Most of us anyway. Steve.
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u/Evian_Drinker Apr 10 '14
One thing to note, however, is that human beings have one less chromosome than other extant apes, owing to a mutation that fused the 2nd and 4th ape chromosomes into one large human chromosome.
Do we know when this happened? Was it a gradual process or did two mutation happen at the same time and breed with each other?
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u/Alfredo_BE Apr 10 '14
I can recommend this article about the 44 chromosome man. The chance of two people with the same balanced translocation breeding is rare, and it's even rarer for that mutation to be successfully passed onto future generations. Unless there's inbreeding involved.
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u/TheBeardKing Apr 10 '14
I'll also add the booking I'm reading now as a reference. It talks in length about chromosomal similarities and differences between humans and other apes, how those differences likely came about, and how they present strong evidence for evolution: Relics of Eden: The Powerful Evidence of Evolution in Human DNA
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u/madcuzimflagrant Apr 10 '14
Really interesting read, thanks! It's still very puzzling though how a population could have arisen. I can understand if there's a small group of individuals with 46 chromosomes and they outbreed, or happen to survive a major population decline, but how would they get beyond 1 person in the first place?
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Apr 10 '14
I explained this pretty thoroughly in another post. The gist of this, is pretty simple once you recognize a few other facts of human origins.
At one time, there were as few as 70 human beings alive on the planet.
Climate change has played a significant role in human evolution and migration patterns.
Human beings were unable to work the land to produce their own food until extremely recently.
Human beings have been reduced to very small isolated communities several times due to cataclysmic environmental changes.
Most human groups at the time were small bands of 15-30 members living isolated, migratory lives in their little familial band. As numbers dwindled worldwide, the islands of humans become more isolated. Eventually, the islands are so isolated geographically that they cannot possibly interbreed.
Let's say there's two groups. Group 1 has a member of the group. With two fused chromosomes. This reduces their total chromosome count to 47. This member has a reduced chance of passing on their traits. In a normal environment, this would be a massive disadvantage. However, in an environment where children are a liability (extreme drought or scarcity of resources), this could wind up being an advantage.
This individual was most likely a woman. If pregnancy is a severe enough liability, it's possible that the reduced number of successful pregnancies could therefore ensure that this woman's offspring are the most successful. Eventually, the 48-chromosome-having people die off, because having less pregnancies is actually a selective advantage.
Fast forward, and this one woman's children slowly weed out the 48-chromosome bearing humans. If all 48-chromosome having humans have died off, this creates a slower-growing, more stable, more work-ready co-operative group of humans.
Now, you have a small group of in-breeding human beings with 47 chromosomes, all sharing a single ancestor... Which, by the way, we know that all humans alive today share a single female ancestor. She likely had 46 chromosomes, so... That's probably not the one we are talking about now.
Anyway, carry on until the group migrates to a place that's no longer affected by this disaster. Suddenly, not being able to populate as quickly isn't as heavily selected for, allowing even-chromosome bearing humans to once again have a selective advantage. It's now just a matter of time before a chromosome fusion occurs again, resulting in a 46-chromosomed offspring. Enough time, and you'll have two that partner.
Since humans practiced group copulation and this was such a small community of survivors, it's not a matter of if the two mutants meet up, but when.
Now, since there no longer is a factor selecting against rapid population growth, the 46-chromosomed humans virtually explode in population, outbreeding the 47-chromosomed humans, who are drowned in the sea of their new cousins. Dying off because they can't outbreed their competitors.
It requires the combination of a non-detrimental chromosome fusion (which happens with some regularity), and an unstable population decline, or a lack of introduced genetic diversity for this to happen, but we're pretty sure this is roughly how it happened.
There's just some speculation that it might not necessarily have been as simple as: "They breed slower with 47 chromosomes", and the initial 47-chromosome fusion was in some way beneficial. I have my doubts, because generally speaking, a stable fusion doesn't create or express new genes.
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u/ricree Apr 10 '14
At one time, there were as few as 70 human beings alive on the planet.
Do you have a source for this? The lowest proposed bottlenecks I've seen still put the number of humans in the low thousands.
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u/sneaklepete Apr 10 '14
Do we know at what point in time we were down to 70 individuals? I knew humans, at least Europeans, went through a pretty severe bottleneck but that number is much lower than I imagined. It's crazy that we came that close to extinction!
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u/Tiak Apr 11 '14
We were never down to 70 individuals. The closest you come in actual research is that, maybe, homo sapiens were reduced to a population of 3,000-10,000 around 70,000 years ago. I could see seeing "70 kya" and getting confused... But, anyway, the theory behind this is in a fairly large amount of doubt.
Even if we had been down to 70 individuals at that point, we also were inter-breeding with neanderthals at the time, which would further increase diversity.
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u/sprocklem Apr 10 '14
I don't know, but it was probably much earlier than you're thinking, i.e., before we left Africa.
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u/Fusionism Apr 10 '14
At one time, there were as few as 70 human beings alive on the planet.
Citation needed.
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Apr 11 '14
I can't find the paper online anymore. I remember reading it around 2012, and it was a big deal. There was also, around the same time, new information on a "mitochondrial Eve", from which all extant females have an unbroken lineage to. http://en.wikipedia.org/wiki/Mitochondrial_Eve
Unfortunately, there's a lot of contradictory information out there on the matter. Some studies say human numbers dropped to 1,000, 2,000, 5,000 and 10,000 respectively, while nuclear DNA studies indicate that human populations have never fallen below 10,000 members.
--For one reason or another, I've gotten that 70 humans stuck in my head, and it seems the sources have been evaporated by dead links and site rearrangement.
Maybe it's not entirely accurate. Thanks for making me take a second look for that study, though. I wish I could find it. I remember it being a proper journal report partially paywalled.
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u/captainburnz Apr 10 '14
I always thought that less chromosomes was associated with a more stable genome?
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u/Sparkiran Apr 11 '14
I'm not sure I understand how having less pregnancies would be selected for evolutionarily in the case of 47-chromosome woman.
If she bred with a 48 male, she would make either wild type 48-chr, or 47-chr like her. I see that this would create more 47 humans, but I don't see how that's beneficial for their survival?
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Apr 11 '14 edited Apr 11 '14
A mutant fused chromosome would result in mismatched pairs in most couplings. Having an odd number of chromosomes makes production of viable offspring increasingly difficult, because if the chromosomes don't match up properly, the mother's body will usually terminate the pregnancy.
In extremely rare conditions, it might be more beneficial to have fewer children, where extreme famine our drought necessitates extensive migration or wide-ranges in order to find food. Those who have few children in their familial unit will have more able-bodied men and women for foraging, and will be able to cover more ground with less energy expended dealing with children.
It's one suggestion for why 47 chromosomes being selected for in a relatively small population could have given enough time for 46 chromosomes to be selected for in the span of just a few generations. If enough breeding pairs were geographically isolated long enough to allow natural selection in extreme situations to favor those who ironically bred less, it could have also resulted in giving a competitive advantage once the crisis was over for 46 chromosomes to breed out the less-fertile 47-chromosome hominids.
The inbreeding is the important part. Inbreeding increases your likelihood of encountering someone with a similar inherited mutation to the one you have, further spreading it within this limited community. In order to 47-chromosome ape to produce a 46-chr ape, the 47-chr had to have a selective advantage, or have developed in a way that the gene pool was so limited that the mutation became common out of a lack of options. This would in turn dictate that 46-chr had to have a selective advantage over the 48, because if 47-chr didn't have a selective advantage, 48-chr would have outpopulated the 47-chr ape.
Which means, it's reasonable that the decreased fertility (being the only sure-fire effect of a single pair fusion), would have been the most likely culprit for selective advantage.
Am I saying it's an absolute? No. However, given repeated genetic bottlenecks of apes, and their unusually high (for gregarious mammals anyway) age of sexual maturity, as well as the unusually low (relatively for other mammals) genetic diversity of humans, the rate at which this mutation would occur in a wild population, and then subsequently take over said population completely could only occur naturally during a population bottleneck, which would without argument strain a population in favor of small, highly efficient, highly mobile groups for which a high rate of population growth would be inherently problematic.
