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 cloned from many insect species, as well as other arthropods and non-arthropods. We have cloned partial cDNAs of two engrailed homologues, which we call engrailed-related genes, from the phylogenetically basal insect, Thermobia domestica (Order Thysanura) and possibly as many as four engrailed-related genes from the phylogenetically intermediate insect, Oncopeltus fasciatus (Order Hemiptera). Previous to our findings, only single engrailed-related homologues had been found in phylogenetically intermediate insect species (Tribolium and Schistocerca) and in the crustacean Artemia, while two engrailed-related homologues have been found in more derived orders (Hymenoptera and the engrailed and invected genes of lepidopterans and dipterans). Consequently, we performed a phylogenetic analysis of insect engrailed-related genes to determine whether insects ancestrally had one or two engrailed-related genes. We have found evidence of concerted evolution among engrailed-related 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, which will require cloning the full length cDNAs from more species. In addition, we examined the embryonic expression pattern of the two Thermobia engrailed-related genes; like Drosophila engrailed and invected, they are expressed 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.