Caspar modulates primordial germ cell fate both in an Oskar-dependent and Oskar-independent manner.

Primordial germ cell (PGC) formation and specification is a fundamental conserved process as PGCs are the progenitors of germline stem cells (GSCs). In Drosophila melanogaster, maternally deposited Oskar (Osk) and centrosome dynamics are two independent determinants of PGC fate. Caspar, Drosophila homolog of Fas-associated factor 1 (FAF1), promotes PGC formation/specification and maintains the PGC count by modulating both the Osk levels and centrosome function. Consistently, casplof PGCs display reduction and inefficient release/transmission of germ plasm. Defective centrosome migration and behavior are evident even prior to PGC formation engineered by Osk and its targets. Taken together with the inability of Osk to regulate nuclear and centrosome migration, our data demonstrate that Casp encodes a novel bi-modal regulator of PGC fate as it controls Osk levels likely by downregulating translational repressor, Smaug (Smg) and also influences nuclear/centrosome migration during early mitotic nuclear division cycles (NCs 6-9), which are Osk-independent. We discuss dual functionality of Casp vis-à-vis germline/soma segregation as it helps acquire both the PGCs and the surrounding soma their individual identities.