The MS blastomere, born at the 7-cell stage of embryonic development, gives rise to many mesodermal cell types, including the posterior half of the pharynx and many body muscle cells. We have previously reported that the MED-1,2 target gene
tbx-35, a T-box transcription factor, is important for specification of MS. At 20C, embryos homozygous for the putative null allele
tbx-35(
tm1789) arrest at less than 2x elongation and lack most of the major tissues made by MS (
glp-1-independent pharynx and
pal-1-independent body muscle; Broitman-Maduro et al., 2006). However, approximately 10% of
tbx-35 embryos elongate to greater than 3x, hatch and subsequently arrest as L1s, and contain a recognizable posterior pharynx; the proportion of such
tbx-35(-) embryos is even higher at 15C. These results suggest that a parallel, temperature-sensitive factor can partially compensate for loss of
tbx-35 function. To investigate other aspects of MS development, we are using a
cup-4::GFP strain (a gift from Johnny Fares, University of Arizona) to mark the four embryonic coelomocytes, all derived from MS (Sulston et al., 1983). We have found that the degree of partial rescue of development in
tbx-35(-) embryos is correlated with production of coelomocytes: At 15C, most embryos develop four coelomocytes, while at 20C, most contain only two to three coelomocytes. These observations support the notion that
tbx-35 mutants grown at lower temperatures undergo a more complete specification of MS. However, although
tbx-35 is clearly dispensable for coelomocyte specification, heat-shock overexpression of a
tbx-35 transgene is sufficient to cause generation of multiple ectopic coelomocytes (>20) in addition to pharynx and body muscle (Broitman-Maduro et al., 2006). We are also investigating the role of the Wnt/MAPK effector POP-1 in MS specification. The activity of maternal POP-1 is required to repress endoderm fate in MS: In the absence of
pop-1, MS adopts an E-like fate, concomitant with the activation of the endoderm-specifying genes
end-1,3 in MS (Lin et al., 1995; Maduro et al., 2005). We have reported the persistence of
tbx-35 activation in MS in the absence of
pop-1 (Broitman-Maduro et al., 2006), which suggests that MS fate might be restored in embryos lacking both
pop-1 and
end-1,3. Indeed, we found that while
pop-1(RNAi) embryos lack coelomocytes,
pop-1(RNAi);
end-3(
ok1448) embryos are able to make coelomocytes. Hence, at least one aspect of MS specification is restored in the absence of POP-1. Our results suggest that at least one additional factor functions in parallel with TBX-35 in MS specification, and that the role of POP-1 in MS specification is primarily to repress the E-promoting end genes.