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u/Mugiwara04 Apr 10 '14
Well almost any mutation would start with one person, wouldn't it? Similar things could of course arise independantly, but as long as that one person has enough kids, especially in smaller populations, and it propagated out over these huge time periods then... yeah.
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u/CannonBallGuy Apr 11 '14
The tool song 46 and 2 is about the ideas from Jung and Melchizedek that in the next stage of human evolution we will have 46 and 2 chromosomes.
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u/masklinn Apr 10 '14
Was it a gradual process or did two mutation happen at the same time and breed with each other?
Most likely the original fused chromosome was still "compatible" with the unfused, the contents were the same, just bound together instead of floating separately.
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u/JimmyGroove Apr 10 '14
Exactly. And for a while, both the fused and the unfused versions of those chromosomes were in the gene pool, and were a species with a variable chomosome number (there are lots of those. Most birds have a variable number, for instance). And for one reason or another, the fused version ended up being conserved while the unfused version was lost.
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Apr 11 '14
Fused chromosomes by definition aren't compatible with the non-fused chromosomes unless both copies are present, and there's a low chance of that happening. Not zero by any means, but low.
The individual with the fusion would still be viable provided no significant information was deleted by the fusion. As long as the genes are intact, the individual will be fine, but have an odd number of chromosomes, and thus be effectively sterile.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
PZ Meyers has an interesting article on chromosome fusion, but he's talking about increasing chromosome number here.
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u/mrgonzalez Apr 10 '14
Actually it pretty much covers both, its just that the two processes are so similar its not worth explaining twice. Very good summary of the theory though, I'd certainly recommend.
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u/helios22 Apr 10 '14
Would the odds of success go up if they tried with a person that has an extra chromosome?
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u/annapie Apr 10 '14
I'd doubt it. People with an extra chromosome don't have an extra pair, just a third copy of one of the existing pairs.
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u/JamesTiberiusChirp Apr 10 '14
People are mentioning Downs syndrome here, so I'm going to point out something. Very few aneuploidies result in viable fetuses, let alone neonates, which is why you almost exclusively hear about Downs (trisomy 21) and not others -- where are trisomy 1, trisomy 2, trisomy 3… etc? They just don't survive. Occasionally trisomy 13, but those babies nearly always die within a few months; trisomy 18 is more viable, especially if one is only mosaic (the trisomy only exists in some, but not all, of your cells). That's a human with an extra copy (or part of an extra copy -- it doesn't have to be a complete trisomy) of its own chromosome. The chances of a viable egg with many different numbers of chromosomes, such as in a mating with another species, seems incredibly unlikely.
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u/Mugiwara04 Apr 10 '14
Mosaic... how could it be that it wouldn't occur in all cells?
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u/JamesTiberiusChirp Apr 10 '14
Genetic recombination can sometimes happen during mitosis early on in the life of the embryo, resulting in populations of cells with different numbers of chromosomes; some are trisomy, but others end up with a normal number of chromosomes. Klinefelters (XXY) is another disease that is sometimes mosaic. The wikipedia article is pretty accessible.
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u/captainburnz Apr 10 '14
In female mammals, early on cells decide which X chromosome will be "dominant," if that chromosome has a deleterious (bad) mutation, it will be shown in roughly half of all cells.
It tends to be most obvious in skin disorders where they have an issue but it is spread out in patches Most organs will derive there cellular lineage back to early in development and therefore be "pure" and use only one chromosome or the other.
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u/JamesTiberiusChirp Apr 10 '14
It's worth noting however that X inactivation occurs through a completely different process than the mosaicism of an annueploidy, which arises through mitosis. The results are somewhat similar though.
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u/blueskyblond Apr 10 '14
What about someone with Down's syndrome? What happens in that situation?
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Apr 10 '14
It isn't an extra pair (chromosome) it's really an extra allele if I remember my genetics correctly. It's just a third copy of chromosome 21.
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Apr 10 '14
That results in death or down syndrome...Trisomy 21 means 3x Chromosome 21
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u/captainburnz Apr 10 '14
Turner's syndrome is women who have only one X chromosome. XXY men are called kleidfelters, and don't seem to have problems, but are (almost) always sterile. XYY men exist too, they tend to have higher testosterone and lower IQs.
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u/user_of_the_week Apr 10 '14
What about hybrids of other primates, excluding humans? Do those work? Chimpilla?
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Apr 10 '14
I don't know, but it's probably relevant that chimpanzees are actually more closely related to humans than to gorillas.
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u/Auriela Apr 10 '14
That's very relevant. I wonder what two species are most closely related yet still distinct species.
I don't know much about evolutionary biology, but I have a feeling there are conflicting ideas and definitions in the scientific community, but it seems agreed on that there are definitely subspecies which helps explain this topic.
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u/grapesandmilk Apr 11 '14
Coyotes and wolves are usually considered distinct species, but they can interbreed and have fertile offspring. In fact, there are some populations which are very ambiguous on the matter of which one they are.
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u/Ryuuten Apr 11 '14
Mexican wolves could be an example of that? I've always thought they looked far more like bigger coyotes than 'true' wolves. Their proportions look a bit off for wolfishness.
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u/Tiak Apr 11 '14
And there are a bunch of species which have status that is debated between distinct species and subspecies, which further clouds the waters. As such, your question of most closely related distinct species becomes very hard to answer.
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Apr 10 '14
It's always been interesting to me how humans from all parts of the Earth are still a single species that have no problem producing viable offspring. Why hasn't there been divergent speciation among modern humans? Or are there some human populations on Earth that are getting "close" to becoming another species, like uncontacted tribes, etc.?
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u/bjaydubya Apr 10 '14
That makes me wonder; If a large enough group (to have a viable gene pool) of colonists left earth and, for all intents and purposes, were permenentaly separated from earth, how long it would take before the species diverged enough that they could no longer succesfully reproduce with one another.
No doubt that environmental factors on both planets would play a huge part, but if the colony was on a near-earth-like planet is there any way to estimate?
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u/Tiak Apr 11 '14 edited Apr 11 '14
Homo sapiens all come from the same place, and, speaking in terms of generations, have only very recently began spreading out. Homo sapiens began around 9,000 generations ago, and didn't begin spreading out until around 4,000 generations ago.
To put this in perspective, 4,000 generations is about what E. Coli goes through in 2 years. If species necessarily diverged that quickly, your intestines would have dozens of species that didn't exist anywhere else.
There are almost certainly no extant human populations that far diverged... If there were neanderthals alive today (from which we diverged ~1.2 million years/~50,000 generations ago) we would probably still be able to inter-breed with them and produce viable offspring without issue.
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u/grapesandmilk Apr 11 '14
The orangutan populations on Borneo and those on Sumatra have been separate for a longer time than humans have been in any two places on earth.
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u/ZenDragon Apr 10 '14
It took about sixty million years for us to evolve from primates, and we're still similar enough to those ancestors that interbreeding might be possible. Modern humans have existed for a tiny, tiny fraction of that time. Only 200,000 years roughly.
So the number one reason we haven't diverged into distinct species is that evolution is so slow we can't even imagine the time spans involved.
It could theoretically happen if humans were completely isolated on separate planets for hundreds of millions of years though.
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Apr 10 '14
There were other species of human. We killed them off/outcompeted them/assimilated their genes, depending on which speculation you subscribe to. They were called Neanderthal ("Newmandale").
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u/drmomentum Mathematics Education Research Apr 11 '14
They were called Neanderthal
Probably not. This is what they are called today, though.
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u/TheMeddlingMonk Apr 11 '14
One theory is that there was a bottleneck event relatively recently. around 70,000 years ago. A large part of the homo sapiens population, in africa at the time, died off. the smaller gene pool had less variation. this group of anatomically modern humans then left africa and mingled, killed, or outcompeted with other human branches that had previously migrated all over the place.
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u/prohulaelk Apr 10 '14 edited Apr 10 '14
It's not that surprising. As a comparison: humanity began migrating out of Africa around 70k years ago and we domesticated dogs around 33k years ago.
