The MS blastomere, born at the 7-cell stage of development, gives rise to cells that are primarily mesodermal, including cells of the posterior pharynx, and many body muscles. We have previously reported that MS is specified in large part by the activity of the T-box factor TBX-35 (Broitman-Maduro et al., 2006). Embryos homozygous for a putative loss-of-function mutation of
tbx-35,
tm1789, show strong defects in production of posterior pharynx cells and MS-derived body muscles. We are interested in identifying genes that might act in the MS gene network to specify these tissues. Using a candidate gene approach, looking for genes expressed in the early embryo as identified by Baugh et al. (2003), we have found that the homeobox gene Y80D3A.3/dlx-1 may be a factor that acts downstream of TBX-35, and upstream of genes that specify muscle fates. First, by in situ hybridization and GFP reporter,
dlx-1 is activated in the MS lineage, shortly after
tbx-35. This suggests that
dlx-1 may be a target of TBX-35. Indeed, a bacterially-expressed T-box domain of TBX-35 is able to interact with fragments of the
dlx-1 promoter in vitro, suggesting that TBX-35 activates
dlx-1 directly. Secondly, overexpression of
dlx-1 by a heat-shock driven transgene results in widespread specification of muscle cells, as detected by activation of the muscle-specifying gene
unc-120 (one of the three factors, along with
hlh-1 and
hnd-1, that have been shown to act together to specify C. elegans muscle; Fukushige et al., 2006), and by in situ hybridization to detect expression of the body muscle myosin gene
myo-3. Third, RNAi of
dlx-1 using a feeding strain produces a strong uncoordinated phenotype, consistent with defective muscle development. Our results suggest that Y80D3A.3/dlx-1 is a component of the mesoderm gene network that acts to specify muscle fates.