Antenna and all gnathal appendages are similarly transformed by homothorax knock-down in the cricket Gryllus bimaculatus

Our understanding of the developmental mechanisms underlying the vast diversity of arthropod appendages largely rests on the peculiar case of the dipteran Drosophila melanogaster. In this insect, homothorax (hth) and extradenticle (exd) together play a pivotal role in appendage patterning and identity. We investigated the role of the hth homologue in the cricket Gryllus bimaculatus by parental RNA interference. This species has a more generalized morphology than Oncopeltus fasciatus, the one other insect besides Drosophila where homothorax function has been investigated. The Gryllus head appendages represent the morphologically primitive state including insect-typical mandibles, maxillae and labium, structures highly modified or missing in Oncopeltus and Drosophila. We depleted Gb’hth function through parental RNAi to investigate its requirement for proper regulation of other appendage genes (Gb’wingless, Gb’dachshund, Gb’aristaless and Gb’Distalless) and analyzed the terminal phenotype of Gryllus nymphs. Gb’hth RNAi nymphs display homeotic and segmentation defects similar to hth mutants or loss-of-function clones in Drosophila. Intriguingly, however, we find that in Gb’hth RNAi nymphs not only the antennae but also all gnathal appendages are homeotically transformed, such that all head appendages differentiate distally as legs and proximally as antennae. Hence, Gb’hth is not specifically required for antennal fate, but fulfills a similar role in the specification of all head appendages. This suggests that the role of hth in the insect antenna is not fundamentally different from its function as cofactor of segment-specific homeotic genes in more posterior segments.