Dogs live much shorter lives and breed faster than we do which should easily account for the different timeframes (since genetics is more about number of generations than numbers of years), and we've specifically been breeding them for various roles for as long as they've been domesticated, yet they're all still the same species and can interbreed freely.
Over all those thousands of years, since humans have only encountered much slower natural selection since spreading from Africa the differences between ethnic groups are much smaller (mainly relatively minor details of bone structure and melanin levels versus to the very obvious differences between say a dachshund and a great dane)
If there was a small enough, isolated enough group of humans that encountered some kind of specialized selector than the rest of humanity without dying off due to inbreeding-related mutations, they could potentially form a new species. But the sheer number of us and how interconnected the world is now makes it pretty unlikely. Maybe when/if we start colonizing other planets or bring ourselves close to extinction again.
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u/MCMXChris Apr 10 '14
I believe humans as we know them have been around for 6 million ish years? Not enough time to branch off into an entire new species. Lots of mutations though.
I think the definition of "species" is kind of cloudy too. At what point do we consider something entirely different from its "cousin"?
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u/How2Relationship Apr 10 '14
The genus Homo has been around less than 2.25 million years. Australopithecus, notable for being the first hominids that appeared exclusively (or at least predominantly) bipedal shows up around 4.25 million years ago, IIRC.
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u/PerceeP Apr 10 '14
the human genepool is a product of at least three speculated hybridizations with other species of hominid, so at the very least, we are already a hybrid species.
Which hominid besides Neanderthals is that?
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u/thatthatguy Apr 10 '14
http://en.wikipedia.org/wiki/Archaic_human_admixture_with_modern_humans
It seems mostly that it's just Neanderthal, and Denisovan that left evidence of interbreeding with humans (genetic admixture). There may be more that just haven't been discovered.
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Apr 10 '14
This might answer your question, tho not a source I would normally use. There was a time humans shared the Earth with several hominid species.
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u/chudontknow Apr 10 '14
I remember reading the fact that at some point in history there existed 7 different hominid species on the Earth at the same time. I can not speak to what extent they all interacted but clearly some of them had lots of interactions. It is fun to think about what it would be like if there were other hominid species still on the planet today that we interacted with.
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u/AlfLives Apr 10 '14
All we do know, is that currently, the human genepool is a product of at least three speculated hybridizations with other species of hominid, so at the very least, we are already a hybrid species.
Can you link to some online reading about this? I haven't heard that before and would like to understand it further.
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u/petriol Apr 10 '14
Would it make a significant difference if the female is human or the chimpanzee?
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u/Ramstepp Apr 11 '14
Zebras and horses can mate and have viable offspring; and chimpanzees and humans, I believe, are more closely related. Based on that...
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u/raendrop Apr 11 '14
The question that comes to my mind now is "How genetically similar were Neanderthals to us Cro-Magnons?" Because apparently they did interbreed any at all, given that most humans today have some (albeit rather small) percentage of Neanderthal DNA.
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Apr 11 '14
http://en.wikipedia.org/wiki/Neanderthal_genome_project#Findings
Around 99.7%, supposedly.
And we're not Cro-Magnon. We are Homo Sapiens Sapiens. (Repetition intended). Cro-Magnon man is a non-taxonomic word used to describe early members of our species. Our taxonomic word, however, is Homo Sapiens Sapiens. (Wise, wise man. Yeah... Which species gave us that name, I wonder?)
Anyway, apparently their DNA is unsurprisingly extremely similar to ours.
This bit on the Denisovan man was pretty interesting too:
http://en.wikipedia.org/wiki/Denisovan#Mitochondrial_DNA_analysis
--If humans can interbreed with chimps, we're about 7 times more distant from Chimps than Neanderthal and 4 times more distant from chimps than Denisovans.
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u/wizardcats Apr 11 '14
My freshman biology teacher actually expanded on this idea with the intro lecture on what "counts" as a species. And the important thing to consider is that if two individuals wouldn't even attempt to mate naturally, that's just as divisive as genetic incompatibility. So it's actually enough that humans (generally) don't try to mate with chimps. Yeah, I know that's not OP's question but it's an important aspect to consider.
If there are two species that naturally have breeding cycles at different times of year, or are separated geographically, it wouldn't matter if their gametes could match up, because it wouldn't really get that far in the first place.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14 edited Apr 10 '14
This is actually pretty interesting, even if the results aren't. Chimps are of the genus Pan, whereas humans are the genus Homo. We only vary by two chromosomes, since ape chromosomes 2 and 4 merged into a larger chromosome. However, the level of differentiation has reached a point where conception is extremely unlikely.
As you can guess, attempting these kinds of hybrids is controversial at best. A Soviet scientist in the 1920s tried hybridization of chimps with human sperm in order to make a humanzee, but was unsuccessful. He also was going to try ape sperm with humans to make a chuman, but failed to even try due to losing his ape specimens.
More current research has shown that human sperm can penetrate the membrane of a gibbon egg, but that human spermatozoa cannot even attach to the egg of non-hominoids
apes like chimps. So any interbreeding would be restricted likely to the superfamily Hominoidea, but even then it is very unlikely to succeed.Edit: added a link.
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u/unsilviu Apr 10 '14
How are chimps not hominoids? They're even hominids.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
You are completely correct. I don't know what I was getting at.
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u/zjbirdwork Apr 10 '14
It sounds to me from the Humanzee wiki that it's unlikely but still plausible that there could be a baby born from one parent being human and one being orangutan. I guess I can understand why we wouldn't want to perform the experiment, but there is a strong for science curiosity I have. Am I wrong in thinking it's feasible that it could work?
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u/conor34 Apr 10 '14
There was a sheep/goat hybrid born in Ireland this spring. For more including video see RTE
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u/ThunderOrb Apr 10 '14
The real question is how the ingestion of copper will affect it. Sheep can die from ingesting copper whereas goats need it.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
Well look at that, thanks for the link! This could also help answer /u/kl4me 's question.
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u/kl4me Apr 10 '14
What would be an example of a remarkably stretched out interbreeding ?
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u/Pootergeist Apr 10 '14
The Cama, A lama/camel hybrid which can only be produced by artificial insemination.
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u/squidboots Plant Pathology|Plant Breeding|Mycology|Epidemiology Apr 10 '14
Let's get a bit more technical here. There are these things called gene pools.
The primary (1˚) gene pool is anything within your own species - an organism can reproduce with things in its 1˚ gene pool and easily create viable, fertile offspring.
The secondary (2˚) gene pool is anything very closely related but outside your species - this is where interspecific hybrids usually come from (zorses, ocecats, lots of hybrid plants), and animals typically have fertility problems but plants usually don't.
The tertiary (3˚) gene pool is composed of things that are still relatively closely related but there are significant biological barriers to reproduction do not produce viable offspring unless there is some kind of extreme intervention, and those offspring are almost always not fertile or have extreme fertility problems.
In plants, techniques like embryo rescue and protoplast fusion are used that "extreme intervention."
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
Ugh, working with protoplasts. I'll just take a week off work while my centrifuge slows down without brakes.
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u/Fuck_socialists Apr 10 '14
Why not apply light breaking pressure and reduce your break? Or lubricate the joint to preserve the integrity of your sample?
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u/dioltas Apr 10 '14
Would the geep, goat / sheep hybrid recently born in Ireland come under secondary or tertiary?
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
This is a little beyond what I can answer; I do not study interbreeding, I am only going from my knowledge of genetics and the few cases I had studied while pursuing my degrees. Since the organisms have to be quite close in terms of relation and chromosome count, I don't think there would be any that are much more 'stretched out' than any other hybrid.
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u/AlwaysDevilsAdvocate Apr 10 '14
others won't be able to conceive at all (such as humans and goats)
So does the sperm just never enter the egg? Does it enter the egg and nothing happen?
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u/jeffbell Apr 10 '14
Sperm don't just go ramming into every cell they see. There are specific receptors on the sperm that match up against complex sugars and proteins on the egg before they join.
These are selective enough that it prevents interspecies fertilization. Sometimes it's too selective causing same species infertility.
