In Drosophila, sex-determination is under the control of the “switch” gene Sex lethal (Sxl). While in some species the sex of the soma is sufficient to determine the sex of the germline via inductive signaling, sex-determination in the Drosophila germline also occurs cell-autonomously via intrinsic signaling dictated by the germline sex chromosome constitution. Interestingly, when Sxl is expressed in XY germ cells, these germ cells are able to produce eggs upon transplantation into a female somatic gonad, demonstrating that even in the germline, Sxl is the “switch” and is sufficient to activate female identity [1]. In both the germline and soma the presence of two X chromosomes leads to Sxl expression. However, its control is different in the germline than in the soma at both, cis- and trans- regulatory levels, and we are studying how this is regulated in the germline. The DNA elements responsible for the activity of Sxl’s sex-specific promoter (SxlPe) in the germline have not been identified. To identify these elements, we are cloning different DNA fragments covering the entire region upstream of the Sxl transcriptional start site into an enhanced GFP (eGFP) reporter vector to test for sex-specific expression in the germline. Further, we are investigating the trans- acting factors that control Sxl expression in the germline. Based on previous studies, the X chromosome “counting genes” important for activating Sxl in the soma are not required in a dose-dependent manner in the germline. However, we have found that knocking down sisterlessA (sisA) specifically in the female germline results in an ovarian tumor phenotype, similar to masculinization of the germline due to loss of Sxl function. Encouragingly, Sxl expression is lowered in sisA RNAi ovaries. Previously, our lab has found that germ cells without SXL exhibit a derepression of Phf7, a male germline sexual identity gene. We are investigating if Phf7 is expressed in sisA RNAi ovaries but have already found that Phf7 RNAi rescues the ovarian tumor from sisA RNAi. Finally, we are searching for additional trans-regulators of Sxl through an RNAi screen of genes expressed sex-specifically in the undifferentiated germline. Our studies will further our understanding of how the ‘sex’ of the germline is coordinated with the ‘sex’ of the soma. Since a failure to match germline and somatic sex leads to defects in gametogenesis in both flies and humans, this work is important for our understanding reproductive biology and human health.
[1] Hashiyama, K., Y. Hayashi, and S. Kobayashi. "Drosophila Sex Lethal Gene Initiates Female Development in Germline Progenitors." Science 333, no. 6044 (2011): 885-88.