MPK-1 ERK signaling is necessary for pachytene progression and oocyte maturation (Church et al., 1994; Lee et al., 13th International C. elegans Meeting). Strong loss-of-function(lf)/null mutations in
lin-45 RAF,
mek-2 MAPKK and
mpk-1 ERK result in germ cells arrested in pachytene. These pachytene arrested nuclei and surrounding plasma membranes are found in clumps, often in the center of the gonadal tube. We call this a Pac phenotype, for pachytene arrest and clumped nuclei and membranes. A similar Pac phenotype is observed in mutant males. Under certain partial lf conditions, where pachytene progression and oocyte differentiation is normal, we observed hermaphrodites with feminized germlines (proximal germ cells developing as oocytes instead of sperm). These results raise the possibility that the Pac phenotype observed in
lin-45,
mek-2 and
mpk-1 mutants is the result of the combined effect of a sexual fate transformation followed by pachytene arrest during oogenesis. In situ hybridization using a probe to
rme-2 mRNA, which encodes the oogenesis specific yolk receptor, was used to test this possibility. In wild-type L4 hermaphrodites, germ cells in the proximal gonad, most of which are in pachytene, are undergoing spermatogenesis and lack
rme-2 mRNA while germ cells in the proximal third of the distal gonad contain
rme-2 mRNA. By contrast, germ cells throughout the proximal gonad and into the distal gonad have
rme-2 mRNA in L4
mpk-1 null hermaphrodites. In
mpk-1 null males,
rme-2 mRNA is found throughout the region showing the Pac phenotype, unlike in wild-type males. Thus,
mpk-1 is required for the male germ cell fate in both hermaphrodites and males. Epistasis experiments indicate that
mpk-1 is acting downstream or in parallel to
tra-2 in the sex determination pathway. Accumulation of MPK-1 and the appearance of activated MPK-1 is sexually dimorphic. MPK-1 is found at high levels throughout the germline of wild-type adult hermaphrodites. By contrast, in wild-type males, MPK-1 is found at lower levels and only in the distal most germ cells, up to about the distal third of the pachytene region. Activated MPK-1 is found at high levels in the 2 to 3 most proximal oocytes and at moderate levels in the proximal half of pachytene in hermaphrodites. In males, activated MPK-1 is only found at very low levels in the transition zone and the first few pachytene germ cells, a region where activated MPK-1 is not observed in wild-type adult hermaphrodites. Thus, MPK-1 is activated sex specifically and to different extents in three spatially distinct regions of the C. elegans germline.