Functional analysis of germ cell less during germ cell formation and specification in Drosophila melanogaster embryos

Abstract

Transcriptional quiescence is an evolutionarily conserved trait that distinguishes the embryonic Primordial Germ Cells (PGCs) from their somatic neighbors. In Drosophila melanogaster embryos, Germ cell less (Gcl) protein is required for proper pole cell formation and the establishment and maintenance of transcriptional quiescence in PGCs. Thus, PGCs from germ cell less (gcl) embryos ectopically express several somatic genes including two x-linked numerator elements, sisterless-A (sisA) and sisterless-B (sisB). These two proteins are shown to activate transcription of Sex-lethal (Sxl), a sex-determination gene that orchestrates female identity in the somatic nuclei. Importantly, wild type (WT) pole cells from blastoderm stage embryos do not express Sxl and are naïve with respect to their sexual identity. Here, we have examined Sxl expression in gcl PGCs. Consistent with precocious activation of numerator elements, Sxl is inappropriately activated on a transcriptional level and in a sex-nonspecific manner in gcl PGCs. Reciprocally, ectopic expression of gcl in the soma is sufficient to inhibit Sxl expression. Precocious expression of Sxl in PGCs results in consistent reduction in the number of PGCs in early embryos and disrupts their migration during mid-embryogenesis. Supporting the conclusion that Sxl is a critical target of Gcl, simultaneous removal of Sxl and gcl mitigates the loss of PGCs observed in gcl embryos. These observations underscore the biological relevance of transcriptional quiescence in the embryonic PGCs and establish Sxl, the master determinant of female somatic fate, as a critical target of silencing mechanisms in PGCs. Finally, considering the findings of two recent papers, we propose two possible models for the activity initiated by Gcl.