The evolution of arthropod segmentation has been studied by comparing expression patterns of pair-rule and segment polarity genes in various species. In Drosophila, the formation and maintenance of the parasegmental boundaries depend on the interactions between the wingless (wg), engrailed (en) and hedgehog (hh) genes. Until now, the expression pattern of hh has not been analysed to such a great extent as en or wg. We report the cloning and expression analysis of hh genes from Euscorpius flavicaudis, a chelicerate, and Artemia franciscana, a branchiopod crustacean. Our data provide evidence that hh, being expressed in the posterior part of every segment, is a segment polarity gene in both organisms. Additional hh expression sites were observed in the rostrum and appendages of Euscorpius and in the gut of Artemia. From the available data on hh expression in various bilaterians, we review the various hypotheses on the evolution of hh function and we suggest an ancestral role of hh in proctodeum specification and gut formation.

Members of the orthodenticle (otd/Otx) and empty spiracles (ems/Emx) gene families are head gap genes that encode homeodomain-containing DNA-binding proteins. Although numerous studies show their central role in developmental processes in brain specification, a surprisingly high number of other developmental processes have been shown to involve their expression. In this paper, we report the identification and expression of ems and otd in two chelicerate species: a scorpion, Euscorpius flavicaudis (Chactidae, Scorpiona, Arachnida, Euchelicerata) and a spider, Tegenaria saeva (Aranea, Arachnida, Euchelicerata). We show that both ems and otd are expressed not only in an anterior head domain but also along the entire anterior-posterior axis during embryonic development. The expression patterns for both genes are typically segmental and concern neurectodermal territories. During patterning of the opisthosoma, ems and otd are expressed in the lateral ectoderm just anterior to the limb bud primordia giving rise to respiratory organs and spinnerets (spider). This common pattern found in two divergent species thus appears to be a conserved character of chelicerates. These results are discussed in terms of evolutionary origin of respiratory organs and/or functional pathway recruitment.

The evolutionary success of insects is in part attributable to the tremendous diversification of their mouthparts, which permitted insects to radiate into novel food niches. The developmental genetic basis of mouthpart development has been well studied in at least two insect taxa possessing derived mouthparts, the hemipteran Oncopeltus fasciatus and Drosophila. However, much less is known about the regulation of mouthpart differentiation of the presumed ancestral mandibulate type. Here we aim to extend current insights into the patterning of mandibulate mouthparts through a functional genetic analysis of three leg gap genes, homothorax (hth), dachshund (dac), and Distal-less (Dll), in the dung beetle Onthophagus taurus, a species whose mouthpart arrangement has in part retained, as well as diverged form, the ancestral mandibulate mouthpart type. We specifically include in this study a first functional genetic analysis of the adult labrum, an enigmatic mouthpart whose appendicular origin has been the subject of a long-standing debate. Our results support a functional role of all three patterning genes in the development of the labium, maxilla, as well as the labrum. In contrast, mandible development appeared to rely only on the patterning functions of hth and dac, but not Dll. Here, our results raise the possibility that evolutionary changes in the dac-patterning may have played an important role in the evolutionary transition from a short, triangular mandible adapted for chewing to the elongated, flat, and blade-like mandible of modern filter-feeding scarabaeine beetles. In general, our results contribute to a growing body of studies that suggest that basic patterning genes can contribute to morphological evolution of adult features while maintaining traditional patterning responsibilities at earlier developmental stages or in other body regions.

Proteaceae is a remarkable family in terms of the number of transitions in floral symmetry. Furthermore, although CYC-like genes in Grevillea have unusual sequence characteristics, they display patterns of expression that make them good candidates for playing a role in the establishment of floral symmetry.