Drosophila switch gene Sex-lethal can bypass its switch-gene target transformer to regulate aspects of female behavior

Abstract

The switch gene Sex-lethal (Sxl) was thought to elicit all aspects of Drosophila female somatic differentiation other than size dimorphism by controlling only the switch gene transformer (tra). Here we show instead that Sxl controls an aspect of female sexual behavior by acting on a target other than or in addition to tra. We inferred the existence of this unknown Sxl target from the observation that a constitutively feminizing tra transgene that restores fertility to tra(-) females failed to restore fertility to Sxl-mutant females that were adult viable but functionally tra(-). The sterility of these mutant females was caused by an ovulation failure. Because tra expression is not sufficient to render these Sxl-mutant females fertile, we refer to this pathway as the tra-insufficient feminization (TIF) branch of the sex-determination regulatory pathway. Using a transgene that conditionally expresses two Sxl feminizing isoforms, we find that the TIF branch is required developmentally for neurons that also sex-specifically express fruitless, a tra gene target controlling sexual behavior. Thus, in a subset of fruitless neurons, targets of the TIF and tra pathways appear to collaborate to control ovulation. In most insects, Sxl has no sex-specific functions, and tra, rather than Sxl, is both the target of the primary sex signal and the gene that maintains the female developmental commitment via positive autoregulation. The TIF pathway may represent an ancestral female-specific function acquired by Sxl in an early evolutionary step toward its becoming the regulator of tra in Drosophila.

Fig. 1.

Lesions in partial-loss-of-function alleles that are central to the present study: Sxl^f7,M1, Sxl^M1,fΔ33, and Sxl^f18,f32. Putative translation starts (AUG) and stops (S) are indicated in the context of the establishment promoter (P_e) that transiently responds very early to the primary sex-determination signal, the maintenance promoter (P_m) that operates thereafter, and the various exons, among which are the male-specific translation-terminating exon 3, which is responsible for the gene's sex-specific functioning, and exons 9 and 10 that encode alternative C-terminal isoforms important for germ-line but not somatic functioning. The two RNA-binding domains (RRM) are shown, as is the location of a proline-rich domain (black line) essential for germ-line activity that is just proximal to the site of alternative splicing out of exon 8 that is blocked in the female-sterile allele, Sxl^f18. M1 is a transposon insertion that, by itself, leads to semiconstitutive feminizing expression of Sxl and hence dominant, male-specific lethality. Sxl^f7,M1 and Sxl^M1,fΔ33 were selected for suppression of that male-specific lethality, and Sxl^f18,f32 was selected for suppression of Sxl^f18 sterility. Sxl^fΔ33 is an intragenic deletion that interferes only partially with female functioning, whereas P{SxlΔPm} is a duplication of the entire gene minus the P_m region. It provides wild-type P_e function but nothing more. Details regarding mutations that are not referenced in the text are given in Materials and Methods.

Fig. 2.

A powerful genetic screen used to generate a constitutively feminizing tra allele for a test of the TIF/TSF distinction and to explore the basis for cis-dominant suppression of Sxl^f18 sterility by Sxl^f32. Among 300,000 somatically masculinized chromosomal daughters (Ψ males) from mutagenized fathers that were screened, one mosaic and one nonmosaic phenotypic female were recovered. Because only Ψ males normally survive this screen, rare phenotypic females arising by either intergenic or intragenic suppression were obvious. Dominant temperature-sensitive–lethal balancers (whose derivation is described in Materials and Methods) were used in conjunction with the recessive temperature-sensitive–lethal shibire^ts to facilitate generation of the large numbers of parents needed and to eliminate the superfluous classes of progeny.

Fig. 3.

Possible origin of the TIF-pathway branch in the current Drosophila sex-determination regulatory gene pathway from a time when tra was the master autoregulating sex-determination gene with Sxl under its sex-specific control. (A) It is proposed that an early step in the switch from tra to Sxl as the positively autoregulating target of the female sex-determination signal was the acquisition by Sxl of a gene target with a female-specific function, with Sxl then becoming a regulatory target of tra. (B) Siera and Cline (13) suggested that redundancy in positive feedbacks involving tra and Sxl might have led to the present-day regulatory arrangement with tra downstream rather than upstream of Sxl. After the switch from A to B, female-specific targets of Sxl that originally were part of the TSF pathway would stand out as part of a TIF pathway. (C) Redrawing of B that presents the same regulatory relationships in a more conventional way.