Insights into insect wing origin provided by functional analysis of vestigial in the red flour beetle, Tribolium castaneum

Abstract

Despite accumulating efforts to unveil the origin of insect wings, it remains one of the principal mysteries in evolution. Currently, there are two prominent models regarding insect wing origin: one connecting the origin to the paranotal lobe and the other to the proximodorsal leg branch (exite). However, neither hypothesis has been able to surpass the other. To approach this conundrum, we focused our analysis on vestigial (vg), a critical wing gene initially identified in Drosophila. Our investigation in Tribolium (Coleoptera) has revealed that, despite the well-accepted view of vg as an essential wing gene, there are two groups of vg-dependent tissues in the "wingless" first thoracic segment (T1). We show that one of these tissues, the carinated margin, also depends on other factors essential for wing development (such as Wingless signal and apterous), and has nubbin enhancer activity. In addition, our homeotic mutant analysis shows that wing transformation in T1 originates from both the carinated margin and the other vgdependent tissue, the pleural structures (trochantin and epimeron). Intriguingly, these two tissues may actually be homologous to the two proposed wing origins (paranotal lobes and exite bearing proximal leg segments). Therefore, our findings suggest that the vg-dependent tissues in T1 could be wing serial homologs present in a more ancestral state, thus providing compelling functional evidence for the dual origin of insect wings.serial homology | morphological novelty | Hox | appendage evolution

Origin and diversification of wings: Insights from a neopteran insect

“…Previous work in beetles revealed that the prothorax (T1) contains vgdependent tissue, leading to the proposal that the affected T1 regions represent wing serial homologs (18,37). Our present results show that, in addition to vg, several other genes involved in wing development also have a function in the wingless prothorax of Oncopeltus.…”

“…No such effects were previously observed in beetles (18,37), possibly because radical reconfiguration of the thorax during the pupal state in holometabolous insects may override any such effects. In Oncopeltus vg RNAi adults, the scutellum lost its tip, and its distinct triangular morphology was changed to a shield-like shape (Fig.…”

“…S2 E-H and E′-H′). Although these two regions are completely missing in Tribolium vg RNAi adults (18), the only change observed on the ventral prothorax in Oncopeltus is restricted to an area surrounding the leg base (compare Fig. S3B with Fig.…”

“…The present insights from Oncopeltus show, to our knowledge for the first time, that vg, which can be thought of as a master wing regulator, also has a function in the T2 notum, namely its central region: the scutellum. Hence, the scutellum may also represent part of the wing program based on the interpretation from previous studies where vg-dependent tissues are considered to be wing serial homologs (18,37).…”

“…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.…”