In wild-type animals, FOG-1 protein was observed in the germline and was predominantly cytoplasmic. In L2s, FOG-1 became detectable, but staining was faint. In L3s, the level of FOG-1 remained low distally in proliferating germ cells, but FOG-1 was abundant more proximally in germ cells that had entered meiosis and were destined for spermatogenesis. Temperature shift experiments with a
fog-1(ts) allele showed that FOG-1 specifies spermatogenesis in L3 when germ cells enter meiosis, consistent with the idea that the abundant FOG-1 in early meiotic germ cells is specifying the sperm fate. In adult male germlines, FOG-1 was spatially graded: FOG-1 was either not detected or barely visible in the distal half of the mitotic region, became detectable in the proximal half of the mitotic region where some germ cells have entered pre-meiotic S phase, intensified in the transition zone and remained high in distal pachytene germ cells; no FOG-1 was detected in more proximal pachytene cells.