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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.
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