The function of RNA Binding Protein 9 in germ cell differentiation in Drosophila ovary

Abstract

Germ cell development is an essential process to ensure continuity of species. Drosophila oogenesis has long been served as a model system to study germ cell development. Previously, the Elav-Hu family protein RNA-binding protein 9 (rbp9) has been reported important for germ cell differentiation in Drosophila ovary, but its mechanism of function is largely unknown. In this study, we aimed to further study the function of Rbp9 in germ cell differentiation. Firstly, we confirmed rbp9 mutant phenotype by using rbp9 germline specific RNAi knock-down. We further confirmed that Rbp9 is highly expressed in sixteen-cell cyst and early egg chambers by using BAC transgenic reporter. Rbp9 mutant germarium contains accumulated early cysts. BMP signaling and Bam regulation is not affected in rbp9 mutant, but early differentiation cysts expressing Nanos, Sxl and Bam are accumulated in rbp9 mutant. Moreover, sixteen-cell cysts are failed to further differentiate in rb9 mutant. Both Bruno and Orb expression cysts are present in rbp9 mutant, but they are blocked before oocyte specification and begin to degenerate. And meiosis is also failed initiated in rbp9 mutant. The function of rbp9 in germ cell differentiation is required for both adult stage and before adult stage. Though Rbp9 is critical for cyst differentiation, over-expression of Rbp9 does not affect germ cell development in germarium region. But Rbp9 over-expression will affect number of cyst division times and mid stage egg chamber development. Moreover, we identified that rbp9 genetically interacts with bam in regulating cystoblast differentiation. Rbp9 functions as downstream of Bam in germline differentiation, as Rbp9 over-expression can partially drive germ cell differentiation in bam mutant, but lack of Rbp9 blocks Bam driving germ cell differentiation. In all, this study will help better understand function of rbp9 in germ cell differentiation in Drosophila ovary, and may also provide insights in general germ cell development.