Drosophila Musashi is required in the ovary to regulate follicle stem cell behaviour and maintain niche homeostasis

Recent studies have shown that the Musashi (Msi) family of RNAbinding translational repressors are expressed in proliferative cells of the mammalian central nervous system, intestine, skin, blood, mammary gland and stomach, leading to the proposal that Msi proteins are stem cell markers and regulate stem and progenitor cell biology. Using the Drosophila testis as a model system, we previously demonstrated that Drosophila Msi is required intrinsically in germline stem cells (GSC's) for maintenance of the GSC fate. In this study, we further investigate whether loss of Msi function affects stem cell regulation in the ovary. We found that GSC regulation in the Drosophila ovary was not dependent upon Msi function, since GSC clones lacking Msi function were maintained at the same frequency as control clones over a period of 3 weeks. However, we found that follicle stem cell (FSC) clones lacking Msi function were slowly lost over this same period. Positively labelling msi-/- cells revealed that FSCs did not always behave as expected, with some mis-localized and others lacking progeny. In msi mutant adult animals, dissected ovarioles exhibited an increase in the number of germline cysts in the posterior region of the germarium, which is the FSC niche region. Additionally, immunostaining with antibodies that label the niche region reveal an expansion of the niche area in msi mutants. Together, these results lead us to conclude that Msi plays a role in regulating the behaviour of FSCs in the Drosophila ovary and also may be required to maintain niche homeostasis.