Functional analyses in the milkweed bug Oncopeltus fasciatus (Hemiptera) support a role for Wnt signaling in body segmentation but not appendage development

Abstract

Specification of the proximal-distal (PD) axis of insect appendages is best understood in Drosophila melanogaster, where conserved signaling molecules encoded by the genes decapentaplegic (dpp) and wingless (wg) play key roles. However, the development of appendages from imaginal discs as in Drosophila is a derived state, while more basal insects produce appendages from embryonic limb buds. Therefore, the universality of the Drosophila limb PD axis specification mechanism has been debated since dpp expression in more basal insect species differs dramatically from Drosophila. Here, we test the function of Wnt signaling in the development of the milkweed bug Oncopeltus fasciatus, a species with the basal state of appendage development from limb buds. RNA interference of wg and pangolin (pan) produce defects in the germband and eyes, but not in the appendages. Distal-less and dachshund, two genes regulated by Wg signaling in Drosophila and expressed in specific PD domains along the limbs of both species, are expressed normally in the limbs of pan-depleted Oncopeltus embryos. Despite these apparently paradoxical results, Armadillo protein, the transducer of Wnt signaling, does not accumulate properly in the nuclei of cells in the legs of pan-depleted embryos. In contrast, engrailed RNAi in Oncopeltus produces cuticular and appendage defects similar to Drosophila. Therefore, our data suggest that Wg signaling is functionally conserved in the development of the germband, while it is not essential in the specification of the limb PD axis in Oncopeltus and perhaps basal insects.

Mechanisms of tentacle morphogenesis in the sea anemoneNematostella vectensis

“…Even though arthropod appendages and vertebrate limbs are not homologous, both exhibit distalless/Dlx expression at the presumptive distal portion of the appendage (Angelini and Kaufman, 2005;Kraus and Lufkin, 2006). The published RNA expression pattern of the Nematostella Dlx homolog does not show expression at the tentacle tips or in any tentacle tissue during development (Ryan et al, 2007).…”

A Pathway Analysis of Melanin Patterning in a Hemimetabolous Insect

“…Although there has been limited success in RNAi silencing by direct body cavity injection in Drosophila, this approach has proven to be highly effective in other insects . This is especially the case in Oncopeltus, which is characterized by a strong systemic gene depletion following RNAi treatment Kaufman, 2004a, b, 2005;Angelini and Kaufman 2005). The recent RNAi analysis of the essential enzymes for the production of black melanin in this species also showed a systemic reduction in coloration encompassing the entire body (Liu et al 2014).…”

“…The black coloration has been confirmed recently to be produced via the melanin pathway, whereas most of the orange coloration comes from another source (Liu et al 2014). The RNA interference (RNAi) approach in Oncopeltus is also highly efficient and generates phenotypes displaying systemic responses Kaufman 2004a, b, 2005;Angelini and Kaufman 2005). This allows for the functional analysis of pigmentation genes at a whole-body scale, as was highlighted by a previous study (Liu et al 2014).…”

The T‐box genes H15 and optomotor‐blind in the spiders Cupiennius salei, Tegenaria atrica and Achaearanea tepidariorum and the dorsoventral axis of arthropod appendages

“…Alternatively, omb is activated via an as yet unknown mechanism. It is important to note that in more basal insects like the beetle Tribolium castaneum (Sanchez-Salazar et al 1996) and the milkweed bug Oncopeltus fasciatus (Angelini and Kaufman 2005a) dpp is expressed similar to the millipede Glomeris and the spiders Cupiennius and Achaearanea (Prpic et al 2003;AkiyamaOda and Oda 2003; no dpp expression data is available yet for Tegenaria). The expression domain of dpp along the entire dorsal side is thus not characteristic for insects and could be an adaptation to leg development from imaginal discs in flies.…”