32 



BIOLOGIC BASIS OF SEX 



mosomal constitutions. Sciara is like Dro- 

 sophila and many other genera in its basic 

 sex-determining mechanism. There is a sex- 

 indifferent period when the maternal geno- 

 type may influence development through 

 cytoplasmic substances. This is followed by 

 active differentiation when the sex pheno- 

 types may be influenced by various agencies 

 but when passed becomes fixed for pheno- 

 typic sex. The indifferent period may repre- 

 sent a fairly large number of early cell gen- 

 erations as in amphibia or fish. Somatic 

 and germ cells are labile but are normally 

 susceptible to control only by forces de- 

 veloped within the organism as present 

 knowledge indicates for Sciara. In Sciara, 

 only the somatic cells conform in chromo- 

 some number to their expected phenotypes. 

 The germ cells show not only a regulated in- 

 stability of their chromosome number but 

 the number is different from that of the 

 supporting somatic cells. The fact that sex 

 mosaics in Sciara may develop both an 

 ovary and testis in the same animal (Metz, 

 1938; Grouse, 1960a, b) shows localized 

 phenotypic sex determination occurring 

 fairly late in development rather than the 

 generalized determination that would oc- 

 cur if sex were established at fertilization. 



The germ line chromosomal differentia- 

 tion from that of the somatic tissue has 

 parallels in a number of fairly widely dis- 

 persed species. The significance of these 

 types to soma line determination and to the 

 reversibility or irreversibility of differen- 

 tial gene activation in differentiated cells 

 subsequent to embryogenesis has been con- 

 sidered by Beermann (1956). Although 

 these types are yet in the fringe areas of 

 our knowledge surrounding the general 

 paths taken in the evolution of sex deter- 

 mining mechanisms, they promise to have 

 more generality as clarification of gene ac- 

 tion becomes more exact. 



B. APIS AND HABROBRACON 



Hymenopteran interi:)retations of sex de- 

 termination have largely turned on studies 

 of the honey bee. Apis mellijera Linnaeus 

 and Habrohracon juglandis Ashmead. Dzi- 

 erzon early established that the drones of 

 honey bees come from unfertilized eggs, 

 whereas the females, queens, and workers 

 come from fertilized eggs so that sex differ- 



entiation is marked by an N set of chromo- 

 somes for the male and 2N for the female. 

 This mechanism was satisfactory until it 

 was realized that on a balanced theory the 

 doubling of a set of chromosomes, if truly 

 identical, should not change the gene bal- 

 ance and consequently not the sex. Further 

 doubt was cast on the N-2N hypothesis for 

 sex determination by the discovery of dip- 

 loid males by Whiting and "Whiting (1925). 

 These difficulties were resolved by Whiting 

 (1933a, b) with the suggestion that the two 

 genomes in the female were not truly alike 

 but actually contained a sex locus linked 

 with the gene for fused which was occupied 

 by different sex alleles as Xa and Xb . In 

 this A'iew the heterozygous condition would 

 lead to the production of females, whereas in 

 the haploid or homozygous condition either 

 of these genes would react to produce males. 

 Difficulties with this hypothesis became evi- 

 dent in that the ratios of males to females in 

 certain crosses were significantly divergent 

 from those which were expected. Evidence 

 was collected and trials made to interpret 

 these difficulties (Whiting, 1943a, b) by as- 

 suming that on fertilization by Xa-bearing 

 sperm the polar body spindle would turn 

 so as to cast out the Xa chromosome and re- 

 tain the complementary Xb in the majority 

 of cases instead of in a random half. Snell 

 (1935) offered the hypothesis that there 

 could be several loci for sex genes, such that 

 the X locus might be a master locus, but 

 when this locus was in homozygous condi- 

 tion the sex would then be controlled by 

 genes in other loci in the genome. As pointed 

 out by Bridges (1939) this hypothesis would 

 satisfy that of sex gene balances postulated 

 in Drosophila and of change in emphasis on 

 loci for sex genes as observed by Winge 

 (1934) in Lebistes. However, the work of 

 Bostian (1939) called both hypotheses into 

 question. Consideration of these further re- 

 sults led to the multiple allele hypothesis of 

 complementary sex determination for 

 Habrobracon (Whiting, 1943a) taking a 

 similar form to that of the sterility relations 

 observed for a single locus in the white 

 clover of INIelilotus. Under this system the 

 sex locus would be occupied by multii)le al- 

 leles, any one of which or any combination 

 of identical alleles would be male-producing, 

 but any combination of two different al- 