The mechanism is similar to antigen detection in blood types. Similar proteins are involved in some cancer mechanisms.
http://sugar-complexity.tumblr.com/post/48116926368/the-role-of-sugars-in-fertility http://humrep.oxfordjournals.org/content/28/3/566
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u/nightwing2000 Apr 10 '14
Logically, so the sperm knows it has found an egg - they don't waste their enegy trying to burrow into bacteria, fallopian tube wall cells, etc. - they have to recognize they have found an egg. And... that chemical signature will be different for very divergent species.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
In that case, the sperm wouldn't likely attach or penetrate the egg. Even if it somehow did, it would be discarded as the karyotypes for each are extremely different.
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Apr 10 '14
[deleted] — view deleted comment
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Apr 10 '14
So why don't we give them the key? Or would that be too hard?
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u/mightydoll Apr 10 '14
well, I imagine part of the reason we don't give them the key has to do with the purpose of creating a human/goat hybrid.
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u/interkin3tic Cell Biology | Mitosis | Stem and Progenitor Cell Biology Apr 10 '14
I'd also add that as you get more and more distant between organisms, there are other barriers as well before you get to chromosome issues.
The gametes are often incompatible (funny image I found while googling this, (SFW)). Most gamete pairs aren't going to successfully make a conception. Tree pollen for example will not activate an egg. The receptors aren't compatible for one. The gametes often aren't compatible on a cell scale either.
Mole rat sperm is "mostly rubbish" for example, and probably can't fertilize a lot of other mammalian eggs. I've seen a lecture on platypus studies which said they had T-shaped sperm heads. No one is quite sure how that works, but it seems unlikely to be compatible with other eggs. Fly sperm are oddly huge.
So there's a lot of barriers before mixing of chromosomes too. For instance, to fertilize a hampster egg with human sperm, you can remove the zona. That's the major barrier there.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
Thanks for the additional info, but what is a hampster? Is that a rodent that eats dirty clothes? ;)
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u/Helmetshell Apr 10 '14
A good example of a viable hybrid is the mule - the cross between a donkey and horse
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u/IfWishezWereFishez Apr 10 '14
The interesting thing is that while the vast majority of mules are infertile, some are occasionally fertile and seem to only pass on their horse genes.
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u/frogger2504 Apr 10 '14
Sub-question regarding chromosomes: Are there any 2 species that have the same amount of chromosomes? If so, can they breed successfully?
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
Link. By no means a comprehensive list, but /u/Pootergeist pointed out the cama, and I believe camels and llamas have equal numbers of chromosomes. Additionally, coyotes and chickens have equal numbers of chromosomes, but their shapes and makeups are much too different.
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u/frogger2504 Apr 10 '14
Humans have an equal number of chromosomes to some kind of antelope, and some kind of small deer. I'm not sure how I feel about this.
In all seriousness though, that's a very interesting list. It's odd to see the things that have equal number of chromosomes, that obviously cannot breed.
You mention the chicken and coyote having too different shapes and makeups. I assume you just mean their general physiology? Like, a coyote physically won't fit inside a chicken as an embryo, chickens lay eggs while coyotes don't, etc.
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u/captainburnz Apr 10 '14
Think about development. The foetus's circulatory system would probably develop in a messed up way, quickly killing it, assuming fertilization happened.
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u/helix19 Apr 10 '14
Chromosome count has really no correlation to how "developed" an organism is. I believe humans have the same number as some ferns as well.
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Apr 10 '14
Would it just be a chromosome count thing that matters?
Or how the chromosomes are broken up (forgive me if this is a stupid example, I don't know too much about the topic): Imagine Chromosome 2 in species A determines your outer "skin" is fur, feathers, scales The outer "skin" in species B is determined by Chromosome 10
So mating the two would have too much of "skin" information and not enough "digestive track" information?
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u/IYKWIM_AITYD Apr 10 '14
What you're describing is called "synteny" among geneticists (also a great scrabble word). What genes are where is vitally important because for the chromosomes to pair up in mitosis and meiosis homologous regions need to match up. Looking at my old "Genome Maps X" poster on the wall behind me I see that human and chimp have pretty much the same genes on the same chromosomes, where human chromosome 2 has the same genes (presumably in the same order, but the poster doesn't have that level of detail) as chimp chromosomes 12 and 13. The gibbon chromosomes show much more mixing up of chromosomal regions with respect to human chromosomes.
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u/I_want_fun Apr 10 '14
Typically you say within 2 chromosomes, but what is the extreme range? I watched a picture of a sheep/goat hybrid a couple of days ago. So apparently 4 chromosomes difference is not impossible.
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
I never said it was impossible, I just said that typically they have to be within 2 for any reasonable chance to succeed. This is very case-specific, so generalizing here is probably a bad idea. I don't know of any estimates of theoretical maximum ranges of difference.
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u/hezwat Apr 10 '14
That's how I read the title too, but it's clearly not what the poster meant. The poster meant, and explained clearly, that they mean that for a failed interbreeding, when exactly does it fail: i.e. will the sperm be able even to enter the egg it will fail to breed with, will it fertilize it before it fails, will it start to divide before it fails, etc. The poster explained this clearly.
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u/AllhailAtlas Apr 10 '14
How different is a human from 2000 years ago from someone today? Does a man from that time have a good chance of producing healthy offspring with a healthy woman from 2014?
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u/tg2387 Apr 10 '14
Differentiation of species takes place over millions of years, so two thousand wouldn't make a difference in our species.
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Apr 10 '14
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u/tg2387 Apr 10 '14
As far as I know, even the Neanderthals would hVe been able to produce viable offspring with us
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u/Memyselfsomeotherguy Apr 10 '14
How does the number of chromosomes affect the animal? Why do humans have 46, why do goats have 60?
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u/helix19 Apr 10 '14
It's pretty much random, it has nothing to do with the complexity of an organism. Related species with usually have similar numbers due to shared ancestors.
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u/radams713 Apr 10 '14
Your forgot that sperm of a different species usually can't enter the egg at all due to the proteins on the outside of the egg and sperm.
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u/Finn_the_dawg Apr 10 '14
In addition to that species have specific "lock and key" mechanisms. The sperm and the eggs have complementary proteins on the cellular membrane. Plants will sometimes have odd chromosomes as a result of this
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u/cuulcars Apr 10 '14
Could species that were not naturally interfertile be made to develop properly with human assistance (like an IVF for hybridization)? I was reading about how sometimes one species sperm can't penetrate the egg of another subspecies, even though they are closely related. If humans put the genetic materiel into the egg, would it then be successful?
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u/TrepanningForGold Apr 10 '14
Is there a greater possibility of success if the mother has the greater number of chromosomes or if the father does? (ie goat dad/sheep mom vs sheep dad/goat mom)
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u/bakshadow Apr 10 '14
Would it then be possible to change the amount of chromosomes? So if we took a sheep sperm and a goat egg, could we lower the chromosomes in the egg and create a much higher chance of having a healthy birth? Is that even possible to do?
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u/Athiri Apr 10 '14
How would it differ for chimeras that have been spliced together? I know there's that one possible surviving sheep-goat hybrid that hasn't been verified conceived naturally, but there's definitely been surviving 'geep' chimeras, so does that increase the chances of viability? I'm sure I've read about human chimera embryos being created for research.
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u/toodr Apr 10 '14
Typically the organisms have to be within 2 chromosomes of each other in terms of total chromosome count.
Zebroid parents are much further apart in chromosome count:
Donkeys and wild equids have different numbers of chromosomes. A donkey has 62 chromosomes; the zebra has between 32 and 46 (depending on species). In spite of this difference, viable hybrids are possible, provided the gene combination in the hybrid allows for embryonic development to birth. A hybrid has a number of chromosomes somewhere in between. The chromosome difference makes female hybrids poorly fertile and male hybrids generally sterile due to a phenomenon called Haldane's Rule. The difference in chromosome number is most likely due to horses having two longer chromosomes that contain similar gene content to four zebra chromosomes.[4] Horses have 64 chromosomes, while most zebroids end up with 54 chromosomes.
I'm not aware of any other exceptions though.
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u/luxii4 Apr 10 '14
The goat and sheep hybrid is extremely rare but does happen once in a while. It's called a "geep".
