POLYPLOIDY AND POLYSOMY 



extra set in a triploid to give spores or gametes with, as a rule, ever^' 

 number up to the haploid. In consequence haploids are highly sterile 

 and their rare progeny are diploids. Entirely homoz)^gous diploids 

 have been produced in this way in the tomato and in Datura. 



In plants, and perhaps even more in animals, changes in chromo- 

 some numbers are a continual source of new variation which may 

 be recognized by change in size, form or fertihty of the variants. 

 Their frequency will depend largely on the conditions of temperature 

 at the time of formation of the parental germ cells and of their 

 fertilization, so that the results recorded in Table 9 might no doubt 

 be multiphed or divided by ten in special circumstances. 



In plants the polyploid, at least the tetraploid, mutants may estab- 

 lish new races. 



TABLE 9 



WHOLE CHROMOSOME MUTANTS IN POPULATIONS OF 



PLANTS AND ANIMALS: LYCOPERSICUM FROM RICK 



(1945) AND TRITURUS FROM FANKHAUSER (1941) 



Amongst animals on the other hand, polyploidy is of little account. 

 Where the sexes are separate, obviously the first polyploid has no 

 mate of its own kind. In experiment healthy and vigorous polyploids 

 can be readily produced in Drosophila, species of Triton and elsewhere. 

 They cannot maintain themselves by reproduction. In nature, there- 

 fore, although they may occur in all species, they establish them- 

 selves only where sexual reproduction has been abandoned (as in 



99 



