Transcriptional repression of reaper by Stand still ensures female germline development in Drosophila

Abstract

Apoptosis plays a central role in shaping tissues and preserving cellular integrity across developmental stages. In the germline, its precise regulation is critical to ensure both the elimination of aberrant cells and the maintenance of reproductive capacity. However, the molecular mechanisms that control apoptotic susceptibility in germline cells remain poorly defined. Here, we identify stand still (stil) as a female germline-specific regulator of apoptosis in Drosophila. Loss of stil leads to nearcomplete depletion of germline cells at the time of eclosion, associated with upregulation of the pro-apoptotic gene reaper (rpr) and activation of caspase-dependent cell death. Reporter assays in S2 cells show that Stil directly represses rpr transcription through its N-terminal BED-type zinc finger domain. The Dietera-restricted conservation of stil and rpr is consistent with a functional association. Despite the absence of stil, undifferentiated germline cells remain resistant to apoptosis. Analysis of publicly available chromatin data reveals that the rpr locus in these cells resides in a closed, H3K9me3-enriched chromatin state, suggesting a Stil-independent mode of transcriptional silencing. Together, our findings uncover two distinct mechanisms that protects the female germline from rpr-dependent apoptosis: Stil-mediated transcriptional repression that operates in both undifferentiated and differentiated germline cells, and an additional chromatin-based silencing mechanism that functions specifically in undifferentiated cells. This work provides new insights into the interplay between transcriptional and chromatin-based regulations that maintain germline cell identity and survival. Author summary Apoptosis eliminates unnecessary or abnormal cells, yet its inappropriate activation in germline cells would be highly detrimental to reproduction. A finely tuned mechanism that permits cell death only when necessary while preventing its harmful activation is indispensable for maintaining fertility and species continuity. In this study, we identified stand still (stil) as a factor that prevents inappropriate activation of the pro-apoptotic gene reaper (rpr) in the female germline of Drosophila. Loss of stil resulted in near-complete depletion of ovarian germline cells at eclosion, accompanied by elevated rpr expression and activation of caspase-dependent apoptosis. Furthermore, we demonstrate that Stil directly represses rpr transcription through its N-terminal zinc finger domain. In addition, undifferentiated germline cells, including germline stem cells, exhibit a closed chromatin state at the rpr locus, suggesting that rpr is intrinsically less accessible for transcription in these cells. Together, this two-layered mechanism suggests that susceptibility to cell death is tuned according to developmental stage, providing a conceptual framework for understanding reproductive and developmental abnormalities as well as diseases associated with dysregulated cell death.