Establishing initial germline and somatic fates in the early Drosophila embryo

Abstract

In Drosophila, somatic and germline fate specification commences during early embryogenesis, with primordial germ cells (PGCs) sequestering germline determinants. The PGCs are distinguished by special properties including transcriptional quiescence and limited mitosis. PGCs from embryos compromised for germ cell-less (gcl) misexpress somatic genes, resulting in PGC loss. Recent studies documented a requirement for Gcl during degradation of the terminal patterning determinant, Torso. Intriguingly, ectopic expression of a degradation-resistant form of Torso recapitulates phenotypes of gcl mutant embryos: somatic transcription in PGCs and aberrant spreading of pole plasm. This suggests that antagonism between Gcl and Torso ensures controlled release of germ plasm underlying germline/soma distinction. Somatic specification, however, depends on zygotic genome activation (ZGA), which is directed by the pioneer factor Zelda, likely with other factors. We explored involvement of Chromatin-Linked Adapter for MSL Proteins (CLAMP) during ZGA. We found that many zygotically-activated genes are similarly influenced in either clamp or zelda compromised embryos. Furthermore, clamp knockdown embryos recapitulate several morphological phenotypes observed in Zelda-depleted embryos. Thus, we propose that CLAMP cooperates with Zelda to regulate zygotic transcription. As ZGA initiates before PGC formation, we wondered whether young PGCs are truly insulated from somatic influences. We observe that zelda- or clamp-compromised embryos display reduced PGC counts and germ plasm spreading. Since centrosomes trigger release of germ plasm during PGC formation, we examined centrosome behavior and discovered defects throughout the soma and in those associated with nuclei invading the germ plasm. Additionally, these PGCs activate somatic genes while somatic nuclei exposed to ectopic germ plasm reduced transcription, suggesting that both PGC and somatic identities require ZGA regulators. As ZGA impacts germline/soma distinction, somatic signals clearly affect developing PGCs, challenging the exclusively cell autonomous model of germline determination in Drosophila embryos. In this regard, the bone morphogenetic protein (BMP) signaling pathway is a promising candidate. We demonstrate that PGCs can respond to BMP signals, which can impact germline determination. Thus, we identify a novel role for the BMP pathway in Drosophila PGC specification. As BMP signaling is critical for mammalian germline determination, our data evidence a conserved mechanism underlying germ cell development.