FOUNDATIONS FOR SEX 



21 



in females largely of D. repleta constitution 

 and that the foundation female for the 

 more fertile line came as a result of a cross- 

 over between an infertility gene and that 

 responsible for the unusual sex ratios. Con- 

 tinued back crosses of females of this line 

 to white D. repleta males have been made. 

 Out of 33 fertile cultures, 16 gave approxi- 

 mately equal ratios of wild-type and white 

 females, wild-type and white males; and 

 17 gave 472 wild-type females, 5 white fe- 

 males, 63 white intersexes, 482 wild-type 

 males, and 339 white males. The white fe- 

 males presumably represented crossovers 

 between the loci of white and the critical 

 gene in the X derived from D. neorepleta. 

 The intersexes were of extreme type with 

 gonads very small (rudimentary ovaries in 

 those cases where they were found at all). 

 External genitalia were missing or of ab- 

 normal male type. Other somatic character- 

 istics included weakness which prevents 

 emergence and accounts for the loss of 

 about 88 per cent of the flies expected in 

 that class. The intersexual condition was 

 suggested as being caused by an autosomal 

 dominant gene derived from D. neorepleta 

 which so conditions the eggs before meiosis 

 that two D. repleta X chromosomes result 

 in the development of intersexes rather 

 than females. The action of this gene occurs 

 before meiosis and may in fact be absent 

 from the intersexes themselves. This was 

 confirmed by crosses of white brothers of 

 the intersexes to pure D. repleta females 

 when the offspring were normal for both 

 sexes; but when these Fi daughters were 

 mated to D. repleta males only intersexes 

 and males resulted. This last cross further 

 showed that although this gene was derived 

 from D. neorepleta in D. neorepleta cyto- 

 plasm, the D. neorepleta cytoplasm was not 

 necessary for the intersexes to result. The 

 case also has an important bearing on the 

 location of the sex-determining factors, for 

 in this cross the characteristics were only 

 secondarily governed by the cytoplasm 

 through earlier determination by genes of 

 the mothers' nuclei. 



Significant parallels are found between 

 the autosomal gene of D. neorepleta and 

 the third chromosome Ne gene of D. mela- 

 nogaster (Gowen and Nelson, 1942) de- 

 scribed in the section on high male sex 



ratio. The D. neorepleta gene caused the 

 cytoplasms of the eggs laid by mothers 

 carrying it to become more male potent. 

 The female potencies of two X chromosome 

 D. repleta zygotes were unable to balance 

 these male elements. Many died late in de- 

 velopment. Those able to emerge became 

 intersexes. The Ne gene also sensitizes the 

 cytoplasms of all eggs of mothers carrying 

 it causing any 2X + 2A, 3X + 3A, 3X + 

 2A or 2X -h 3A intersexes of female type to 

 die in the eggs at 10 to 15 hours whereas 

 males XY + 2A and male-type intersexes 

 live. 



Other mechanisms for causing sex and 

 sex ratio changes are known, i.e., Cole and 

 Hollander (1950), but few are as well 

 worked out as that of D. repleta x D. neo- 

 repleta. New mechanisms will certainly be 

 found for the opportunities for genetic anal- 

 ysis of sex in hybrids are many. 



B. MOSAICS FOR SEX 



Recent genetic work has emphasized the 

 fact that individual D. melanogaster may 

 be composed of cells of more than one genie 

 or chromosome constitution. The main type 

 of sex mosaic is the gynandromorph com- 

 posed of cells of female constitution on one 

 side, XX + 2A, and male, X + 2A on the 

 other, the loss of the X chromosome coming 

 at an early cleavage (Morgan and Bridges, 

 1919; L. V. Morgan, 1929; Bridges, 1939). 

 The mosaic areas are large since the cells of 

 each type may be in nearly equal numbers. 



At the other extreme Stern (1936) has 

 shown that phenotypic mosaics may de- 

 velop as a consequence of the somatic chro- 

 mosome pairs crossing over at late stages 

 in embryologic development. Special genes, 

 Minutes, materially increase the frequen- 

 cies of these crossovers. The proportion of 

 the body occupied by the cross-over type 

 cells is small because crossing over takes 

 place so late in development. 



Recently another agent in the form of a 

 ring chromosome has been discovered which 

 greatly increases the production of sex mo- 

 saics. Some ring chromosomes are relatively 

 stable whereas others are quite unstable, 

 the instability depending to some extent on 

 aging of the eggs and environmental factors 

 (Hannah, 1955). The instability is manifest 



