Abstract:

Understanding how genes control sexual dimorphism is a fundamental problem in Developmental Biology. Over the last thirty years, geneticists and developmental biologists have uncovered a sex determination hierarchy for Drosophila melanogaster . It is now well-established that the initial choice of sexual identity is determined by the X chromosome to Autosome ratio which activates the transcription of Sex-lethal (Sxl ) in females (XX) but not males (XY). Sxl expression is maintained in females throughout development via an autoregulatory alternative splicing event and is necessary for female differentiation. As Sxl also functions to prevent male-specific dosage compensation, loss of Sxl is female lethal.

Since the maintenance of Sex-lethal expression is dependent upon regulated alternative splicing, mutations in general splicing regulators often lead to female-specific lethality. For example, mutations in fl(2)d lead to male-specific Sxl splicing and female lethality. Although fl(2)d is needed for the alternative splicing of other pre-mRNAs in Drosophila and its human homolog was isolated from functional spliceosomes, the precise function of fl(2)d is unknown. We attempted to determine the function of F1(2)d by characterizing its ability to interact with other well-defined splicing regulators. We found that Fl(2)d forms a stable complex with Sxl and the general splicing regulator Snf. Our experiments suggest that fl(2)d functions during the early stages of spliceosome assembly.

During our analysis of fl(2)d , we serendipitously found that fl(2)d1 ovaries have egg chamber packaging defects. We next tried inducing Sxl- follicle cell clones and found that this also led to the same phenotype. Further analysis revealed that Sxl negatively regulates the Notch (N ) signaling pathway in ovaries. In theory, however, Sxl could potentially regulate the N signaling pathway in tissues that are common to both sexes since both genes are expressed in common tissues. This immediately led us to ask, why would N be regulated differently in females compared to males? Using a series of genetic assays, we showed that the regulation of N by Sxl contributes to sexual dimorphism. Finally, we have evidence indicating that the interaction between Sxl and N might also be important in the female germline and in evolutionarily related species.