THE TEMPERATURE-SENSITIVE MUTATION VIRTS (VIRILIZER) IDENTIFIES A NEW GENE INVOLVED IN SEX DETERMINATION OF DROSOPHILA

When XX animals homozygous for the temperature-sensitive mutation vir(ts) of virilizer (2-103.9) are raised at the restrictive temperature of 29-degrees-C, they are transformed into sterile intersexes with a morphology comparable to XX flies mutant at the sex-determining gene doublesex (dsx). The gonads of the vir(ts) intersexes are ovaries in which the germ cells undergo abortive oogenesis. At the permissive temperature of 25-degrees-C or below, XX vir(ts) animals develop into marginally fertile females. The temperature-sensitive period of vir(ts) is within the third larval instar. XY males are not affected by the mutation. Animals that are homozygous for vir(ts) and either transformer (tra) or tra2 develop as pseudomales; on the other hand, constitutive expression of a female-specific tra product rescues XX animals from the effect of vir(ts), but these females are sterile. The data show that tra and tra2 are epistatic to vir. Animals with only one wildtype copy of either tra or tra2 and mutant for vir(ts) are already transformed into intersexes at 25-degrees-C. Conversely, the presence of three copies of the tra+ gene largely prevents the effect of vir(ts) at 29-degrees-C; such flies are practically female, but sterile. Animals homozygous for vir(ts) and heterozygous for dsx(D)/+, raised at 29-degrees-C, are transformed into severely masculinized intersexes or almost pseudomales. The observations suggest that vir acts above and via tra and tra2 to achieve proper female-specific expression of the dsx gene in XX zygotes.