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Posted by chris_harper2 on March 22, 2003 at 18:38:01:
In Reply to: Re: probably a dumb question (but theres no avialable info) posted by meretseger on March 21, 2003 at 22:55:47:
::The thing that keeps mules from reproducing is that horses and donkeys have different numbers of chromosomes,
If this were true then hybrids between wild horses (33 chromosome pairs) and domestic horses (31 chromosome pairs) should also be infertile like mules, right? But these hybrids are always fertile.
And there have been many fertile female mules, but that is certainly not the norm.
::In snakes, for whatever reason, closely related species seem to have the same number of chromsomes,
There have been cases where snakes with differing chromosome numbers have produced fertile offspring. I don't recall what the species in question were, however. I'll do a google search later and see if I can track it down.
This has happened in frogs as well.
::so if it's possible for them to hybridize, the hybrids are fertile.
Only based on the examples we currently have. We don't know that all hybrids specimens are fertile nor do we know how all of them will hold up with multiple generation breeding. In some birds F1 hybrids are fertile but then F2's are not. Similar mechanisms may be at place in snakes and other reptiles (lizard and turtle hybrids have also been produced).
::I don't think I've heard of any infertile snake hybrids.
I also have not hear of any infertile snake hybrids. But the fact we have not found any does not prove in any way that they do not exist. For years many species of birds were thought to be monogamous and this was accepted as scientific "fact". But with modern DNA techniques we've found that monogamy does not exist in ANY of the bird species previously thought to be monagamous.
So hybrid fertility in snakes in not an acceptable null model. At least not yet.
::Maybe there's something about mammal cells and evolution that makes their chromosomes change number more frequently, I'm not sure.
One current thought is that pre-mating isolation mechanims are more important in reptiles and post-mating isolation is more important in mammals. This certainly seems robust given that most reptiles have very limited ranges. And sure enough mammal species that have minimal migration and/or movement seem more likely to produce fertile hybrid offspring.