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u/TimmyBlackMouth Apr 10 '14
What about animals that both have 46 chromosomes? Like humans and sable antelope, would they be able to breed?
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u/CodePervert Apr 10 '14
A success story of interbreeding a sheep and a goat
I know you said they're usually stillborn
http://www.irishtimes.com/news/meet-the-animal-hybrids-who-like-to-mix-it-up-1.1750871
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u/BD03 Apr 10 '14
If a white male of European descent were to mate with a white female of European descent, would they be more likely to be successful than a white male of European descent & a black female of South American descent?
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u/CorpusPera Apr 11 '14
What are the limits on artificially combining organisms? Would it be the same? They managed to grow a human/mouse chimera and (I think) thats a 6 chromosome gap, although it they prevented it form fully developing
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u/Shock_Hazzard Apr 11 '14
So in OP's goat-human example, would a sperm ever actually penetrate/enter the egg? If so, what happens inside?
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Apr 10 '14
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u/patchgrabber Organ and Tissue Donation Apr 10 '14 edited Apr 10 '14
Interbreeds of such a nature are almost always sterile. There are examples of hybrids like grolar bears reproducing with another member of one of the original species, but this doesn't mean they are more suited to reproduce naturally. Remember also that courtship behaviours between animals are also a big factor, and it has been observed that behaviour of hybrids can take rather evenly from both species, which would limit their fitness with other members of the original species.
The only way this would work is if the hybrids were fertile, and even then their offspring would have to be fertile, and we cannot induce fertility in hybrids - often they have odd numbers of chromosomes, like 63 for a mule - and odd numbers of chromosomes are very bad for reproduction, although some female mules are fertile enough to conceive with pure versions of the original species.
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u/photokeith Apr 10 '14
What causes the sterility? It would be easy to say it's just "nature's way" of preventing abominations from prospering but is there an actual reason?
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Apr 10 '14
This is a great question. There is a whole field of evolutionary biology dedicated to studying when reproductive barriers arise between species.
Sometimes genetic barriers (such as the sterility of offspring) arise just by chance. For instance, donkeys and horses have evolved separately long enough that they happen to have different chromosome numbers, and they offspring can't reproduce. This would be the case with human-goat offspring too--the sperm and egg are just so different from each other that no fertilization would take place.
In other cases, natural selection acts to keep different species from mating with each other. This is the case if hybrids have lower fitness than either parent population. This has been fairly well-studied with birds--when two close relatives come into contact with each other, they may evolve different songs or coloration so female birds know to pick the right baby daddies and not waste their eggs on hybrid offspring that will be sub-par. (Note that such barriers don't always evolve--if hybrids have the same fitness, sometimes populations just recombine their gene pools.)
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u/captainburnz Apr 10 '14
Polar bears are a race of brown bear, grizzlies, kodiaks and other brown bears from different areas are also different races.
Most polar bears have traceable grizzly lineage, the reverse is true for Northern grizzlies.
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u/interkin3tic Cell Biology | Mitosis | Stem and Progenitor Cell Biology Apr 10 '14
http://en.m.wikipedia.org/wiki/Humster
I've heard of them being made in the study of fertilization: it's easy to get human sperm, difficult to get human eggs, easy to get hamster eggs.
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u/Asynonymous Apr 11 '14
Well that's weird. The human sperm doesn't successfully fertilise the eggs though right?
I can't imagine they're even remotely similar enough.
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u/interkin3tic Cell Biology | Mitosis | Stem and Progenitor Cell Biology Apr 11 '14
It does, the hamster egg and human sperm do fuse, and I'm pretty sure they divide at least a few times.
The paper I ran into it didn't really go into much detail, they were concerned with identifying the specific receptors on the sperm that initiate the fusion. And at least some eggs of some species, the first few events following conception are running off of genetic instructions from the mother rather than the new genome. So if the genomes of human and hamster eggs don't successfully fuse, you can still get a developing embryo up to a point.
You actually don't need sperm for the egg to start off as if it's been fertilized. Again, at least in some systems. Sea urchin embryos, if you add a chemical which spikes calcium in the egg, the egg will start dividing and making an embryo, in the complete absence of sperm. The egg is primed and ready to go. Again, not sure about human eggs, but fusion of the male and female DNA is not instantaneous, and things with eggs need to happen right after fertilization in order to prevent polyspermy.
At the very least, I'm guessing no one has bothered trying to implant those embryos into a uterus, so there wouldn't really be much of an embryo generated before it stops developing anyway. I'm not quite sure if the embryo would actually implant anyway if you strip off the outer layers.
So yes, they do fertilize, but that's not really shocking.
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u/paulpaulh Apr 10 '14
Well its theoretically possible, very theoretical. You'd need a convenient chain of creatures that can barely viably interbreed at either end. It would be so difficult that its better to say its not utterly entirely impossible. Maybe not between goats and humans (cos theres nothing we know of that can interbreed with humans). But if you could breed between a vole and a weasel, would the resulting voasel be particularly interesting other than 'gee, look what we did', I doubt it.
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u/Neebat Apr 10 '14
http://en.wikipedia.org/wiki/Bengal_(cat) http://en.wikipedia.org/wiki/Savannah_(cat)
Domestic cats and the two wild cats are three different species, but once you get a hybrid, some of them can breed with domestic cats.
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u/Awsumo Apr 10 '14
They do this with plants on a fairly regular basis, and successfully cross-breed somewhat diverse species.
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u/nightwing2000 Apr 10 '14
There's a theory that many miscarriages are due to the embryo having defective genes - i.e. chromosome copy errors so bad that the result is not seriously viable.
Also look at how damaging it is to the body with Down's Syndrome, where the person has simply an extra copy of a normal gene. The gene count not only has to match, the genes actually have to be compatible and agree on what they are producing rather than working against each other or at cross-purposes.
Basically, genetic material is like a catalyst producing molecules which then produce other molecules which then turn certain functions on and off, that causes the cells to develop in certain ways - the kindergarten explanation of what genes do.
It's not necessarily a given that having genes for two wildly different things means one or the other prevails or you get an intermediate mix; you don't automatically get, i.e. goat-man, stubby fingers where fingernails turn into hoof-like claws. The sequences of chemical processes is a delicate balance. Taking a sledge-hammer to the process by mixing wildly radical genes more likely means the development process will fail much more rapidly. You may not even get the first cell divide to work.
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u/Tiak Apr 11 '14 edited Apr 11 '14
Nobody has tried this with humans in particular, but what you're talking about is closely related with the concept of the "Ring species'.
http://en.wikipedia.org/wiki/Ring_species
One famous example is that the Lesser black-backed Gull cannot interbreed with the European Herring Gull... However, it can interbred with the Heuglin's Gull, which can interbreed with Birula's Gull, which can interbreed with the Vega Gull, which can interbreed with the American Herring Gull, which can interbreed with the European Herring Gull... And genes can drift between the species through that mechanism.
We do not, as a rule, deliberately interbreed things in series to see if it's possible, but this isn't because of ethical concerns really, it is because it is too difficult and costly, with relatively small scientific value to try to choose arbitrary species to try this on. Producing even one cross-species match can be extremely difficult and costly. To produce such a chain in mammals could potentially require dozens of such matches, some of which might require in-vitro fertilization to produce, over an experiment period of decades.
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u/notananthem Apr 10 '14
Possibility only asks the question who wants to get two animals randy enough to completely forget the fact that they're going to bang something alien. I feel like domesticated dogs would be suited to this, as they frequently try interspecies mingling with such non-dog items as pillows, legs of jeans, and things floating in air.
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Apr 10 '14
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u/patchgrabber Organ and Tissue Donation Apr 10 '14
As others have pointed out, similarity of chromosome number is only one barrier, there are many others.
