Spatial regulation of Drosophila ovarian Follicle Stem Cell division rates and cell cycle transitions

Drosophila ovarian Follicle Stem Cells (FSCs) present a favorable paradigm for understanding how stem cell division and differentiation are balanced in communities where they can be regulated independently. Many key extracellular signals for FSCs have been identified, including inversely graded Wnt and JAK-STAT pathway activators. FSCs also exhibit interesting functional spatial heterogeneity; posterior FSCs become proliferative Follicle Cells, while anterior FSCs become quiescent Escort Cells at a much lower rate. Here, by using live imaging and FUCCI cell-cycle reporters, we measured absolute division rates and found that posterior FSCs cycle 3-4 times faster than their anterior neighbors, matching their increased differentiation rate. We also found evidence for FSC G2/M cycling restrictions and G1/S restriction that increases more anteriorly, especially beyond the FSC domain. JAK-STAT signaling promotes both transitions but graded JAK-STAT signaling alone does not explain the graded cycling of FSCs. Genetic interaction tests and FUCCI reporter assays suggest that JAK-STAT signaling acts partly through Yorkie and can largely substitute for stimulation of division by Hh signaling. PI3 kinase signaling, in contrast to Hh signaling, acts largely independently of Yorkie induction and stimulates the G2/M transition.