CROWDING AND SEX DETERMINATION 303 



same host will have the same effect as if all were of the same species; 

 nor do we know whether the effect of malnutrition or other adverse 

 environmental influences upon the host will affect the sex ratio of 

 the parasites. Christie has noted that with heavy infestations there 

 is a reduction in size of individuals present, whether males or fe- 

 males, but that there is no tendency toward a differential suppression 

 of the gonads. He finds that there is a tendency for the parasites in 

 a heavily infested host to complete their parasitic development more 

 rapidly than those in a lightly infested animal. While this applies 

 to both sexes, it is more apparent with the females. 



SEX IN CLADOCERA 



The sex situation in the cladoceran Crustacea differs from the 

 instances previously discussed in that with these animals the usual 

 method of reproduction is parthenogenetic : the females produce 

 eggs that develop without fertilization to form other females. In 

 nature, after a period of purely parthenogenetic reproduction of this 

 sort, there may occur an outbreak of bisexuality in which parthe- 

 nogenetic eggs develop into males,' and at the same time, or, more 

 usually, slightly later, eggs are produced which require fertilization 

 before development. The eggs so produced are resistant to many 

 adverse conditions, such as drying or freezing. With favorable con- 

 ditions some of them, after a dormant period, produce partheno- 

 genetic females, and the usual type of reproduction begins again. 

 In the other cases cited, the effect of crowding upon sex dealt with 

 the effect on the expected sex ratio; here we are interested in its 

 effect on the transfer from the production of parthenogenetically 

 produced, self-sufficient females to the production of parthenogene- 

 tically produced males. 



' It is interesting to note that Banta and Wood (1928) report genetic evidence that 

 males of the cladoceran, Daphnia longispina, are diploid, and that E. Allen (1928) finds 

 in cytological studies of Moina macrocopa: "After the egg is laid, the first division occurs 

 in the parthenogenetic egg without reduction in the number of chromosomes. In the 

 sexual egg, the first maturation division results in the haploid number, which is eleven. 

 The diploid number is twenty-two in both t3^es of egg. In the eggs of crowded mothers, 

 which should produce a high percentage of males, no evidence has yet been obtained 

 indicating that the male number is haploid. Several such crowded mothers have been 

 studied." 