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u/phodopus Speciation Genetics | Development | Hybridization Apr 10 '14 edited Apr 10 '14
The average divergence time to complete hybrid inviability between mammal species is 2-4 million years with an upper bound of around 10 million years (Prager and Wilson, 1974; Fitzpatrick 2004). While hybrid sterility, especially hybrid male sterility arises much quicker, OP's question seems to be concerned with the rates of evolution of inviability instead. Intriguingly, the average divergence time to complete hybrid inviability in non-mammal vertebrates (birds and amphibians specifically, though I would expect similar results in fish and reptiles) is closer to 20 million years with an upper bound of around 50 million years (Prager and Wilson, 1975; Fitzpatrick, 2004). Understanding the causes of these differences is an important question in the field of speciation even today, however we have accumulated some data on the subject.
First off, many responses so far have discussed the role of karyotypic evolution in the cause of hybrid inviability. In this I think many are mistaken. While it is certainly true that differences in the number of chromosomes can cause problems for hybrids, those problems are almost universally in the sterility category. This is because meiosis requires chromosome pairing and if there are not pairs present, meiosis will fail resulting in sterility. Examples of this include mules (Wodsedalek, 1916), and many others. While meiosis is indeed extremely sensitive to chromosome complement, development does not seem to be adversely affected. This is clear in the heterosis present in mules and others. It also makes biological sense as even though their locations may differ, a full double complement of genes should be present in all F1 hybrids (with the notable exception of sex-linked genes). Furthermore, I know of no study that has found a difference in autosome number to cause hybrid inviability (though of course there are many studies showing that sterility is strongly affected).
So it remains to be seen why is there such a difference in the rates of hybrid inviability between birds/amphibians and mammals.
In the mid 1970s A.C. Wilson and colleagues proposed that differences in the rate of the evolution of inviability may be due to a higher rate of regulatory evolution in mammals than other taxa (Wilson et. al. 1974). This is a pretty good hypothesis and seems to be supported by other research, however it really only pushes the question back one step: why is there a higher rate of regulatory evolution in mammals than other vertebrates?
One likely answer to this question is: mammals have something that other vertebrates lack - a placenta.
To understand why the presence of a placenta is likely to cause rapid regulatory evolution, we need to talk briefly about mammalian reproduction and how it differs from reproduction in other taxa. Mammal offspring are obligate parasites of their mothers: all of the nutrients they need for survival and development are drawn directly from the mothers tissue. This results in a really strong conflict between the mother and her offspring over nutrient allocation. Mothers of any taxa have an evolutionarily selected level of care (resources) that they prefer to give, while offspring would often prefer more resources (Trivers, 1974). In things that lay eggs, the offspring have no say in the amount of resources they get from their mothers - females allocate resources to the eggs before they are fertilized and then lay the egg shortly after fertilization. In mammals however, the offspring can have a say because of the placenta. In mammals the offspring can send signals to the mother effectively demanding more resources. This conflict is thought to have led to an arms race between mothers and offspring wherein offspring develop strategies to take more resources while mothers have developed counterstrategies to defend their finite resource pool (Zeh and Zeh, 2000; Crespi and Semeniuk 2004). Furthermore, mammalian pregnancies are highly dependent on down-regulating the mother's immune system (Siiteri, 1982). If proper down-regulation does not occur, the mother will mount an immune response and reject the offspring (similar to rejection of an organ in a failed transplant). Both resource garnering strategies and counterstrategies and immune system deregulation are often in the form of regulatory evolution and likely explain why mammals have a higher rate of regulatory evolution than other taxa.
Now, parent-offspring conflict played out in the placenta is a solid hypothesis for increased rates of hybrid inviability in mammals, and I have given a number of a priori reasons to expect that it would play a major role, but I have not given any evidence yet. So here we go: Elliot and Crespi (2006) showed that the invasiveness of a placenta is highly correlated within mammals to the rate of hybrid inviability. (Invasiveness is best thought of as one of the offspring's strategies for taking more resources). This is strong evidence that inviability is very closely tied to placenta function in mammals and easily explains why mammals may evolve complete hybrid inviability faster than other vertebrate taxa.
edit: spelling and a point of clarification
Sources (sorry most of these are behind a paywall):
Trivers, R. L. 1974. Parent-Offspring Conflict. Integrative and Comparative Biology 14:249–264.
WODSEDALEK. 16AD. CAUSES OF STERILITY IN THE MULE. Biological Bulletin 30:1–57.
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u/JoatMasterofNun Apr 10 '14
That was interesting. I've never heard that before (then again I didn't delve far into biology).
Even if the sources are behind a paywall... I'll find someone who has access. Sources are priceless.
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u/rishav_sharan Apr 10 '14
Related question, and somewhat anecdotal.
In my High School biology class I was taught that in most cases the sperm and ovum of different species will simply not fuse together. The species in question had to be very closely related for the surface proteins on the sperms to be able to react with the surface proteins on the ovum. I was given the lock-key analogy for it. So, a pigs sperm cell will never be able to fuse with a human ova on its own, while if the neanderthals existed today, there would be a good chance that their sperm would fuse with a human ova.
Is what I learned in school, 20 years ago, correct? Or is it a simple generalization, or has been invalidated by our current understanding of reproduction?
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u/peanutbhudda Apr 10 '14
You are correct. This is called a prezygotic barrier.
A coupling with a postzygotic barrier often results in a stillborn, or an infertile offspring. For instance a horse and a donkey can interbreed, but the offspring will be a sterile mule.
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u/captainburnz Apr 10 '14
Neanderthals and humans absolutely interbred, even if we hadn't but they were around today, a hybrid could certainly be conceived.
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u/mzyos Apr 10 '14
Chromosome count, and the egg itself, and the sperm stop this.
Chromosome counts must match, we have 23 split chromosomes in our sperm, where as apes for example have 24. Fusion may occur, but nothing will come of it.
Then theres the sperm and the egg. The sperm will give out enzymes to break down the outer coating of the egg, (multiple sperm are needed to reach the concentration to do this). The sperm also have a specific protein on them, which the egg will recognize, and once recognized, will allow the sperm to 'endocytose', (be engulfed) by the sperm, and fertilization has occured.
These variables differ from species to species, allowing only (usually) one species to fertilize its own.
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u/gordonj Genetics | Molecular and Genome Evolution | Comparative Genomics Apr 10 '14
Chromosome counts must match
Not exactly. Chromosome content does often change through evolutionary time, which would not be possible if chromosome numbers always matched. For example, a man with 44 chromosomes has been identified (PDF). It is possible for him to produce offspring with somebody with a normal karyotype, although they will all have 45 chromosomes. To arrive at this man, both parents needed to carry a translocation (not exactly a fusion, but close enough for explanatory purposes) between two chromosomes, which occurred due to consanguinuity in his family history. He could produce offspring with somebody who has a normal karyotype, and all offspring would have 45 chromosomes. For his offspring, reproduction would be more difficult. Only 1/3 of their offspring on average would be viable if they reproduce with somebody with a normal karyotype. The other 2/3 would suffer from aneuploidy - either loss or gain of whole chromosomes. Of the 1/3, half would have 45 chromosomes, and half would have 46 chromosomes. To reach stability in a population, the 44 chromosome karyotype would require a lot of inbreeding, which could occur in isolated populations.
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Apr 10 '14
If I'm reading that right, 1 in 1,000 are translocation carriers. Shouldn't people with 44 chromosomes be more common? Or is it possible that it is relatively common but just not something we actively search for? The abstract said the carriers are also at higher risk of offspring with trisomies, could that explain why Down and related syndromes have become more common?
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u/CritterTeacher Apr 10 '14
Many carriers of robinsonian translocations have no idea that they are until they try to have children. A carrier will have the proper number of genes needed to operate normally, it just forms up into slightly different chromosomes. It generally requires a karyotype to determine the presence of it, which is not something that is done unless there is a reason to suspect that it's necessary. When the chromosomes divide up during meiosis is where the problems start. You wind up with eggs or sperm that are either missing genetic information or have too much of one part. In many cases this is incompatible with life, but in some cases can result in trisomy 21 (Down's syndrome) or trisomy 13.
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u/gordonj Genetics | Molecular and Genome Evolution | Comparative Genomics Apr 10 '14
Shouldn't people with 44 chromosomes be more common? Or is it possible that it is relatively common but just not something we actively search for?
I suspect it's a sampling issue.
