Meeting Abstract
Arthropods are formed by segments whose identities are determined by hox genes, but anterodorsal head tissues pose an exception to this rule. The bend-and-zipper model proposes that this region originates by upfolding of anterolateral embryonic primordia, suggesting that its developmental control is separate from its ventral counterpart. This could have facilitated evolution of novel dorsal head structures critical for insect diversification. Such structures are typically absent in the larval head, and instead develop during metamorphosis. Yet nothing is known about how the dorsal head is patterned during postembryonic development or how novel structures have become integrated into this process. We used larval RNAi to investigate if otd and six3, two embryonic patterning genes, instruct adult dorsal head formation in the flour beetle Tribolium castaneum and the morphologically divergent horned beetles Onthophagus taurus and O. sagittarius. We show that in Tribolium, otd1 and otd1+otd2 RNAi cause subtle gaps in ventral trunk plates but have no obvious effects in the dorsal head. In contrast, otd1 and otd1+otd2 RNAi in Onthophagus results in deletion of large portions of ventral trunk plates, and spectacular changes in dorsal head structures, including reduction or deletion of horns, novel patterns of horn formation, and ectopic medial eyes. RNAi of six3 results in smaller eyes but has no obvious effect on the head in any species. We characterized expression of otd1, otd2 and six3, and show that otds, but not six3, are upregulated in the dorsal head epidermis of Onthophagus spp. Our results suggest that embryonic head patterning genes play crucial and previously undescribed roles in specifying the identity of medial epidermal head structures during postembryonic development.
