International team uses fossils to shed light on evolution of sexual conflict in scorpionflies

How sexual conflict evolved and shaped mating behavior in scorpionflies is the subject of a recent detailed study.

An international group of scientists studied that the evolution of sexual conflict from a comparison of the morphology of the male scorpionfly abdomen in different time periods and the present. Their research appears in the online journal eLife Feb. 11.

The male scorpionfly fossils date to the Cretaceous and Eocene periods, between 145.5 million and 65.5 million years ago and 56 million to 33.9 million years ago respectively.

Scorpionflies (Mecoptera, Panorpidae) are used as models in insect sexual conflict because they have a large variety of mating practices, ranging from coercion to providing nuptial gifts, including in extreme cases "mouth-to-mouth saliva transfer until the end of copulation," the researchers write.

The scientists note that previous paleontological studies of the subject have been limited because of the lack of fossils.

Some extinct and extant male scorpionflies have a notal organ, a clamp-like structure on the upper part of its abdomen, that holds down one of the female's wings during mating. The notal organ shape on the scorpionfly matches the fold on the female's wing.

The notal organ effectively ensures sperm transfer and copulation time. The longer time means that more sperm can be transferred, leading to a potentially greater number of fertilized eggs. The mating time can last as long as three hours.

The males can have other non-genital structures to help secure the female in copulation. The advantage of secure clamping also allows a male to forgo a nuptial gift.

The researchers mention from recent research that older and weaker males and smaller males tend to use more coercive methods because they are not as able to offer females prey or saliva.

The fossil findings

"Based on disparate abdominal configurations and correlations in extant relatives, we posit that each new fossil taxon had a different mating approach along a nuptial gifting-coercive spectrum," the researchers write.

They show that the two earlier Cretaceous fossil species examined did not seem to have a coercive mating behavior, and that the later Cretaceous fossil species appears to have a mixed mating behavior. The later fossil the researchers examined from the Eocene had the most oppressive approach to mating.

The researchers speculate that the mecopteran mating strategies involving gift giving may have been "evolutionarily favored" and likely "more prone to promote the species' genetic diversity."

They conclude, "Our data provide first steps toward gaining a deep-time perspective able to inform discussion on the evolution of mating-related sexual conflict in Mecoptera and in insects more generally."

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A. Soszyńska-Maj et al. Evolution of sexual conflict in scorpionflies. eLife, Feb. 11, 2022.

DOI: 10.7554/eLife.70508