We previously reported that Rbp9 is expressed in the germarium region as soon as germ cells complete mitosis and form 16 interconnected cystocytes. Mutation of Rbp9 caused over-proliferation of cystocytes and generated an ovarian tumor phenotype. This led us to conclude that Rbp9 either inhibits cell proliferation or is required for cell differentiation. To examine the role of Rbp9, we over-expressed it ectopically in germline stem cells and early stage cystocytes that had not yet formed 16 cell clusters. The egg chambers of the newly eclosed transgenic flies looked normal. However, on day 12, most of the developing egg chambers had disappeared and only the germarium region remained. Staining of the Rbp9 over-expressing transgenic ovaries with mitosis markers revealed almost no mitotic divisions in the transgenic germaria. Further examination of these cystocytes with HTS [hu-li tai shao protein], which labels the fusome structure, revealed that the cells in the germarium region consisted of stem cells and cystocytes that had not yet completed four mitotic divisions. These observations suggest that precocious expression of Rbp9 inhibits cystocyte proliferation. Therefore the precise onset of Rbp9 expression in germarium region 2a is critical for inhibiting their further proliferation and achieving their correct differentiation.
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