gad-1 (gastrulation defective) was first described for its role in E lineage gastrulation. A temperature sensitive mutant and
gad-1 RNAi both lead to a failure to gastrulate and eventual arrest prior to hatching (Knight and Wood 1998). The cellular phenotypes described for
gad-1i are similar to those that seen in
end-3(-) embryos. These phenotypes include delayed gastrulation, abnomral axis of the E2 division and decreased lifespan of E lineage cells. Both
end-1 and
end-3 are GATA transcription factors essential for E lineage fate specification in early embryogenensis. To understand
gad-1's role in E lineage specification we used 4D imaging and automated cell tracing to examine
gad-1 RNAi-treated embryos. In
gad-1i embryos
end-1 and
end-3 RFP reporters had a normal onset of expression even when the gastrulation defect was most severe, indicating
gad-1 does not directly activate
end-1 and
end-3. However, expression of an
elt-2 reporter - a downstream GATA transcription factor known to be a target of
end-1/3 which is also essential for E lineage activation - was delayed in
gad-1i embryos. Further, all phenotypes seen for
gad-1i, including
elt-2 activation, were variable in their penetrance, similar to
end-3(-) embryos. When we examined the cellular phenotypes throughout the rest of the embryo, however, we found a variable, but reproducible delay in the cell lifetimes of all lineages. This delay increases the average cell lifetimes at the 50 cell stage by 12% when excluding the E lineage and coincides with both gastrulation and the onset of zygotic expression. Subsequently the delay became more pronounced and eventually leads to the AB cells at the 250 cell stage living 40% longer when compared to WT. After the initial decrease in lifespan seen in E2 cells the E lineage also shows extended lifespans with the E8 cells living 20% longer in
gad-1i embryos. An RFP reporter of
gad-1 is expressed ubiquitously beginning at the E4-cell stage. These phenotypes, including the failure to gastrulate and a general extension of lifespans, closely resemble those seen in RNAi knockdowns of the RNA polymerase II subunit
ama-1, supporting
gad-1 playing a more general role in zygotic transcription. Many of the phenotypes observed in
ama-1i may be a result of a failure to activate
gad-1. While it is clear that
gad-1 plays a role in E lineage specification, the embryo-wide defects observed point towards it having an important role in general embryonic development. The variable nature of these phenotypes indicates
gad-1 has an important role in maintaining robustness during development.