Dual evolutionary origin of insect wings supported by an investigation of the abdominal wing serial homologs in Tribolium

Abstract

The origin of insect wings is still a highly debated mystery in biology, despite the importance of this evolutionary innovation. There are currently two prominent, but contrasting wing origin hypotheses (the tergal origin hypothesis and the pleural origin hypothesis). Through studies in the Tribolium beetle, we have previously obtained functional evidence supporting a third hypothesis, the dual origin hypothesis. Although this hypothesis can potentially unify the two competing hypotheses, it requires further testing from various fields. Here, we investigated the genetic regulation of the tissues serially homologous to wings in the abdomen, outside of the appendage-bearing segments, in Tribolium. We found that the formation of ectopic wings in the abdomen upon homeotic transformation relies not only on the previously identified abdominal wing serial homolog (gin-trap), but also on a secondary tissue in the pleural location. Using an enhancer trap line of nubbin (a wing lineage marker), we were able to visualize both of these two tissues (of tergal and pleural nature) contributing to form a complete wing. These results support the idea that the presence of two distinct sets of wing serial homologs per segment represents an ancestral state of the wing serial homologs, and can therefore further support a dual evolutionary origin of insect wings. Our analyses also uncovered detailed Hox regulation of abdominal wing serial homologs, which can be used as a foundation to elucidate the molecular mechanisms that have facilitated the evolution of bona fide insect wings, as well as the diversification of other wing serial homologs.