sisterless Ais required for activation ofSex lethalin theDrosophilagermline

Abstract

In Drosophila, sex determination in somatic cells has been well-studied and is under the control of the switch gene Sex lethal (Sxl), which is activated in females by the presence of two X chromosomes. Though sex determination is regulated differently in the germline versus the soma, Sxl is also necessary and sufficient for the female identity in germ cells. Loss of Sxl function in the germline results in ovarian germline tumors, a characteristic of male germ cells developing in a female soma. Further, XY (male) germ cells expressing Sxl are able to produce eggs when transplanted into XX (female) somatic gonads, demonstrating that Sxl is also sufficient for female sexual identity in the germline. As in the soma, the presence of two X chromosomes is sufficient to activate Sxl in the germline, but the mechanism for "counting" X chromosomes in the germline is thought to be different from the soma. Here we have explored this mechanism at both cis-and trans-levels. Our data support the model that the Sxl "establishment" promoter (SxlPE) is activated in a female-specific manner in the germline, as in the soma, but that the timing of SxlPE activation, and the DNA elements that regulate SxlPE are different from those in the soma. Nevertheless, we find that the X chromosome-encoded gene sisterless A (sisA), which helps activate Sxl in the soma, is also essential for Sxl activation in the germline. Loss of sisA function leads to loss of Sxl expression in the germline, and to ovarian tumors and germline loss. These defects can be rescued by the expression of Sxl, demonstrating that sisA lies upstream of Sxl in germline sex determination. We conclude that sisA acts as an X chromosome counting element in both the soma and the germline, but that additional factors that ensure robust, female-specific expression of Sxl in the germline remain to be discovered.

The Regulation of Germline Sex Determination in Drosophila by Sex lethal

“…cDNA representing the Pe transcript was identified in early germ cells [Shigenobu et al, 2006], and expression of an early promoter (Pe-GFP) reporter was observed in early first instar larval germ cells [Goyal et al, 2021]. In addition, when the late Sxl protein was specifically tagged, expression was first observed in second instar larvae, after Pe is active, consistent with a need to activate Pe in order to allow production of functional protein from Pm [Goyal et al, 2021].…”

“…The model of autoregulation in the soma, where the early promoter (Pe) is active only in females to produce a protein that autoregulates the transcript from the maintenance promoter (Pm) is also thought to occur in the germline [Hager and Cline, 1997]. cDNA representing the Pe transcript was identified in early germ cells [Shigenobu et al, 2006], and expression of an early promoter (Pe-GFP) reporter was observed in early first instar larval germ cells [Goyal et al, 2021]. In addition, when the late Sxl protein was specifically tagged, expression was first observed in second instar larvae, after Pe is active, consistent with a need to activate Pe in order to allow production of functional protein from Pm [Goyal et al, 2021].…”

“…In addition, the cis-regulatory regions required for Pe activation in the germline are different from those in the soma. A 3.0-kb fragment upstream of Pe is sufficient for expression in the soma [Keyes et al, 1992], while a much larger region, including sequences both upstream and downstream from Pe, is required for expression in the germline [Goyal et al, 2021]. Despite these differences, it was recently found that one somatic XCE, sisA, is utilized in the germline [Goyal et al, 2021].…”

“…A 3.0-kb fragment upstream of Pe is sufficient for expression in the soma [Keyes et al, 1992], while a much larger region, including sequences both upstream and downstream from Pe, is required for expression in the germline [Goyal et al, 2021]. Despite these differences, it was recently found that one somatic XCE, sisA, is utilized in the germline [Goyal et al, 2021]. sisA is expressed in early germ cells, and loss of sisA in the germline causes a loss of Sxl expression and produces a similar germline defect as loss of Sxl.…”

Trajectory Mapping of the EarlyDrosophilaGermline Reveals Controls of Zygotic Activation and Sex Differentiation

“…4c). Among those, two are X-encoded transcription factors Cut and SisA, the latter being a known X-dosage sensor in the soma and recently shown to regulate germline expression of Sxl, a known regulator of female germline sexual development (Cline and Meyer 1996;Goyal et al 2020). These genes may also contribute to germline sex determination.…”