EVOLUTION OF THE INSECT BODY PLAN AS REVEALED BY THE SEX COMBS REDUCED EXPRESSION PATTERN

The products of the HOM/Hox homeotic genes form a set of evolutionaril y conserved transcription factors that control elaborate developmental processes and specify cell fates in many metazoans. We examined the e xpression of the ortholog of the homeotic gene Sex combs reduced (Scr) of Drosophila melanogaster in insects of three divergent orders: Hemi ptera, Orthoptera and Thysanura. Our data reflect how the conservation and variation of Scr expression has affected the morphological evolut ion of insects. Whereas the anterior epidermal expression of Scr, in a small part of the posterior maxillary and all of the labial segment, is found to be in common among all four insect orders, the posterior ( thoracic) expression domains vary. Unlike what is observed in flies, t he Scr orthologs of other insects are not expressed broadly over the f irst thoracic segment, but are restricted to small patches. We show he re that Scr is required for suppression of wings on the prothorax of D rosophila. Moreover, Sci expression at the dorsal base of the prothora cic limb in two other winged insects, crickets (Orthoptera) and milkwe ed bugs (Hemiptera), is consistent with Scr acting as a suppressor of prothoracic wings in these insects. Scr is also expressed in a small p atch of cells near the basitarsal-tibial junction of milkweed bugs, pr ecisely where a leg comb develops, suggesting that Scr promotes comb f ormation, as it does in Drosophila. Surprisingly, the dorsal prothorac ic expression of Scr is also present in the primitively wingless fireb rat (Thysanura) and the leg patch is seen in crickets, which have no c omb. Mapping both gene expression patterns and morphological character s onto the insect phylogenetic tree demonstrates that in the cases of wing suppression and comb formation the appearance of expression of Sc i in the prothorax apparently precedes these specific functions.