EVOLUTION OF THE INTERACTION BETWEEN HOX GENES AND A DOWNSTREAM TARGET

Segmental identities along the insect body depend on the activities of Hox genes [1,2], In Drosophila melanogaster, one well-studied Hox reg ulatory target is Distal-less (Dll), which is required for the develop ment of distal limb structures [3], In abdominal segments, Dll transcr iption is prevented when Hox proteins of the Bithorax Complex (BX-C) b ind to cis-regulatory elements upstream of the Dll transcription start site [4,5]. Previous evolutionary comparisons of gene expression patt erns suggest that this direct repression is conserved between Diptera and Lepidoptera, but is absent in the Crustacea [6,7], We examined gen e expression patterns in three orders of hexapods, all of which develo p abdominal appendages, in order to determine when the strong repressi ve interaction between BX-C proteins and Dll appeared during evolution . In each of the species examined, Dll expression was initiated in abd ominal cells despite the presence of high levels of BX-C proteins. It appears that the strong repressive effects of BX-C proteins on Dll exp ression arose relatively late in insect evolution. We suggest that the regulatory interaction between the BX-C genes and Dll has evolved wit hin the hexapods in a complex, segment-specific manner, (C) Current Bi ology Ltd ISSN 0960-9822.