Why is it that many plants, like bananas, and some animals, like certain fish, can survive nondisjunction to become triploids or polyploids, but humans (and I imagine mammals in general) can’t?
Basically it’s just that humans are a lot more complex than plants. More proteins are in delicate balance, so it takes less of a nudge to throw the whole thing off kilter.
Good question that I don’t know the answer to. I’m pretty sure that plants only survive triploidy in the individual sense. The organism develops but is sterile, so even in plants being triploid is very harmful, if you take the evolutionary viewpoint that there’s no difference between not being born and being born but not able to pass on your genes.
The only trisomy event (one extra chromosome) that humans can have and still reach birth is Down’s Syndrome, so it’s not surprising that having a whole extra set is going to cause a spontaneous abortion. Why that is, why some normally diploid organisms can develop with three sets of genes while humans can’t, I’ve no clue. It may be like ** smeghead** said and the more complex the organism the easier it is for things to go wrong. Something else I was thinking was that plants, fish, insects and other organisms that can survive being triploid don’t develop in utero. Maybe something about triploid tissue sets of the mother’s immune system? That’s a real WAG, though.
Apparently a polyploid ( tetraploid ) rat has been found in Argentina as of 1999 ( Sept. issue of Nature ). apparently its sperm is immense compared to other rats in order to contain all the genetic material. So it is not unknown in mammals - Just incredibly rare.
- Tamerlane
Not entirely correct. Trisomies of chromosomes 13 and 18 have been observed in live births (Patau and Edward syndroms respectively), but these are very rare and affected individuals rarely live beyond a few weeks. Sex chromosome trisomies (and polysomies) have also been observed (XXY and XXX). These individuals fair better since the body has a mechanism for deactivating extra X chromosomes.
Actually, this is an interesting point, because there are plenty of healthy polyploid plant species out there. Many plants are capable of asexual reproduction - they send out shoots or whatever - and so not being able to produce gametes isn’t that much of a problem for them. Commercial bananas, for instance, are triploid. Wild diploid bananas aren’t really edible, because the part we eat is actually the seed pod, and is normally filled with marble-sized inedible seeds. The triploid plant’s seeds abort very early on, leaving only those tiny black specks we see in the banana. Seedless watermelons are similar, but I think they’re heptaploid.
Generally, though, it’s only odd-numbered ploidy plants that have trouble making seeds. If they’re tetraploid, for instance, they can usually manage it, because each chromosome has a partner in meiosis.