Meeting Abstract
The ancestral, polyramous arthropod appendage is comprised of three components: epipod, exopod, and telopod. The telopod has been homologized to the walking legs of all modern arthropods, whereas epipods have been proposed to have given rise to insect wings, crustacean gills, horseshoe crab book gills, spider book lungs, and spider spinnerets. To test this homology statement, we compared embryonic gene expression of the proximal-distal (PD) axis patterning gene dachshund (dac) in the polyramous appendage of the amphipod crustacean Parhyale hawaiensis and the dac-1 homolog in the spider Parasteatoda tepidariorum. Specifically, we aimed to investigate whether dac expression was conserved in the bona fide epipods of P. hawaiensis and its putative epipodal homolog, the spider spinneret. Here we show that canonical dac expression is observed in structures of telopodal origin, such as the pereopods of P. hawaiensis walking legs, and the pedipalps and walking legs of the spider. We did not observe dac expression in the developing epipods or exopods of P. hawaiensis. By contrast, expression of the spider dac homolog in the spinneret strongly resembles the expression pattern observed in the walking legs of P. tepidariorum and the pereopods of P. hawaiensis. Together with other characteristics of spinnerets, such as internal muscle attachment sites and the incidence of proximo-distal segmentation of spinnerets in basal spiders, these data suggest that spinnerets are not homologous to crustacean gills, but rather to arthropod walking legs. Our data thus counter the notion that spinnerets have an epipodal or gill-like origin.
