Positive regulation of Sex-lethal in the germline and soma of Drosophila melanogaster.

摘要

Drosophila somatic cells establish and maintain their female identity by cell-autonomously engaging a positive feedback loop of the master feminizing gene, Sex-lethal (Sxl). SXL-F protein is induced in response to X-chromosome dose, and establishes this positive feedback loop by directing the splicing of its own pre-mRNA from a default, presumably non-feminizing isoform into a functional, female-specific isoform. Although Sxl autoregulatory splicing is also required for germline sex determination, it is initiated by a different mechanism in the germline than the soma. Here I describe my studies of two different aspects of Sxl positive regulation.;First, I describe the identification and characterization of sex-lethal helper (sxh), which was identified in a mutagenesis screen for functional partners of Sxl in the germline and soma. SXH is a conserved RS-domain-containing protein that specifically facilitates Sxl's autoregulatory splicing in the germline and soma. Unlike the known facilitators of Sxl autoregulatory splicing virilizer(vir) and fl(2)d, sxh does not regulate the splicing of SXL-F'S primary somatic sex determination target, transformer(tra). Although sxh lies on the X-chromosome, it does not appear to be part of the mechanism by which X-chromosome dose is signaled in the soma or germline.;Second, I describe how I made the surprising discovery that maternal expression of SXL-F'S target, traF facilitates engagement of the Sxl positive feedback loop in the gonadal soma. I made this discovery while studying Sxl-mutant females in which the autoregulatory feedback loop specifically fails to be triggered in ovarian somatic cells. The Sxl-mutant female cells produce feminizing SXL-F continuously, but at a level below the threshold that would fully engage the positive feedback loop. Although all mutant larval ovaries appear normal, most become nearly unrecognizable during metamorphosis. I show that this disorganization of the gonadal soma is due to intersexuality, because it can be rescued by manipulating tra or doublesex(dsx) to produce a uniformly female or male somatic sexual phenotype. Maternal overexpression of tra has the same rescuing effect, but does not affect sons in any way. Additionally, I found that a pair of TRA-F binding sites is conserved in the critical splicing regulatory region of 12 Drosophila Sxl orthologs, suggesting a mechanism by which TRA-F, a target of SXL-F, might directly augment Sxl autoregulatory splicing.