Caenorhabditis elegans T-box genes tbx-9 and tbx-8 are required for formation of hypodermis and body-wall muscle in embryogenesis.

Transcription factors containing the DNA binding motif, T-box, play an important role in the embryonic development of metazoans. There are 20 T-box genes in the nematode Caenorhabditis elegans, three of which reportedly have postembryonic functions. We characterized two T-box genes, tbx-9 and tbx-8, that are phylogenetically related to each other. tbx-9 is expressed in a subset of embryonic cells that are precursors of the intestine, body-wall muscle, and hypodermis. The expression pattern of tbx-8 is markedly similar to that of tbx-9. Both tbx-9 mutants and tbx-8 mutants show incomplete penetrant morphogenetic defects in embryogenesis, but the malformations of the tbx-9 and tbx-8 mutants are observed in different parts of their bodies. In embryos with both tbx-9 and tbx-8 inactivated, the body structure is severely disorganized, more so than the sum of the separate mutant phenotypes. Further analysis shows that the hypodermis and body-wall muscle show abnormalities at the site of morphogenetic defects of these mutants. Together, these data indicate that tbx-9 and tbx-8 do not only contribute individually to formation of the hypodermis and body-wall muscle, but also suggests functional redundancy between tbx-9 and tbx-8 in embryonic morphogenesis.