At the 4-cell stage of embryogenesis, the blastomere EMS produces intestine and pharynx, and its sister blastomere, P2, produces skin, muscle and germline. Several genes are known to be involved in specifying the proper fates of EMS and P2.
skn-1 is a gene required for the proper fate of EMS, and it encodes a probable transcription factor localized to the nucleus of both EMS and P2. The gene
pie-1, whose protein product is localized to the nucleus of P2, appears to act as a negative regulator of transcription in P2, thus preventing expression of
skn-1-dependent cell fates. In
pie-1 mutant embryos, the P2 blastomere produces
skn-1-dependent pharynx and intestine.
pie-3(
zu277ts) was isolated in a screen for temperature-sensitive maternal-effect mutants. Ablation analysis of embryos from mothers homozygous for the
pie-3 mutant indicates that the P2 blastomere produces pharynx and intestine, as observed in
pie-1 mutants, suggesting that
skn-1 may be active in P2. Unlike
pie-1 mutants,
pie-3 mutants produce germline from P2.
pie-3 mutant embryos stained with the PIE-1 antibody show a normal pattern of PIE-1 expression suggesting that in wild-type embryogenesis
pie-3 may be required to act with
pie-1 to negatively regulate
skn-1 activity in the P2 blastomere.
pie-2(
zu199) was identified in a screen for maternal-effect lethal mutants.
pie-2 mutant embryos make extra gut and pharynx as do
pie-1 and
pie-3 mutants. A phenotypic characterization of
pie-2 (
zu199) is currently underway.