FOUNDATIONS FOR SEX 



19 



of two kinds. Triploid females were out- 

 crossed separately to males of two diploid 

 stocks. The triploid daughters from these 

 crosses were again crossed to males like 

 their fathers. This repeated outcrossing to 

 the different stocks resulted in a shift in 

 the grade of the intersexes in both cases, in 

 one case to a very high proportion of ex- 

 treme male-like intersexes and in the other 

 to nearly as high a proportion of extreme 

 female-type intersexes. These results were 

 interpreted as showing that the grade of 

 development of the sexual characters was 

 dependent on genetic modifiers. The second 

 experimental test consisted of subjecting a 

 triploid stock to selection toward a line 

 which produced a high proportion of ex- 

 treme male type intersexes and to another 

 line which would have a high proportion of 

 extreme female type intersexes, each being 

 much higher than the original stock. The 

 l)rocedure established a line with a high 

 proportion of extreme male type and an- 

 other line which was not so extreme in its 

 proportions but was definitely higher in fe- 

 male type intersexes than the original stock. 

 Again these results w'ere interpreted as in- 

 dicating the selection of modifying genes 

 of unknown positions within the inheritance 

 complexes. 



This evidence had been preceded by 

 Bridges' (1921) discussion in which he 

 wrote "the fourth-chromosome seems to 

 have a disproportionately large share of the 

 total male-producing genes; for there are 

 indications that triplo-fourth intersexes are 

 predominately of the 'male-type', while the 

 dijjlo-fourth intersexes are mainly 'female- 

 type'." In 1932, Bridges concluded for Dro- 

 sophila intersexes that, in spite of the fa- 

 vorable genetic checks, in repeated and 

 varied tests, it has been impossible to state 

 with any assurance whether the 4th chro- 

 mosome is or is not a large factor in the 

 variability encountered. 



In our own work a stock of attached X 

 triploids has for many years consistently 

 produced only male type intersexes. This is 

 in contrast to what we frequently see within 

 other lines of triploids as made up utilizing 

 the cIIIG gene (Gowen and Gowen, 1922). 

 Lines established from these triploids or- 

 dinarily have three intersexual types: male, 



intermediate, and female. These lines, how- 

 ever, may be subjected to selection in both 

 directions. In our experience, male intersex 

 lines are established rapidly and remain 

 relatively permanent. On the other hand, 

 female intersex lines take many more gen- 

 erations and are less stable. These lines 

 have been extensively examined for their 

 4th chromosome constitutions (Fung and 

 Gowen, 1960). The male intersex lines sel- 

 dom show more than two 4th chromosomes. 

 On the other hand, the female intersex lines 

 rarely show two 4th chromosomes but gen- 

 erally have more than three, the number 

 sometimes going as high as four. More tests 

 are needed but the evidence would seem to 

 indicate that the fourth chromosome does 

 have sex genes. These genes, contrary to the 

 first notion of Bridges, are more frequently 

 of the female determining type than of the 

 male determining type. This would make 

 the 4th chromosome like the X in that it 

 carries an excess of female influencing genes 

 and is not like the rest of the autosomes 

 which have an excess of male determining 

 genes. These observations are of particular 

 interest in view of Krivshenko's (1959) pa- 

 per. In this investigation on D. busckii, cy- 

 tologic and genetic evidence was presented 

 for the homology of a short euchromatic 

 element of the X and Y chromosome with 

 each other and also with the 4th chromo- 

 some or microchromosome of D. melano- 

 gaster. This conclusion is based on (1) ob- 

 served somatic pairing of the X and Y of 

 D. busckii by their proximal ends in gan- 

 glion cells and the conjugation of the short 

 euchromatic elements of these chromosomes 

 at their centromeric regions in the salivary 

 gland cells; (2) the presence in the short 

 Y chromosomal element of normal allelo- 

 morphs to four different mutant genes of 

 the short X chromosomal element; (3) the 

 presence in the short element of the D. 

 busckii X chromosome of chromosome IV 

 mutants: Cubitus interruptus. Cell and 

 shaven of D. melanogaster. These consider- 

 ations furnish proof for the homology of 

 this X chromosomal element with the 4th 

 chromosome of Drosophila. 



These observations of Krivshenko sup- 

 port our findings that the 4th chromosome 

 of D. melanogaster has an excess of female 



