Multiple ovarian follicle stem cells reside in the germarium and contribute stochastically to follicle cell daughters. Amy Reilein¹, Ari Berg¹, David Melamed¹, Natania Field¹, Elisa Cimetta², Nina Tandon², Gordana Vunjak-Novakovic², Daniel Kalderon¹. 1) Biological Sciences; 2) Biomedical Engineering, Columbia University, New York, NY.

   Follicle stem cells (FSCs) provide a very interesting paradigm for how stem cells compete and function as a community. FSCs reside midway along the anterior-posterior axis of the germarium of the Drosophila ovary and give rise to epithelial follicle cells that surround developing germ cells. We used lineage analysis and live imaging to examine the behavior of FSCs. We generated ovaries with up to 6 distinguishable genotypes of FSC clones using flies of the genotype yw hs-flp/yw; ubiGFP FRT40A FRT42B ubiRFP/tub-lacZ FRT40A FRT42B to produce lineages that were GFP ⁺lacZ⁺, GFP⁺lacZ^- or GFP^-lacZ⁺, each with or without RFP. We defined a FSC clone as one persisting for at least 7d after clone induction and containing a fasciclin III-negative cell at the 2a/b border of the germarium and a clone of matching genotype in the ovariole. Single ovarioles often contained four or more distinguishable genotypes. Putative FSCs of multiple genotypes were located around the circumference of the germarium at the 2a/b border. We also counted the number of distinct lineages present in each egg chamber and found that egg chambers were comprised most often of 2 or 3 FSC lineages and that genotypes frequently changed from one egg chamber to the next. For any given lineage there was a random pattern throughout the ovariole, indicating stochastic contribution of follicle cell daughters by individual FSCs. A second population of somatic cells, Escort cells (ECs), resides in the anterior half of the germarium. ECs are postulated to renew through self-duplication (Kirilly et al., 2011). Our lineage experiments showed that the genotypes of ECs largely matched the genotypes of FSC lineages, leading us to postulate that new ECs may derive from FSCs. Live imaging and positive marking experiments support this idea. Live imaging showed that cells in the 2a/b region move in all directions, including radially around the circumference of the germarium and in posterior and anterior directions.