OMA-1 and OMA-2 are two CCCH zinc-finger-containing proteins whose functions are redundantly required for oocyte maturation in C. elegans (see abstract by Detwiler et al). With antibodies specific to either OMA-1 or OMA-2, we have shown that both proteins are expressed at a very high level in the cytoplasm of maturing oocytes and in one-cell embryos. The embryonic expression drops to below detectable levels by immunofluorescence in 2-cell embryos. However, with an OMA-1-GFP fusion construct, we could detect GFP up to approximately the 4-cell stage. OMA-1-GFP is preferentially segregated to the germline precursors (P1 and P2) and is P-granule-associated, in addition to being cytoplasmic. We believe that detection with OMA-1-GFP is more sensitive than our antibody staining. Because
oma-1;
oma-2 mutant worms are sterile, we are unable to examine the function, if any, of OMA-1 and OMA-2 in early embryos. A gain-of-function mutation in
oma-1,
zu405 , results in a temperature-sensitive, maternal-effect, embryonic lethality. We can show with antibody staining that OMA-1 protein is detected in
zu405 embryos from the 1-cell to approximately the 8-cell stage. The staining is specific to the germline precursors and is cytoplasmic as well as P-granule-associated. The level of OMA-2 protein is not changed in
zu405 animals. Embryos from
zu405 homozygous mothers produce extra intestinal cells as well as extra pharyngeal and body-wall muscles, tissue types normally derived from wild-type EMS. Laser ablation analysis indicates that the C blastomere adopts the fate of EMS in
zu405. Instead of producing skin and muscles, it produces muscles and intestinal cells. We have also shown that C in
zu405 expresses an EMS-specific marker: MED-1-GFP. When wild type EMS divides, intestine is derived from the posterior daughter E and this specification requires a Wnt signal from P2. When C divides in
zu405 embryos, intestine is always derived from the posterior daughter Cp. We could show that the intestine formation from C is dependent on
wrm-1 activity, suggesting that a Wnt-like signaling pathway is required for C to produce gut in
zu405 . In wild-type embryos, the C blastomere fate is dependent on the transcription factor PAL-1 whereas the EMS blastomere fate depends on SKN-1, despite the expression of both SKN-1 and PAL-1 in both EMS and C. Normally, SKN-1 level is low in 8-cell and not detectable in 12-cell embryos but PAL-1 protein lasts past 28 cells. It has been suggested that C expresses PAL-1-dependent cell fates because PAL-1 outlasts SKN-1 in C. If this model were correct, over-expression or endurance of SKN-1 in the C blastomere would allow it to develop like a wild-type EMS blastomere. Consistent with this notion, we detect a high level of SKN-1 protein up to the 8-cell and a low level in the 12-cell stage
zu405 embryos. SKN-1 protein appears to last one cell division longer in
zu405 than in wild type embryos. In 30% of
zu405 embryos, the P2-to-EMS signaling is abnormal and both daughters of EMS produce mesoderm. The C to EMS fate transformation and the defect in P2 to EMS signaling are similar to what was described with the
gsk-3(RNAi) embryos. The expression of PIE-1 and APX-1 appears unaffected in
zu405 embryos. We are currently investigating the expression patterns of other maternal factors in
zu405 mutants.