Tergal and pleural wing‐related tissues in the German cockroach and their implication to the evolutionary origin of insect wings

Abstract

The acquisition of wings has facilitated the massive evolutionary success of pterygotes (winged insects), which now make up nearly three‐quarters of described metazoans. However, our understanding of how this crucial structure has evolved remains quite elusive. Historically, two ideas have dominated in the wing origin debate, one placing the origin in the dorsal body wall (tergum) and the other in the lateral pleural plates and the branching structures associated with these plates. Through studying wing‐related tissues in the wingless segments (such as wing serial homologs) of the beetle, Tribolium castaneum, we obtained several crucial pieces of evidence that support a third idea, the dual origin hypothesis, which proposes that wings evolved from a combination of tergal and pleural tissues. Here, we extended our analysis outside of the beetle lineage and sought to identify wing‐related tissues from the wingless segments of the cockroach, Blattella germanica. Through detailed functional and expression analyses for a critical wing gene, vestigial (vg), along with re‐evaluating the homeotic transformation of a wingless segment induced by an improved RNA interference protocol, we demonstrate that B. germanica possesses two distinct tissues in their wingless segments, one with tergal and one with pleural nature, that might be evolutionarily related to wings. This outcome appears to parallel the reports from other insects, which may further support a dual origin of insect wings. However, we also identified a vg‐independent tissue that contributes to wing formation upon homeotic transformation, as well as vg‐dependent tissues that do not appear to participate in wing formation, in B. germanica, indicating a more complex evolutionary history of the tissues that contributed to the emergence of insect wings.

Broad Complex and Wing Development in Cockroaches

“…To have another reference, we also determined the expression of Distal-less (Dll), a gene that typically promotes appendage development. As expected, Dll localizes in the distal end of the appendages, whereas vg mainly localizes at the edge of the three thoracic segments, as well as in the abdominal segments, as previously reported (Clark-Hachtel et al, 2021). In contrast, Br-C transcripts localize in the developing central nervous system, where the signal is especially clear in the cerebral ganglion, but also all along the ventral nerve cord (Fig.…”

“…The procedure was originally reported by Clark-Hachtel et al (2021) and is described in detail in Supplementary Table S2. Briefly, for embryo fixation, oothecae were removed from the female and placed in PBT in a microcentrifuge tube.…”

“…Previous in situ hybridization studies in B. germanica embryos had shown that the first signs of vg expression can be observed in the brain on day 3 of development, while on day 4, vg expression becomes apparent at the edge of the dorsal terga throughout the thorax and abdomen (Clark-Hachtel et al, 2021). Thus, we chose day 4 embryos to compare the localization of expression of this typical wing-related gene with that of Br-C in early embryogenesis.…”

Acis-regulatory sequence of the wing selector gene,vestigial, drives the evolution of scaling relationships inDrosophilaspecies

“…In order to study wing-to-body allometric relationships in Drosophila species, we selected four representative species based on the following criteria. Since vg is evolutionarily conserved as a wing selector gene in insects (Abouheif and Wray, 2002; Clark-Hachtel et al, 2013; Clark-Hachtel et al, 2021; Zhang et al, 2021), we first used the vg coding sequence (CDS) from eight species closely related to D. melanogaster to construct a phylogenetic tree (Fig. 2).…”

TheDaphniacarapace and the origin of novel structures

“…Intriguingly, the posterior spine of the Daphnia carapace, which expresses both pnr and vg, may be positionally homologous to the insect scutellum, which is also a dorsal posterior protrusion of the tergum that expresses vg [19][20][21] .…”