(A) Drosophila germarium. GSCs (red) reside in a niche, comprising somatic cells such as cap cells (orange), terminal filament, and escort stem cells. GSCs are identifiable by their typical spectrosome morphology (yellow) and their location (adjacent to the cap cells). GSC division produces one self-renewing daughter and one cystoblast that differentiates into a germline cyst. (B) Representative examples of the germaria of wild-type flies containing one, two, or three GSCs in each germaria. Samples were stained with 1B1 (green) and anti-DE-cadherin (magenta) antibodies, which visualized GSCs (dotted circles) and overall cell membranes, respectively. (C) Protocol for GSC analysis for all experiments in this study. 3-day-old females were mated with males and used for assay 1 day after mating. (D) Frequencies of germaria containing one, two, and three GSCs (left y axis), and average number of GSCs per germarium (right y axis) in virgin and mated females in wild-type flies. Mated females showed increased GSC numbers as compared with virgin females. (E) Frequencies of GSC expressing phosphorylated Mad (pMad), which is a stem cell marker [4], were almost the same in mated female flies as compared with virgin female flies. (F) Frequency of mitotic GSCs was counted by staining with anti-phospho-histone H3, which is a marker for mitotic cell division. Mated females showed an increased rate of mitotic GSCs as compared with virgin females. (G) The number of cap cells was counted by staining with anti-Lamin-C antibody, which is a marker for the cap cells. The number of germaria analyzed is shown in parentheses in E, and inside bars in D, F, and G. (H, I) Temporal change in GSC numbers in virgin and mated females. Females were mated with males for the first time 3 days after eclosion (1st mating) (H) and in the second time 7 days after 1st mating (2nd mating) (I). Also see S1 Fig. The number of germaria analyzed for H and I are shown in S2 Table. Values are presented as the mean with standard error of the mean in G, H, I. For statistical analysis, a Mann-Whitney U test was used for D, H, I. Chi-square analysis was performed for F. A Student’s t-test was used for G. ***P ≤ 0.001, **P ≤ 0.01, n.s., non-significant (P > 0.05).