The expression of two engrailed-related genes in an apterygote insect and a phylogenetic analysis of insect engrailed-related genes

Homologues of the Drosophila segment polarity gene engrailed have been clon ed from many insect species, as well as other arthropods and non-arthropods . We have cloned partial cDNAs of two engrailed homologues, which we call e ngrailed-related genes, from the phylogenetically basal insect, Thermobia d omestica (Order Thysanura) and possibly as many as four engrailed-related g enes from the phylogenetically intermediate insect, Oncopeltus fasciatus (O rder Hemiptera). Previous to our findings, only single engrailed-related ho mologues had been found in phylogenetically intermediate insect species (Tr ibolium and Schistocerca) and in the crustacean Artemia, while two engraile d-related homologues have been found in more derived orders (Hymenoptera an d the engrailed and invected genes of lepidopterans and dipterans). Consequ ently, we performed a phylogenetic analysis of insect engrailed-related gen es to determine whether insects ancestrally had one or two engrailed-relate d genes. We have found evidence of concerted evolution among engrailed-rela ted paralogues, however, that masks the true phylogenetic history of these genes; the phylogeny may only be decipherable, therefore, by examining the presence or absence of engrailed-specific and invected-specific motifs, whi ch will require cloning the full length cDNAs from more species. In additio n, we examined the embryonic expression pattern of the two Thermobia engrai led-related genes; like Drosophila engrailed and invected, they are express ed in very similar patterns, but show one temporal difference in pregnathal segments that correlates with the tentative phylogenetic placement of the genes. Thermobia engrailed-related expression also confirms that the dorsal ridge is an ancient structure in insects.