Ultrabithorax function in butterfly wings and the evolution of insect wingpatterns

Background: The morphological and functional evolution of appendages has pl ayed a critical role in animal evolution, but the developmental genetic mec hanisms underlying appendage diversity are not understood. Given that homol ogous appendage development is controlled by the same Hox gene in different organisms, and that Hox genes are transcription factors, diversity may evo lve from changes in the regulation of Hox target genes. Two impediments to understanding the role of Hox genes in morphological evolution have been th e limited number of organisms in which Hox gene function can be studied and the paucity of known Hox-regulated target genes. We have therefore analyze d a butterfly homeotic mutant 'Hindsight', in which portions of the ventral hindwing pattern are transformed to ventral forewing identity, and we have compared the regulation of target genes by the Ultrabithorax (Ubx) gene pr oduct in Lepidopteran and Dipteran hindwings. Results: We show that Ubx gene expression is lost from patches of cells in developing Hindsight hindwings, correlating with changes in wing pigmentati on, color pattern elements, and scale morphology. We use this mutant to stu dy how regulation of target genes by Ubx protein differs between species. W e find that several Ubx-regulated genes in the Drosophila haltere are not r epressed by Ubx in butterfly hindwings, but that Distal-less (Dll) expressi on is regulated by Ubx in a unique manner in butterflies. Conclusions: The morphological diversification of insect hindwings has invo lved the acquisition of different sets of target genes by Ubx in different lineages. Changes in Hox-regulated target gene sets are, in general, likely to underlie the morghological divergence of homologous structures between animals.