Genetic Systems II | 189 



that began the cycle are restored. In embryos that will become males 

 the X's from the father are eliminated, whereas in embryos that will 

 become females one of the paternal X's is also eliminated and de- 

 generates in the cytoplasm. There are many fascinating details not 

 included in the above description and many problems of interest to 

 geneticists, embryologists, and evolutionists that have not been 

 solved. One cannot help but be impressed by the very large number 

 of phenomena observed by cytogeneticists which are unknown vari- 

 ables in an overall picture of the evolutionary process. 



Eupolyploidy 



Eupolyploidy as a genetic system occurs very frequently in plants 

 but seems to be quite rare in animals. It has been suggested that 

 increase in chromosome number in animals would upset the sex- 

 determination mechanism and for this reason it has been selected 

 against. Only in parthenogenetic animals are there polyploid series 

 comparable to those in plants. Chromosome numbers have been 

 sampled, however, for relatively few organisms, and conclusions 

 about the occurrence of polyploidy are dangerous. On the basis of 

 available numbers in plants, polyploidy appears to be most common 

 among the vascular plants. For this reason, the following discussion 

 will be limited to that group. 



The study of chromosome numbers and behavior in wild forms 

 and in hybrids made in the laboratory usually enables one to identify 

 polyploid organisms. It is even sometimes possible to synthesize in 

 the laboratory a species recognized in nature. If the diploid hybrid 

 can be made, chemicals, such as colchicine, which upset the spindle 

 mechanism can be employed to double the chromosome number. If 

 a plant has a high chromosome number and is impossible to cross 

 with other forms, one can only infer its polyploid nature. If pairing 

 is perfect in such a plant and if its fertility is high, it must be assumed 

 that it has become functionally diploid. Only the techniques of 

 comparative morphology (including comparisons of proteins) offer 

 hope of deducing its origin. 



In discussing this sort of genetic system it is customary to draw a 



Fig. 9.4 I (see opposite page) Chromosome cycle of Sciara coprophila. 

 Discussion in text. (Modified from White 1954, Animal Cytology and 

 Evolution, 2d ed., Cambridge University Press, and after Metz, 1938, Am. 



Nat. 72. ) 



