Figure 5.

Evolution of Ubx regulation of Dll in the insect lineage. (Upper) The expression domains of Ubx are different in the onychophoran (Left) and Drosophila (Right). OUbx is expressed on the extreme posterior of the onychophora, overlapping the expression of Dll in the terminal limb of the homonomous trunk. In contrast, DUbx is expressed in the third thoracic segment of Drosophila, where it patterns the haltere and the T3 leg, and throughout much of the Drosophila abdomen, where it represses Dll expression and limb development. (Lower) Ubx repression of Dll in the insect abdomen may have evolved because of changes in Ubx protein function and in cis-regulatory sequences of the Dll target gene. The OUbx protein (Left) shares some functional elements with DUbx (Right), including the homeodomain and the Exd interaction domain. However, evolutionary modifications to Ubx protein function in the insect lineage may have created a novel cofactor interaction domain (shown in orange) or a novel posttranslational regulation of Ubx activity required for Ubx repression of Dll. Ubx regulation of Dll in the insect lineage also involved the evolution of Ubx binding sites in the Dll early limb enhancer.