Diverging functions of Scr between embryonic and post-embryonic development in a hemimetabolous insect, Oncopeltus fasciatus

Abstract

Hemimetabolous insects undergo an ancestral mode of development in which embryos hatch into first nymphs that resemble miniature adults. While recent studies have shown that homeotic (hox) genes establish segmental identity of first nymphs during embryogenesis, no information exists on the function of these genes during post-embryogenesis. To determine whether and to what degree hox genes influence the formation of adult morphologies, we performed a functional analysis of Sex combs reduced (Scr) during post-embryonic development in Oncopeltus fasciatus. The main effect was observed in prothorax of Scr-RNAi adults, and ranged from significant alterations in its size and shape to a near complete transformation of its posterior half toward a T2-like identity. Furthermore, while the consecutive application of Scr-RNAi at both of the final two post-embryonic stages (fourth and fifth) did result in formation of ectopic wings on T1, the individual applications at each of these stages did not. These experiments provide two new insights into evolution of wings. First, the role of Scr in wing repression appears to be conserved in both holo- and hemimetabolous insects. Second, the prolonged Scr-depletion (spanning at least two nymphal stages) is both necessary and sufficient to restart wing program. At the same time, other structures that were previously established during embryogenesis are either unaffected (T1 legs) or display only minor changes (labium) in adults. These observations reveal a temporal and spatial divergence of Scr roles during embryonic (main effect in labium) and post-embryonic (main effect in prothorax) development.

Origin and diversification of wings: Insights from a neopteran insect

“…Injections and dsRNA synthesis were performed as previously described (16). Detailed information including primer sequences and RT-PCR controls, are in SI Material and Methods and Fig.…”

“…Depletion via RNAi of the homeotic gene Sex combs reduced (Scr), which regulates T1 segmental identity (8), results in the formation of a small ectopic wing on T1 (16). Similar small T1 wings are present in diverse early insect fossils (2).…”

“…Such an opportunity exists in Oncopeltus, where the depletion of Scr at late nymphal stages leads to the development of both ectopic scutellum and wings on the prothorax (Fig. 4B) (16), thus allowing us to examine the functions of vg while these structures are developing de novo. First, we performed the double Scr/vg RNAi.…”

“…Specifically, the T1 winglets were always much smaller in fossils and apparently lacked hinge articulation whereas T2 (fore-) and T3 (hind-) wings were fully operational in adults, featuring muscles, venation, and size that rendered them capable of flapping flight (14). T1 winglets were subsequently repressed in multiple lineages (15)(16)(17)(18) whereas T2 and T3 wings acquired morphology similar to modern day Paleoptera (mayflies and dragonflies) and other extinct paleopterous orders. The transition from Paleoptera, which rest with wings extended from the body, to Neoptera, which rest with wings folded flat against the body, required changes in the hinge mechanism, with many orders also evolving a precise mechanical fit between wing margins and the adjacent body wall of the dorsal thorax.…”

Insights into the molecular mechanisms underlying diversified wing venation among insects

“…Moreover, recent gene functional analyses of beetles (Coleoptera) and the milkweed bug (Hemiptera) demonstrated that serially homologous structures of wings in thoracic and abdominal segments are promoted or inhibited to differentiate into wings depending on the regulation of Hox genes and the wing gene network [10][11][12][13][14]. This scenario might be applied to basal winged species, the mayfly Ephoron eophilum (Ephemeroptera) and even to non-winged species, the bristletails Pedetontus unimaculatus (Apterygota, Archaeognatha) [15].…”