the basis of pairing relationships between the several genomes, are con- 

 sidered by Stebbins to be autoallohexaploids of the type AAABBB. 

 Most alloploids are either tetra- or hexaploids, but higher levels of 

 ploidy exist or are suspected. Obviously autoploids and alloploids rep- 

 resent extremes of relationship between genomes, and any kind of 

 intermediate may be expected — indeed most cases will represent some 

 deviation from the ideal type. 



Polyploidy in Animals 



In contrast to plants, polyploidy is very rare in animals. Nonetheless, 

 some animal groups do contain polyploid members. For example, poly- 

 ploid species or races have been quite definitely established in flatworms, 

 segmented worms such as the earthworm, crustaceans such as Artemia, 

 the Lepidoptera, and the sawfly, Diprion simile. Individual animals in 

 many groups may be polyploid, but for a variety of reasons are unlikely 

 to establish a polyploid race. Cases of individual polyploidy have been 

 definitely established in the urodeles, and less satisfactorily in pigs, rab- 

 bits, mice, and some birds. 



Why the incidence of polyploidy should be so much greater in plants 

 than in animals is still an open question. Certainly there is little reason 

 to believe that the possibility of becoming polyploid is any greater at the 

 cellular level in the former than in the latter. The most commonly ac- 

 cepted explanation is the one originally offered by H. J. Muller in 1925. 

 The essential part of the argument is that polyploidy would be expected 

 to upset the sex chromosome mechanism in bisexual organisms by abol- 

 ishing the heterogamety on which sex determination depends. Therefore, 

 both in animals and plants, polyploidy would become established only 

 in species which are parthenogenetic, hermaphroditic, or reproduce vege- 

 tatively. In general this restriction seems to apply, but there are a number 

 of dioecious species of plants which are polyploid and a number of 

 animal groups, such as the self-fertilizing hermaphroditic molluscs, 

 which appear to be devoid of any polyploid members though there is no 

 obvious cytological barrier to the establishment of such races. A second 

 explanation for the difference in incidence in polyploidy in the two king- 

 doms is that offered by Stebbins (1950) who suggests that cellular dif- 

 ferentiation and presumably intercellular relationships are more complex 

 in animal than in plant embryogeny and thus more liable to reflect dis- 

 turbance due to polyploidy. For this point of view there is some evidence 

 of the existence of polyploids at pregastrula stages, e.g., in mice (Fisch- 

 berg and Beatty, 1950), but against it is the fact that polyploidy has 

 become established in animals in which embryogeny is at least as com- 



178 / CHAPTER 8 



