Maternally-encoded factors are required to repress zygotic
pes-10 expression in the embryonic germ lineage. Geraldine Seydoux and Andy Fire. Carnegie Institution, Baltimore MD 21210. In the previous WBG, we described the expression pattern of the
pes-10 gene, one of the first genes to be transcribed in embryos (WBG 13.3, p 33). Starting in the four-cell stage,
pes-10 RNA is expressed transiently in each somatic founder lineage.
pes-10 RNA was never detected in the germ lineage (Pl -P4). To test whether regulation of
pes-10 expression is dependent on maternally-encoded products, we have analysed by in situ hybridization the distribution of
pes-10 or pes-l O::lacZ transcripts in embryos derived from mothers homozygous for Maternal-effect mutations that affect the fate of embryonic blastomeres. The observed
pes-10 expression patterns are summarized below. skn-l
(zu67): SKN-l is a maternally-encoded putative transcription factor required for the specification of the fate of the EMS blastomere' . (In skn-l (-J embryos, EMS descendants adopt fates resembling those of the somatic descendants of P2.) In skn-l (-) embryos,
pes-10 expression appears unaffected at least to the 8-cell stage. This result indicates that skn-l (+) activity is not required for
pes-10 transcription in EMS. pie-l (
zul54): PE- 1 is required for the specification of the fate of the P2 blastomere2. (In pie-l (-) embryos, the P2 germline blastomere adopts a fate resembling that of its somatic sister EMS.) In pie-l (-) embryos,
pes-10 RNA is ectopically expressed in P2 and P3, in addition to its normal expression in somatic cells. This result indicates that pie-l (+) activity is required to keep
pes-10 expression off in the germ (P) lineage. This observation suggest a model for the mechanism of PE-1 action: PE-1 may function generally to keep zygotic transcription off in the germ lineage. This would counteract the action of transcription factors, including
pes-10 regulator(s) and
skn-1. (SKN- 1 is present in both P2 and EMS but seems to act only in EMS to specify its fate3).
mex-3(zulSS): MEX-3 is required to repress myogenic fates in the AB lineage4. (In
mex-3(-) embryos, AB descendants develop muscle.)
pes-10 expression is abnormal in
mex-3(-) embryos. In particular, we frequently saw
pes-10 expression in P2 (12/19 four-cell embryos), although no
pes-10 expression was seen in P3 (18/18 eight-cell embryos). In the 28 cell stage, a time when
pes-10 expression is normally restricted to the C and D lineages, we observed instead
pes-10 expression in a subset of cells in the AB/EMS lineages. These observations suggest that
mex-3(+) activity is required for the regulation of
pes-10 expression in both somatic and germ lineages. References: 1, Bowerman et al. (1992). Cell 68, 1-20. 2. Mello et al. (1992). Cell 70, 163-176. 3. Bowerman et al. (1993). Cell 74, 443-452. 4. Draper and Priess, pers. communication.