A transitory signaling center controls timing of primordial germ cell differentiation.

Organogenesis requires exquisite spatiotemporal coordination of cell morphogenesis, migration, proliferation, and differentiation of multiple cell types. For gonads, this involves complex interactions between somatic and germline tissues. During Drosophila ovary morphogenesis, primordial germ cells (PGCs) either are sequestered in stem cell niches and are maintained in an undifferentiated germline stem cell state or transition directly toward differentiation. Here, we identify a mechanism that links hormonal triggers of somatic tissue morphogenesis with PGC differentiation. An early ecdysone pulse initiates somatic swarm cell (SwC) migration, positioning these cells close to PGCs. A second hormone peak activates Torso-like signal in SwCs, which stimulates the Torso receptor tyrosine kinase (RTK) signaling pathway in PGCs promoting their differentiation by de-repression of the differentiation gene, bag of marbles. Thus, systemic temporal cues generate a transitory signaling center that coordinates ovarian morphogenesis with stem cell self-renewal and differentiation programs, highlighting a more general role for such centers in reproductive and developmental biology.