Regulation of metamorphosis in neopteran bugs is conserved within the paleopteran Cloeon dipterum (Ephemeroptera)

meaning

Mayflies are the only insects still alive that molt after forming wings in a stage called subimago. Numerous authors have wondered whether this stage is a nymph, an adult, or some kind of intermediate stage. Another question is why mayflies have a subimago stage when molting a wing is risky. While working with Cloeon dipterum, we found that metamorphosis is regulated like in neopteran insects and that it is determined before the formation of the subimago. Hence, it should be viewed as an adult stage. We also found that the forelegs grow dramatically between the final stage of the nymph, the subimago, and the adult. This necessary growth can help explain the functional purpose of the Subimago.

abstract

In the Paleozoic Era, more than 400 Ma, a number of groups of insects shed their skin after forming functional wings. Today, however, flying insects stop molting after metamorphosis when they are fully winged. The only exception are the mayflies (Paleoptera, Ephemeroptera), which molt in the subimago, a flight stage between the nymph and the adult. However, the identity and homology of the Subimago has not yet been adequately researched. A debate remains as to whether this stage represents a modified nymph, an adult, or a pupa like that of butterflies. Another relevant question is why mayflies have the subimago stage despite the risk of molting on fragile membranous wings. These questions have fascinated numerous authors, but clear answers have not yet been found. By combining morphological studies, hormonal treatments and molecular analyzes in the mayfly Cloeon dipterum, we have found answers to these ancient questions. We observed that treatment with a juvenile hormone analog in the last nymph stage stimulated expression of the Kr-h1 gene and reduced that of E93, which suppresses and induces metamorphosis, respectively. The regulation of the metamorphosis thus follows the MEKRE93 path, as with neopteran insects. In addition, the treatment prevented the formation of the subimago. These results suggest that the subimago must be viewed as a stage of the adult mayfly. We have also observed that the forelegs grow dramatically between the final stage of the nymph, the subimago, and the adult. This necessary growth, which was spread over the last two stages, could at least partially explain the adaptive meaning of the Subimago.

Footnotes

• Authors’ contributions: XB designed research; OK, AV-A., IA, and XB conducted research; OK, AV-A., FC, IA and XB analyzed data; and XB wrote the paper.

• The authors do not declare any competing interests.

• This article is a PNAS direct submission.

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• This article provides supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2105272118/-/DCSupplemental.

Data availability

All study data is included in the article and / or supporting information.