During embryogenesis of the fruit fly, Drosophila melanogaster, the homeotic genes are required to specify proper cell fates along the anterior-posterior axis of the embryo. We cloned partial cDNAs of homologues of the Drosophila homeotic gene teashirt and five of the homeotic-complex (HOM-C) genes from the thysanuran insect, Thermobia domestica, and assayed their embryonic expression patterns. The HOM-C genes we examined were labial, Antennapedia, Ultrabithorax, abdominal-A and Abdominal-B. As the expression pattern of these HOM-C genes is largely conserved among insects and as Thermobia is a member of a phylogenetically basal order of insects, we were able to infer their ancestral expression patterns in insects. We compare the expression patterns of the Thermobia HOM-C genes with their expression in Drosophila and other insects and discuss the potential roles these genes may have played in insect evolution. Interestingly, the teashirt homologue shows greater variability between Thermobia and Drosophila than any of the HOM-C genes. In particular, teashirt is not expressed strongly in the Thermobia abdomen, unlike Drosophila teashirt. We propose that teashirt expression has expanded posteriorly in Drosophila and contributed to a homogenization of the Drosophila larval thorax and abdomen.

No abstract

To understand better both the development and evolution of insect mouthparts, we have compared the expression pattern of several developmentally important genes in insects with either mandibulate or stylate-haustellate mouthparts. Specifically, we examined the expression of the proboscipedia (pb) and Distal-less (Dll) gene products as well as three regulators of pb, Sex combs reduced (Scr), Deformed (Dfd), and cap 'n' collar (cnc). These genes are known to control the identity of cells in the gnathal segments of Drosophila melanogaster and would appear to have similar conserved functions in other insects. Together we have made an atlas of gene expression in the heads of three insects: Thermobia domestica and Acheta domestica, which likely exemplify the mandibulate mouthparts present in the common insect ancestor, and Oncopeltus fasciatus, which has piercing-sucking mouth parts that are typical of the Hemiptera. At the earliest stages of embryogenesis, only the expression of pb was found to differ dramatically between Oncopeltus and the other insects examined, although significant differences were observed later in development. This difference in pb expression reflects an apparent divergence in the specification of gnathal identity between mandibulate and stylate-haustellate mouthparts, which may result from a "phylogenetic homeosis" that occurred during the evolution of the Hemiptera.