MicroRNA-dependent roles of Drosha and Pasha in the Drosophila larval ovary morphogenesis.

The Drosophila larval ovary morphogenesis mainly involves coordinated development of somatic and germ cell lineages that is essential for forming a correct number of niche-germline stem cell (GSC) units (ovarioles) in the adult ovary. Ecdysone, Insulin, Activin, Dpp and EGFR signaling pathways form a regulatory network that orchestrates ovarian soma and germ line throughout larval development. Identification and characterization of additional genes or machineries involved in this process will provide more insights into the underlying mechanisms. Here, we show that the core microRNA (miRNA) pathway components Drosha and Pasha are required for coordinated development of somatic and germ cell precursors in the larval ovary. Drosha or pasha mutants display defective proliferation of primordial germ cells (PGCs), the precursors of GSCs prior to late third larval instar (LL3) and promoted PGC differentiation at LL3. In the mean time, loss of Drosha or Pasha function perturbs somatic precursor development, causing defects in formation of terminal filaments (TFs), a major composition of the GSC niche at LL3, as well as in TF precursor accumulation at early larval stages. Comparative analysis of the mutant phenotypes reveals that three other key miRNA pathway components, Dicer-1 (Dcr-1), Loquacious (Loqs) and Argonaute-1 (Ago-1) have similar effects as Drosha and Pasha indicated above, suggesting a role of the canonical miRNA pathway in the ovary development. Furthermore, genome-wide screening and genetic studies identify a set of Drosha-controlled miRNAs including miR-8, miR-14, miR-33, miR-184, miR-317 and let-7-C that function in this gonadogenesis. Taken together, this study provides the first ever demonstration that miRNA-mediated regulation is involved in the Drosophila larval ovary morphogenesis.