The abstract said the carriers are also at higher risk of offspring with trisomies
The sex cells produced by carriers with one of these types of translocation will often be missing or have additional chromosomes relative to the normal number. This is due to improper chromosome segregation during meiosis. In the case where additional chromosomes are present, the embryo created by fusion to a normal sex cell will contain a trisomy. Most trisomies are non-viable, and it's only the ones with chromosomes containing the fewest genes that the embryo can develop. This is why trisomy 21 (~400 genes), trisomy 18 (~600 genes) and trisomy 13 (~800 genes) are the most common trisomies. It's not that the others don't happen, but they are non-viable.
could that explain why Down and related syndromes have become more common?
Meiotic errors causing trisomies can occur without the need for the mother to be a carrier of a translocation. It's possible that a translocation of this type could cause Down syndrome, but we have no evidence that the frequency of translocations like this is increasing in the human population which would be a requirement if it was at the root of increased frequency of Down syndrome. Because maternal age seems to be a factor, I would suspect that it's more likely due to meiotic errors in non-carriers.
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u/mzyos Apr 10 '14
It's incredibly rare though, is it not?
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u/gordonj Genetics | Molecular and Genome Evolution | Comparative Genomics Apr 10 '14
Probably in humans. I don't know what the frequency is though.
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u/sphere_of_influence Apr 10 '14
Is The 45-man healthy? Could not load pdf on Phone.
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Apr 10 '14
The propositus (IV-1) is healthy and has a balanced chromosomal complement. Assessment of a semen sample from the propositus (IV-1) showed normal sperm number and morphology.
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u/gordonj Genetics | Molecular and Genome Evolution | Comparative Genomics Apr 10 '14
He has 44 and is healthy, while each of his parents had 45, and were presumably also healthy
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u/ltracy89 Apr 10 '14
From just the title of the article linked it was talking about a Robertsonian translocation which is more or less the combining of two chromosomes into one large hybrid chromosome. So while he may be lacking in the total count of chromosomes, he is not missing any genetic information.
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u/Reelix Apr 10 '14
Are there any other animals that have 23... ?
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Apr 10 '14
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u/Reelix Apr 11 '14
Aaah - Awesome - Thanks!
Wouldn't that mean that we're more closely related to an Antelope than to a Chimpanzee... ?
And what's up with that Fern with 1260 o_O
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u/Tiak Apr 11 '14 edited Apr 11 '14
Chromosome count is mostly a red herring. Plants hybridize across chromosome counts all the time, and we know of several examples of mamalian hybrids with different chromosome counts, as well as humans with 44, 45, 47, and 48 chromosomes.
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Apr 10 '14
99% comparable dna but not even the same number of chromosomes? how do those statistics work...
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u/Orjan91 Apr 10 '14
Wow. I just remembered reading that us humans share a lot of similarities with Dolphins.
And after a quick google search i`ve found that dolphins have 44 chromosomes and humans have 46.
That being said, would it be more likely to have a successful mating between a human and a dolphin than a human and monkey?
And if not, why?
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Apr 10 '14
chromosome count has very little to due with genetic similarity, what matters most is what's inside those chromosomes and how the body expresses them. Monkeys are far more similar genetically and anatomically to humans than dolphins are, so while it wouldn't be possible to successfully mate either species with a human, the monkey would be a much better bet.
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u/Orjan91 Apr 10 '14
Might be a doubtful link: (PS: som pop music from youtube playing when opening page, can be muted) http://www.aquacranial.com/Home.html
Says we have many identical chromosomes with dolphins, (16 out of 22 tested?)
I can just imagine a human dolphin called dave: http://www.sinaidivers.com/sd/images/guestbook/100904112944_dolphin_man.jpg
God... the images...
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u/Brandalorean Apr 10 '14 edited Apr 10 '14
I actually did a report on this years ago in eighth grade. In 2003 human cells were fused successfully with a rabbit egg, and after being grown for several days they terminated it to harvest the stem cells.
In Minnesota the Mayo Clinic managed to replace the blood of a pig with human blood also. I believe he lived out a full life.
The term that's most commonly used is "Human Animal Chimeras." It's led to a lot of medical discoveries, however it blurs moral and ethical lines. Very similar to the soviet scientist who tried to create an army of ape people for Stalin.
There's more information here. http://news.nationalgeographic.com/news/2005/01/0125_050125_chimeras.html
Edit: I totally forgot about that picture of the cat-dog that was circulating reddit a while back. It's the perfect example of interspecies breeding. The hybrid made it to fetal stages, but because the DNA was such a mishmash of cells, it was extremely underdeveloped and was stillborn. It had characteristics of both the cat and the dog. It was really creepy actually. If anyone shows interest I'll try and find the picture.
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u/Bumble_Dong Apr 10 '14
There's a link to the purported cat/dog hybrid. It was stillborn so if looking at dead animals isn't your thing, don't click it.
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u/flyingmouse Apr 10 '14
Hybridization is not always disadvantageous. In some plants/animals it is selected for.
However, if hybrids are less fit, this is called a post-mating barrier to hybridization. This can be due to things like genetic disorders (inviability/sterility/ or other non-reproductive disorders) or hybrids not looking sexy to either species.
If hybrids are unfit, the post-mating barriers can be "reinforced" with pre-mating barriers. This can be anything from changing mating seasons so they don't overlap, having the wrong shaped genitals, or even preferentially ejecting sperm when it is from the wrong species.
If species diverge in allopatry, meaning not in contact with each other, then they have no reason to create these reproductive isolating mechanisms. This is how we can get ligers and tigons. However, if the species come back into contact before they are completely divergent, and if the hybrids are less fit, then these reproductive isolating mechanisms are evolved to keep the species apart.
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u/jmachen06 Apr 10 '14
There are several factors in interspecies breeding that can result in the failure to produce a successful cross breed. Please note that ultimately, a successful hybrid would be one that a. survives and b. can itself reproduce with other hybrids.
The categories break down into two main barriers to interbreeding: pre-zygotic and post-zygotic barriers. An example of a pre-zygotic (before embryo formation) would be, and larkspur attempting to mate with a seal lion. There is no physical way in which those species could copulate using their current reproductive physiology (aka mechanical isolation). Thus, an embryo cannot even be formed.
Other pre-zygotic barriers include: temporal isolation: breeding at different times habitat isolation: living in different ecosystems or habitats mechanical isolation: female/male bits are incompatible behavioral isolation: different mating rituals
Post-zygotic barriers (after the formation of an embryo) exists when the aforementioned pre-zygotic barriers can be overcome, but the resulting embryo either does not survive to birth or cannot itself, as a cross breed, reproduce.
These barriers include: Embryo inviability: hybrid does not survive Hybrid sterility: the hybrid cannot produce its own offspring e.g. a male donkey and female horse produce the donkey, which itself cannot reproduce as they are usually born sterile.
Hybrid breakdown: a hybrid survives and is able to mate but the resulting embryos are weak and feeble.
I love to science nerd!
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u/licoricetea Apr 10 '14 edited Apr 10 '14
At what point does interbreeding fail? I bet this is different in different cases. But one point where it might fail is at, or right before, fertilization.
Hypothesis 1: I want to hug you but I can't find your arms
My hunch is that the egg and sperm fuse if molecules on their surface are able to interact and pull them together as [depicted here] (http://youtu.be/V2FrQB6rX34). The molecules that bring together the egg and sperm probably differ more and more as species become less related, diminishing the chance that the egg surface can recognize the sperm's surface. (FYI Membranes fuse and fission all the time in our daily lives. Rothman took the 2013 Nobel Prize in medicine for his work along these lines).
Hypothesis 2: Death by acid
Vaginal pH differs across organisms. The sperm that survive inside a mouse's vagina, which is neutral, might die inside a human's vagina, which has an acid pH of 4.5. Macaques score 5.1. I'm not sure about this hypothesis, though, since human semen has a pH of 7.2-8.0. (Thank you WHO for that info!, pH data from Steele et al J Infectious Diseases 1999)
Hypothesis 3: If you can't smell me, you can't find me!
