POLYPLOIDY 



niicrotuherculatus bivalens all are tetraploid forms which reproduce bi- 

 sexually. The golden hamster, Cricetus auratus, may belong here, for it 

 has 22 pairs of chromosomes, while its nearest relatives have 11 pairs. It 

 has been claimed, but not proven, that this species is an allotetraploid of 

 Cricetus cricetus and Cricetulus griseus. Thus it appears that any sexual 

 unbalance which may be caused by random segregation of the sex chro- 

 mosomes in polyploids can be overcome. But that it is not frequently 

 overcome is indicated by the rarity of polyploids among bisexually repro- 

 ducing plants and animals, and by their over-all rarity in the Animal King- 

 dom, where separate sexes are the rule. 



A related phenomenon is fragmentation of the chromosomes, with the 

 result that multiples of a basic number appear in a series of chromosome 

 counts. Vandel lists examples in most of the major groups of animals in 

 which species show chromosome numbers which are twice the number 

 of some more primitive species. Like polyploidy among plants, increase 

 in the number of chromosomes by fragmentation among animals appears 

 to go hand in hand with increasing specialization. Thus, among the lower 

 mammals, the diploid number is generally 24; among the Eutheria, it is 

 generally 48; among the highly specialized eutherians, such as the ungu- 

 lates, it is often 60; and among such highly specialized mammals as the 

 rodents, it may be as high as 84. Why fragmentation of the chromosomes 

 should be related to evolutionary specialization is an obscure point, but 

 it may be that this is based upon position effects, or comparable phe- 

 nomena. 



REFERENCES 



GooDSPEED, T. H., AND M. V. Bradley, 1942. "Amphidiploidy," Botan. Rev., 8, 271- 

 316. 



MuNTZiNG, A., 1936. "The Evolutionary Significance of Autopolyploidy," Hereditas, 

 21, 263-378. 



Stebbins, G. L., 1950. "Variation and Evolution in Plants," Columbia University Press. 

 ( Karpechenko, Winge. ) 



Vandel, A., 1937. "Chromosome Number, Polyploidy, and Sex in tlie Animal King- 

 dom," Proc. Roy. Zool. Soc, London, Series A, 107, 519-541. 



(Three of the above papers are thorough reviews of the subjects stated. Unfortu- 

 nately, none is up to date. Stebbins' book is a comprehensive review of evolution in 

 plants, and it includes a more timely study of polyploidy. ) 



307 



