Section 10 — Developmental Genetics 



sex comb on each basitarsus. The differentiation 

 of extra sex combs could depend on (a) the 

 primary change of the second or third into a first 

 leg, or (b) the autonomous action of esc in the 

 sex-comb area. Which alternative is correct may 

 be decided by producing esc/esc mosaic patches 

 of tissue at the sex-comb sites on the second and 

 third legs in esc + /esc males. 



In order to obtain such mosaics, males having 

 an X chromosome bearing the gene yellow (v), 

 and second chromosomes — one bearing the esc + 

 gene and an insertion of the y+ locus into the left 

 arm, and the other carrying the esc gene — 

 were irradiated during the larval period. Somatic 

 crossing over in a dividing cell yields two daughter 

 cells, one homozygous for the r f insertion and for 

 esc + , and the other homozygous for esc. The 

 former results in tissue which is not yellow and 

 not esc, and the latter in the absence of the y + 

 insertion, and the presence of the y allele on the 

 X chromosome produces yellow bristles which 

 are also homozygous for esc . 



The differentiation of extra sex combs is 

 interpreted as having been caused by the auton- 

 omous action of esc. 



Research performed under the auspices of the 

 U.S. Atomic Energy Commission. 



10.7. Developmental Genetics of the Gene "Sex- 

 combless" in Drosophila melanogaster. Ard- 

 hendu Mukherjee (Berkeley, U.S.A.). 



The mutant "sex-comb/ess" (sx, I) affects 

 differentiation of sexcomb teeth on the basitarsus 

 of the male first leg. Sx is associated with an 

 overlapping inversion, in the X-chromosome of 

 salivary glands, which involves the region 

 between HCand 15F. Studies on the morphology 

 and chaetotaxy of the basitarsus of sat male first 

 leg reveal that the mean number of sexcomb 

 teeth and transverse rows of bristles per basi- 

 tarsus is 1.7 and 6.9 respectively. Bristles, that 

 are intermediate between a typical tooth and a 

 macrochaeta, frequently appear in the sexcomb 

 area. Whenever present, the tooth is always 

 oriented at an angle less than that found in the 

 normal male, and may lie anywhere beyond the 

 fourth transverse row. Studies on gynandric 

 mosaics utilizing ring-X reveal that in a small 

 patch of sx male tissue in the sexcomb area (in an 

 otherwise sx/+ female basitarsus) sx inhibits the 

 formation of any tooth and thus behaves autono- 

 mously. When sx is combined with ey D , en 

 (both differentiate extra sexcomb on the basi- 

 tarsus of the first leg) or tra (tra/tra — transformed 



female differentiates sexcomb on the first leg) 

 the mean number of teeth is greatly reduced in 

 every case. This shows that sx is epistatic over 

 the action of these genes with respect to the dif- 

 ferentiation of sexcomb teeth on the first leg. 



10.8. A Gene which Transforms Males and Females 

 into Intersexes. Philip E. Hildreth and John 

 C. Lucchesi (Berkeley, U.S.A.). 



A recessive gene on the third chromosome of 

 Drosophila melanogaster has been discovered 

 which when homozygous changes both chromo- 

 somal females and males into intersexes. The XX 

 and XY intersexes are extremely similar in 

 appearance, and both exhibit male and female 

 traits. The pigmentation of the tergites is similar 

 to that of the wild-type male. The seventh tergite 

 is present, and at its base is the seventh spiracle, 

 both female characteristics. The ventral part of 

 the eighth abdominal segment has a protuberance 

 similar to the female gonopod while the ninth 

 segment, as in males, bears claspers. The anal 

 plates are situated vertically as in males. Neither 

 the XX nor XY intersex has sex combs; however, 

 on the basitarsus of the forelegs of each, the 

 bristles of the last transverse row are enlarged 

 and rotated toward the area a sex comb would 

 occupy if present. Internally the XX intersex 

 usually has male and female reproductive parts 

 with varying degrees of completeness. Well- 

 developed ovaries are present in some. Frequent- 

 ly a single gonad may be attached to both the 

 male and female systems. The XY intersex usu- 

 ally has a predominantly male internal repro- 

 ductive system, but both a male and female 

 system are present in some. In all cases examined 

 the gonads are poorly developed. 



Research performed under the auspices of the 

 U.S. Atomic Energy Commission. 



10.9. Genetics of Gynandromorph Production in 

 Aedes aegypti. K. S. Rai and G. B. Craig Jr. 

 (Notre Dame, U.S.A.). 



About 100 gynandromorphs have been isolated 

 from Aedes aegypti (L.) during three years of 

 genetical investigations. Although simple genetic 

 ratios are not apparent, frequency of their 

 occurrence in some strains and in particular 

 crosses indicates an hereditary basis. A line of 

 demarcation between male and female tissue 



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