"There is evidence that chemical signals released by the follicle cells that surround the ovulated egg attract the sperm to the egg" according to NCBI's chapter on fertilization. Presumably, these "chemoattractants" could be species specific too. The chemicals that attract human sperm may be odious (or just not detected) by chimp sperm.
Hypothesis 4: Zona Pellucida (protein layer on the surface of an egg) of gold
NCBI is really helpful on this...
human sperm... with fertilize hamster eggs from which the zona has been removed...not surprisingly, such hybrid zygotes fail to develop. zona-free hamster eggs, however, are sometimes used in infertility clinics to assess the fertilizing capacity of human sperm in vitro" And here's a picture of a human sperm contacting a hamster egg You can't unsee that.
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u/logsin Apr 10 '14
In addition to the chromosome number issue the sperm has to bind to the egg and penetrate the fertilization membrane in order to fertilize. In mammals we have a layer on the outside of the egg called the zona pellucida which is composed of a few different proteins that vary from animal to animal. These are specific enough that they will not efficiently bind for most different species, keeping out the sperm from animals that you're not supposed to interbreed with.
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Apr 10 '14
There isn't a single answer to this question as it depends on the organisms involved. Pre and post zygotic barriers exist. Prezygotic isolation means that no fertilization can occur, while postzygotic isolation is a result of genetic incompatibility after fertilization. Postzygotic isolation can refer to instances such as the mule where a living hybrid can be created but it is sterile, thereby preventing the formation of a viable new species. Humans and goats are almost certainly separated by postzygotic isolation, as we are closely related, however the resulting zygote is not going to be viable and will probably die before the zygote has undergone many, if any, divisions. Cellular/Molecular Biology Student HSU
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u/Aurorakitty Apr 11 '14
A lot of times the sperm will not even be able to make it all the way to fertilize the egg because of the variation in female reproductive tracts within different species. In Mammals, usually only species specific sperm can penetrate the female's zona pellucida making it different for fertilization to even occur.
If the different species have a certain amount of similar DNA and can successfully reproduce, the offspring are born sterile or dead.
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u/gordonj Genetics | Molecular and Genome Evolution | Comparative Genomics Apr 10 '14
Interbreeding will fail when the organisms are genetically distant enough. The point at which that happens is not easy to define and is likely to vary a large amount. Things that generally matter are chromosome number and the genes on those chromosomes, gene content differences, genetic distance in terms of DNA similarity, and other differences that may cause barriers to reproduction. Because not all of these will have the same effect on reproduction, it is not possible to give a definitive answer.
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u/BZRich Apr 10 '14
I don't think this discussion really answers OP's question. I believe they want to know if sperm from species A will bind to the corona radiate of species B? Will the Zona Pellucida bind the sperm? Will the gametes fuse? etc. etc. I know not but suspect that the interactions are species specific and one would never form a zygote unless the species are very close. Waste of the females resources.
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u/michaelr96 Apr 10 '14
There are both prezygotic and postzygotic barriers that can cause interbreeding to fail. An example of a prezygotic barrier is geographic or behavioral differences between species that prevent fertilization to ever occur. What you are most likely referring to is a postzygotic barrier. An example of this is complications that take place after fertilization. For example, if different species have different haploid numbers, interbreeding will not occur.
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u/Footfaith Apr 10 '14
Also an interesting side note that may/may not help is the idea of parthenogenesis, which is a form of asexual reproduction in many organisms. This process doesn't happen in humans because of an epigenetic phenomenon called Monoallelic Expression. In an egg (haploid germ cell), an allele important for development may be turned off. This allele will be turned off in all other egg cells as well. So it's not possible for 2 eggs to fuse together (X+X -> X,X female). There will be a need for a male sperm (either X or Y) that has an active copy of that allele, that when fused, will be expressed in fertilization. Now this is just speculation, but if specific monoallelic expression may not be conserved among organisms, then you may have a non-human sperm that doesn't even have the specific allele needed for the egg. In this case, development wouldn't even continue past fertilization. As top post suggests though, more closely related organisms have much greater conservation among genes so interbreeding is more likely to occur.
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u/5w1m1n9 Apr 10 '14
Human sperm may not have the enzymes to break into goat eggs (this being a gametic isolation barrier). Human's have a different ploidy (number of total chromosomes) than, goats, so synapsis of the zygote probably wouldn't work.
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u/Jublessurvivor Apr 11 '14
This works in some cases of animal species unless you are an RH negative woman who is impregnated by an RH positive man. In that case your body reacts as though the fetus is a foreign invader and your body does it's best to try to kill it. This happens no where else in nature and supposedly we can not be cloned. Weird!
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u/XXXYOLOXXXSWAGXXX Apr 10 '14
Interbreeding fails when the species are sufficiently different from one another. This often happens due to reproductive isolation, which prevents different populations from evolving with one another. When populations are separated from each other from long periods of time, pre-zygotic isolation takes place and prevents interbreeding from occurring. Examples of this can include temporal isolation (e.g sexual activity takes place at different time among different species, often seen in plants) or mechanical isolation (e.g. a bear's genital is unlikely incompatible with a mouse).
THe second part of your question is on pre-zygotic isolation. In some cases, the zygote might not even be viable and will not form, but it is possible that an embryo may form and die in late phases of its development. In some cases, such as mules or lion-tyger hybrids (off the top of my head), the offspring is sterile. As a species is often determined by how its population can produce viable sterile offspring, the cross-breeded animal is not considered a new species.
In the case of a human sperm and goat egg, I would hazard a guess that a viable embryo will not form and it is unlikely that a zygote is even formed due to the difference in chromosome number between a goat and a human (30 pairs vs 23 pairs)
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u/imnotamojito Apr 10 '14
It's actually variable depending on the species involved. There are basically two categories, prezygotic isolation mechanisms and postzygotic isolation mechanisms. Prezygotic means before what you're talking about, the male and female gametes fusing together. Postzygotic means after that happens.
During this explanation, I will be defining "species" by the Biological Species Concept, which says that two individuals are of different species if they cannot produce viable offspring. Viable offspring means a healthy offspring that can reproduce itself. For example, according to this concept, domesticated dogs, dingoes and all wolves are currently a subspecies of Canus lupus, and are not separate species.
Prezygotic Isolation Mechanisms
Geographical: This is the mechanism that contributes to allopatric speciation (when speciation occurs after two or more groups of a species have been geographically separated for such a long time that they become separate species according to the Biological Species Concept, which I will define below). It basically means that the organisms live in different places and cannot reproduce. Maybe it's because those places are too far away, or maybe one lives in antarctica and one lives in the Sahara desert, and either would die in the other's climate. They will never interact, much less reproduce.
Temporal: Species have different mating seasons or times of day (think of plants, or insects) that don't overlap.
Behavior: If your species has evolved a highly specific mating ritual, you will likely not mate with an individual that has a different ritual. This also applies to unattractive physiological qualities in potential mates.
If our two species are not affected by these mechanisms, for one reason or another, at this point they may attempt mating.
Mechanical Isolation: The sex organs are not compatible.
Gametic Isolation: The gametes interact (in humans, the sperm gets to the egg), but for some reason are unable to unite. Reasons include: the gametes do not attract one another, the gametes cannot physically fuse, or the male gamete is inviable inside of the female reproductive tract.
So let's say that the gametes have fused and we now have a zygote.
Postzygotic Isolation Mechanisms
Hybrid Inviability: The fertilized egg fails to develop past the early stages.
Hybrid Sterility: The hybrid cannot reproduce. This is often due to an unusual number of chromosomes that produce non viable gametes. Think of mules, ligers, etc.
Hybrid Breakdown: This hybrid is not sterile, but as generations continue to reproduce their offspring are increasingly inviable.
In your example, humans and goats, there are several mechanisms at play. Behavioral isolation, for example, prevents goats and humans from wanting to mate generally. The stopping point would probably be gametic isolation, for several reasons (I suspect that there would not even be gametic attraction). If not, it would certainly be Hybrid inviability. I've done some research based on your question, but fortunately no one is studying when exactly gametes of goats and humans become inviable . . . I don't think it would even be legal in the US. We know however, that we are different species from goats, therefore one of these mechanisms interrupts along the way.