Patterning of Caenorhabditis elegans posterior structures by the Abdominal-B homolog, egl-5

The Caenorhabditis elegans body axis, like that of other animals, is patterned by the action of Hox genes. In order to examine the function of one C. elegans Hox gene in depth, we determined the postembryonic expression pattern of egl-5, the C. elegans member of the Abdominal-B Hox gene paralog group, by means of whole-mount staining with a polyclonal antibody. A major site of egl-5 expression and function is in the epithelium joining the posterior digestive tract with the external epidermis. Patterning this region and its derived structures is a conserved function of Abd-B paralog group genes in other animals. Cells that initiate egl-5 expression during embryogenesis are clustered around the presumptive anus. Expression is initiated postembryonically in four additional mesodermal and ectodermal cell lineages or tissues. Once initiated in a lineage, egl-5 expression continues throughout development, suggesting that the action of egl-5 can be regarded as defining a positional cell identity. A variety of cross-regulatory interactions between egl-5 and the next more anterior Hox gene, mab-5, help define the expression domains of their respective gene products. In its expression in a localized body region, function as a marker of positional cell identity, and interactions with another Hox gene, egl-5 resembles Hox genes of other animals. This suggests that C. elegans, in spite of its small cell number and reproducible cell lineages, may not differ greatly from other animals in the way it employs Hox genes for regional specification during development.