Distinct roles of the homeotic genes Ubx and abd-A in beetle embryonic abdominal appendage development

Insects are easily distinguishable by the absence of legs on the adult abdo men. Studies performed on the Dipteran, Drosophila melanogaster, indicate t hat this is because of the repressive effects of the homeotic genes Ultrabi thorax (Ubx) and abdominal-A (abd-A) on the limb promoting gene Distal-less (DII) during embryonic development. However, in many species appendage-lik e structures are present on abdominal segments in embryonic and juvenile st ages. Here, by using classical genetics and double-stranded RNA-mediated ge ne silencing in the red flour beetle, Tribolium castaneum, a species that d evelops an appendage on the first abdominal segment, we investigate the rol es of Ubx and Abd-A in abdominal limb development. We find that in Triboliu m, Abd-A, but not Ubx, represses early expression of DII in the embryonic a bdomen. Ubx appears to modify the A1 appendage. This difference in the acti vities of Abd-A and Ubx is critical for proper development of this appendag e, We suggest that an ancestral role of Abd-A in insect abdominal appendage development was in the repression of DII initiation and that of Ubx was in modulation of abdominal appendage morphology.