The evolution of hexapod engrailed-family genes: evidence for conservation and concerted evolution

Abstract

Phylogenetic analyses imply that multiple engrailed-family gene duplications occurred during hexapod evolution, a view supported by previous reports of only a single engrailed-family gene in members of the grasshopper genusSchistocercaand in the beetleTribolium castaneum. Here, we report the cloning of a second engrailed-family gene fromSchistocerca gregariaand present evidence for two engrailed-family genes from four additional hexapod species. We also report the existence of a second engrailed-family gene in theTriboliumgenome. We suggest that theengrailedandinvectedgenes ofDrosophila melanogasterhave existed as a conserved gene cassette throughout holometabolous insect evolution. In total 11 phylogenetically diverse hexapod orders are now known to contain species that possess two engrailed-family paralogues, with in each case only one paralogue encoding the RS-motif, a characteristic feature of holometabolous insect invected proteins. We propose that the homeoboxes of hexapod engrailed-family paralogues are evolving in a concerted fashion, resulting in gene trees that overestimate the frequency of gene duplication. We present new phylogenetic analyses using non-homeodomain amino acid sequence that support this view. TheS. gregariaengrailed-family paralogues provide strong evidence that concerted evolution might in part be explained by recurrent gene conversion. Finally, we hypothesize that the RS-motif is part of a serine-rich domain targeted for phosphorylation.