Sex Determination 



111 



diploid having ten pairs of chromosomes 

 like their sisters. Genetic study shows that 

 such diploid males have a biparental origin. 

 Not only are diploid males relatively in- 

 viable, but the hatchability of sibling eggs 

 is very poor. A study of intrastrain and 

 interstrain breeding supports the interpreta- 

 tion 5 that a multiple allelic series determines 

 sex in this form. With respect to this sex- 

 determining locus or chromosome region, 

 haploids are males, diploid heterozygotes 

 are females, and diploid homozygotes are 

 semisterile males. 



Role of Genotype in Sex Determination 



In certain organisms, male and female gam- 

 etes are produced in the same individual. 

 Animals of this type are said to be hermaph- 

 roditic (after Hermes and Aphrodite), and 

 plants, monoecious. The hermaphrodite 

 snail, Helix, has a gonad which produces 

 both eggs and sperm from cells which some- 

 times lie very close together. In the earth- 

 worm, eggs and sperm are produced in sepa- 

 rate gonads located in different segments of 

 the body. In certain mosses, egg and sperm- 

 like gametes are also produced in separate 

 sex organs (located on the same haploid 

 gametophyte). 



In all these cases, the two types of gametes 

 are produced by an organism that has but 

 a single genotype; that is, one that is not 

 genetically mosaic. Nevertheless, it might 

 be supposed, at first, that the haploid geno- 

 type carried by eggs and by the sperm is 

 different and causes the difference in pheno- 

 type and behavior. In the case of the 

 gametophyte of mosses, however, the indi- 

 vidual is haploid and so are both types of 

 gametes it forms. Accordingly, in such or- 

 ganisms we cannot expect differences in 

 gene content to be the basis either for the 

 formation of gametes or for the different 

 types of gametes produced. 



5 See P. W. Whiting (1943). 



Gamete formation in hermaphroditic and 

 monoecious organisms, therefore, must de- 

 pend primarily upon environmental differ- 

 ences. Such differences must exist even 

 between cells which lie close together, as is 

 the case in Helix. It is reasonable to sup- 

 pose that the same kinds of environmental 

 factors which can direct one group of cells 

 to form muscle cells and an adjacent group 

 to form bone cells, can direct the differen- 

 tiation of still other cells to make gonadal 

 tissue in which adjacent cells can further 

 differentiate as sperm and egg. 



Note, however, that sex involves another 

 kind of differentiation, which, at least in 

 organisms like the mosses, is separate from 

 the type of gamete formed. This problem 

 (which will not be discussed in detail here) 

 concerns the genetic and environmental fac- 

 tors responsible for the onset of meiosis, 

 which is, of course, the feature most funda- 

 mental to the success of the sexual process 

 as it presently occurs in many species. 



In the examples already mentioned, the 

 type of gamete differentiated depends upon 

 the different positions which cells have 

 within a single organism; consequently, they 

 are subject to differences in internal and 

 external environments. In the marine an- 

 nelid, Ophryotrocha, the two sexes are in 

 separate individuals, and the sex type formed 

 is determined by the size of the organism. 

 When the animal is small, because of youth 

 or because it was obtained by amputation 

 from a larger organism, it manufactures 

 sperm; when larger, the same individual 

 shifts to the manufacture of eggs. In this 

 case the environment of the gonad is changed 

 by the growth of the organism. 



Finally, consider sex determination in the 

 marine worm, Bonellia, in which the sepa- 

 rate sexes are radically different in appear- 

 ance and activity — females being walnut- 

 sized and having a long proboscis, males 

 being microscopic ciliated forms that live 

 as parasites in the body of the female. Fer- 



