T-box genes are a family of transcriptional regulators unified by a conserved T-box DNA binding domain. There are 20 T-box genes in C.elegans, more than in any other species analysed so far but relatively little is known about their function. Using RNA interference, we have inactivated each of the worm T-box genes and have found obvious phenotypes in only a few cases. The remaining T-box genes give no obvious phenotype when inactivated singly by RNAi. Phylogenetic analysis reveals that several pairs of T-box genes are very similar to each other. Therefore, these gene pairs are likely to be the result of recent duplications and may share overlapping functions. Double RNAi experiments have revealed this to be the case with at least one of the T-box gene pairs,
tbx-8 and
tbx-9, which have been found to have essential, overlapping functions during embryonic morphogenesis. The earliest defect observed is in the process of dorsal hypodermal intercalation, which together with hypodermal ventral enclosure is crucial for embryonic elongation to occur. The dorsal hypodermal migration of cells is aberrant in
tbx-8/9 RNAi embryos resulting in defects in elongation and morphogenesis. These embryos also have defects in muscle cell positioning and intestinal development.
tbx-8 and
tbx-9 are expressed in gut, muscle and hypodermis. Their expression domains largely overlap with both genes being highly expressed in the nuclei of dorsal hypodermal cells undergoing intercalation.
tbx-9 single RNAi produces worms which have a bobbed-tail phenotype that is similar to the
vab-7 mutant phenotype (
vab-7 is an even-skipped homologue required for posterior patterning). This suggests that these genes may be acting in the same pathway. We find that
tbx-8 and
tbx-9 are both necessary and sufficient for
vab-7 expression, and thereby function upstream of
vab-7 to activate its expression in particular posterior cells. Furthermore, we find that
vab-7 in turn represses another T-box gene,
mab-9, in the posterior of embryos. Thus, there are complex interactions between
vab-7 and T-box genes in C. elegans. Positive regulation of eve homeobox genes by T-box genes has been reported in other systems yet this is the first demonstration that T-box genes can also be subject to repression by eve. The potential evolutionary association between the role of
tbx-8 and
tbx-9 in dorsal hypodermal intercalation in the worm and the role of T-box genes in convergent extension movements of cells during morphogenesis of other organisms is an interesting parallel.