VARIATION OF SCUTELLAR BRISTLES IN DROSOPHILA. 3. SEX-DIMORPHISM.

Abstract

HE number of scutellar bristles in unselected wild-type stocks is normally four in both males and females, which contrasts with the marked sexdimorphism of the number of scutellars in scute stocks. The reduction of scutellar bristle number effected by the scute mutations is greater in males than in females. FRASER (1963) has shown that the frequency of extra scutellar bristles is greater in females than in males, in.wild type. He also showed that the frequency of missing scutellar bristles is greater in males than in females, in wild type. These results show that there is a sex-dimorphism of scutellar number in wild type as well as in scute. RENDEL (1959) showed that selection for extra scutellars is more effective in females than in males (see also PAYNE 1918; SISMANIDIS 1942; FRASER ( 1963) ; FRASER, SCOWCROFT, NASSAR, ANGELES and BRAVO, in manuscript). There would appear to be a general sex-modification of the expression of scutellar genes which is only obvious when the development of scutellar bristles has been shifted above or below the canalized path leading to four bristles. The scute locus is located in the left end of the X chromosome, and it is possible that the sex-dimorphism of scutellar number is primarily determined by this locus being present twice in females, and only once in males. The sex-dimorphism may be due to the sex difference of dosage of the scute locus. This can be assayed by adding short sections of the left end of the X chromosome to the Y chromosome. If these short sections of the X chromosome contain s ~ + and sc then comparisons can be made of normal females, with males carrying two scf or sc alleles. Three such duplication Y chromosomes have been studied, and the results are presented and discussed below.