Fig. 7. Model for control of germline fate by FOG-1 abundance. (A) Threshold model for FOG-1 control of two germline fates. [FOG-1], hypothetical FOG-1 concentration; broken line, threshold of FOG-1 concentration; yellow, low FOG-1 promotes mitosis; blue, high FOG-1 that specifies spermatogenesis. x-axis, developmental time. In fbf-1 fbf-2 double mutants (blue line), FOG-1 protein increases with time and ultimately promotes spermatogenesis in all germ cells. In fog-1/+; fbf-1 fbf-2 animals (green line), FOG-1 levels are lower than in fbf-1 fbf-2 double mutants, but they still increase with time; as a result, mitosis continues longer and more germ cells are generated before FOG-1 accumulates to a level that promotes the sperm fate. In fog-1; fbf-1 fbf-2 triple mutants (red line), no FOG-1 is generated, few mitotic divisions occur (presumably under control of some other regulator), and no sperm are specified. The FOG-1 increase with time is depicted as linear for simplicity, but it may well increase non-linearly because of a combination of FBF repression and positive autoregulation (see text). (B) Model for FOG-1 spatial gradient in adult male germline. FOG-1 levels are depicted by font size. We suggest that low FOG-1 in the mitotic region promotes proliferation, whereas high FOG-1 in distal MR and transition zone specifies the sperm fate as germ cells enter meiosis.