Polyploidy is the multiplication of number of chromosomal sets from 2n to 3n (tr
ID: 30659 • Letter: P
Question
Polyploidy is the multiplication of number of chromosomal sets from 2n to 3n (triploidy), 4n (tetraploidy) and so on. It is quite common in plants, for example many crops like wheat or Brassica forms. It seems to be rarer in animals but still it is present among some amphibian species like Xenopus.
As I know in mammals polyploidy is lethal (I don't mean tissue - limited polyploidy). I understand that triploidy is harmful due to stronger influence of maternal or paternal epigenetic traits that cause abnormal development of placenta, but why there is no tetraploid mammals?
Explanation / Answer
Great question, and one about which there has historically been a lot of speculation, and there is currently a lot of misinformation. I will first address the two answers given by other users, which are both incorrect but have been historically suggested by scientists. Then I will try to explain the current understanding (which is not simple or complete). My answer is derived directly from the literature, and in particular from Mable (2004), which in turn is part of the 2004 special issue of the Biological Journal of the Linnean Society tackling the subject.
The 'sex' answer...
In 1925 HJ Muller addressed this question in a famous paper, "Why polyploidy is rarer in animals than in plants" (Muller, 1925). Muller briefly described the phenomenon that polyploidy was frequently observed in plants, but rarely in animals. The explanation, he said, was simple (and is approximate to that described in Matthew Piziak's answer):
animals usually have two sexes which are differentiated by means of a process involving the diploid mechanism of segregation and combination whereas plants-at least the higher plants-are usually hermaphroditic.
The 'complexity' answer...
Another answer with some historical clout is the one given by Daniel Standage in his answer, and has been given by various scientists over the years (e.g. Stebbins, 1950). This answer states that animals are more complex than plants, so complex that their molecular machinery is much more finely balanced and is disturbed by having multiple genome copies.
This answer has been soundly rejected (e.g. by Orr, 1990) on the basis of two key facts. Firstly, whilst polyploidy is unusual in animals, it does occur. Various animals with hermaphroditic or parthenogenetic modes of reproduction frequently show polyploidy. There are also examples of Mammalian polyploidy (e.g. Gallardo et al., 2004). In addition, polyploidy can be artificially induced in a wide range of animal species, with no deleterious effects (in fact it often causes something akin to hybrid vigour; Jackson, 1976).
It's also worth noting here that since the 1960s Susumo Ohno (e.g. Ohno et al. 1968; Ohno 1970; Ohno 1999) has been proposing that vertebrate evolution involved multiple whole-genome duplication events (in addition to smaller duplications). There is now significant evidence to support this idea, reviewed in Furlong & Holland (2004). If true, it further highlights that animals being more complex (itself a large, and in my view false, assumption) does not preclude polyploidy.
The modern synthesis...
And so to the present day. As reviewed in Mable (2004), it is now thought that:
In addition, there are now several new suspected factors involved in ploidy which are currently being investigated: