Ecdysone Signaling Antagonizes EGF Signaling in Germline-Cyst Cell Interactions of Drosophila melanogaster Testes. Ricky W Zoller, Cordula Schulz. Cellular Biology, University of Georgia, Athens, GA.

   The testes of Drosophila melanogaster contain two populations of stem cells: germline stem cells and somatic cyst stem cells. Both stem cell populations produce daughter cells, the gonialblasts and the cyst cells, that together form a cyst composing of one germline cell enclosed by two cyst cells. The enclosed germline cell, the gonialblast, undergoes four rounds of mitotic transit amplification divisions before entering terminal differentiation. The cyst cells continue to engulf the developing germline cells until differentiation, where they become specialized cap and tail cyst cells. This codifferentiation of germ and cyst cells is a highly coordinated process and the mechanisms regulating the development from stem cells to more mature germ cells are not well understood. The programming of amplification and exit into differentiation requires interactions of the germ cells with the two accompanying cyst cells. Signaling via the EGFR regulates germline-soma association and the differentiation of the enclosed germline cells. We recently discovered that reduction of the ecdysone receptor (EcR) in cyst cells via RNAi knockdown promoted the differentiation of cyst and germline cells in EGF mutant testes. Through western blotting and immunofluorescence techniques, we confirmed that EcR is indeed expressed in testes and, specifically, in cyst cell nuclei. We next addressed if mutations in enzymes responsible for the biosynthesis of ecdysone had the same effect on EGF mutants as EcR. We found that several of these so-called Halloween genes are expressed in testes, as evidenced by RT-PCR, and their reduction markedly decreased the severity of the EGF mutant testis phenotype. Currently, we are dissecting the EcR pathway in cyst cells in more detail.