Miami University researchers shed light on insect-wing evolution debate

A recent paper by two scientists who studied crustaceans highlights the discovery that the wingless insects’ gene network was similar to that of insects – showing the gene network preceded the actual development of insect wings.

The paper hypothesizes that since the gene network of the wingless insects preceded insect-wing development, both the leg structures and the body wall may be insect-wing homologues. Scientists have been studying insect-wing evolution for more than 200 years, with some proposing that wings developed from ancestral leg structures. Still others proposed that insect wings developed from the body wall.

“The debate surrounding the evolutionary origin of insect wings has raged off and on for over 200 years, culminating in two possible wing origin tissues: the dorsal body wall (tergum) and ancestral proximal leg structures (pleural plates),” Courtney Clark-Hachtel  and Yoshinori Tomoyasu of Miami University in Ohio wrote in Nature, Ecology and Evolution. “Various approaches, from paleontology, comparative morphology, phylogenomics to biomechanics, have been employed to decipher the mystery of insect-wing origin.”

The scientists studied a wingless insect relative, the evo-devo model Parhyale hawainsis, which they indicated would be an ideal candidate to investigate insect-wing origins.

“The dorso-ventral (DV) body plan of Parhyale remains largely similar to that of insects, with the two tissues corresponding to the two proposed wing origins being present, allowing us to evaluate the evolutionary relationship of these tissues to the insect wing and other wing-serial homologues,” Clark-Hachtel  and Tomoyasu wrote. “In this study we sought to identify the crustacean wing homologues in Parhyale through a functional investigation of Parhyale genes whose insect orthologues are essential for wing formation.” 

The Parhyale vestigial (Ph-vg) gene was deemed to be an important wing gene as it has a unique function that would orchestrate insect-wing development. The scientists in this study and others found Ph-vg to be critical.

“Our expression analysis revealed that the Parhyale vg orthologue is expressed throughout the lateral and posterior tergal edge, as well as in parts of the proximal leg, including the edge of the coxal plate (cp) and part of the basis," Clark-Hachtel and Tomoyasu wrote. “Ph-vg is not expressed in the gill. Knockout (KO) of Ph-vg via [CRISPR] resulted in the deletion of the tissues where Ph-vg is expressed: the tergal edge and proximal leg tissues. 

“The tergal edge was markedly reduced both laterally and posteriorly in the Ph-vg mutants, being most obvious from the failure of the tergal plates to overlap each other and the failure of the tergal edge to partially cover the cp.” 

The gills of Ph-vg KO remained intact, the authors noted, even though crustacean gills had previously been proposed as crustacean-wing homologues. The scientists also found that apterous is another important gene in the origin of insect wings, and was previously identified in wing homologues in insects and crustaceans.

The outcome of the study showed the possibility that it is “not just each wing gene individually but a network of genes” that resulted in insect wings in a common ancestor of an insect/crustacean.