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Sexual morphology of male Sepsis cynipsea (Diptera: Sepsidae): lack of support for lock-and-key and sexually antagonistic morphological coevolution hypotheses

Published online by Cambridge University Press:  02 April 2012

William G. Eberhard
William G. Eberhard
Affiliation:
Smithsonian Tropical Research Institute and Escuela de Biologia, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica (e-mail: archisepsis@biologia.ucr.ac.cr)
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Abstract

Traits that function in male–female sexual interactions tend to diverge rapidly compared with other traits. Several hypotheses attempt to explain this evolutionary pattern. Predictions of two of these hypotheses, lock-and-key and sexually antagonistic morphological co-evolution, were tested by examining how two sets of species-specific male structures fit with female structures during courtship and copulation in the fly Sepsis cynipsea (L., 1758). Contrary to predictions of both hypotheses, neither the species-specific modifications of the male's front legs nor those of his genitalic surstyli were matched by modifications of the female structures with which they meshed (wing bases, 6th abdominal sternite); males damaged small patches of microtrichia on the female's wings with their legs, but the morphology of the female's wings and abdomen showed no sign of the defensive designs expected under antagonistic morphological co-evolution. Data regarding the alternative hypothesis of sexually antagonistic behavioral co-evolution by females in response to male morphology were less conclusive, but this hypothesis failed to explain the sustained, apparently stimulatory rhythmic squeezing by the male genitalia and the lack of female defensive responses to this squeezing. These movements of the male surstyli during copulation suggest that they function to stimulate the female. The wing base of the female has apparent sense organs near the sites contacted by the male, as expected under the alternative hypothesis of traditional female choice to explain rapid divergent evolution. The male's genitalic surstyli were also used in novel precopulatory interactions. A pair of previously undescribed processes at the bases of the surstyli probably grasp and may also rhythmically squeeze the female during copulation.

Résumé

Les caractères impliqués dans les interactions sexuelles entre les mâles et les femelles ont tendance à diverger plus rapidement que les autres caractères. Plusieurs hypothèses cherchent à expliquer ce patron évolutif. L'examen de l'appariement de deux appareils mâles spécifiques aux structures femelles durant la cour et l'accouplement chez la mouche Sepsis cynipsea (L., 1758) a permis de vérifier les prédictions de deux de ces hypothèses, celle de la serrure et de la clé et celle de la coévolution morphologique sexuelle antagoniste. Contrairement aux prédictions des deux hypothèses, les femelles ne possèdent pas de modifications des structures (bases des ailes, 6e sternite abdominal) qui sont en interaction avec les modifications spécifiques des pattes antérieures des mâles, ni avec celles de leurs surstyles génitaux. Les mâles endommagent de petites plages de microtriches sur les ailes des femelles avec leurs pattes; la morphologie de la femelle ne montre, cependant, aucune signe des adaptations de l'aile et de l'abdomen prévues par la coévolution morphologique antagoniste. Les données concernant l'hypothèse de rechange de la coévolution du comportement sexuel antagoniste chez la femelle en réaction à la morphologie du mâle sont encore moins claires; l'hypothèse ne réussit pas à expliquer la compression rythmique soutenue, apparemment stimulatrice, exercée par les génitalias mâles et l'absence de réactions de défense de la part de la femelle à cette compression. Les mouvements des surstyles mâles durant l'accouplement laissent croire qu'ils servent à stimuler la femelle. La base de l'aile de la femelle possède, semble-t-il, des organes sensoriels près des surfaces palpées par le mâle, tel que prévu par l'hypothèse de rechange traditionnelle du choix de la femelle pour expliquer l'évolution divergente rapide. Les surstyles génitaux du mâle servent aussi à des interactions précopulatrices inédites : une paire de processus, encore non décrits, à la base retiennent probablement la femelle et la compriment aussi sans doute rythmiquement durant la copulation.

Type
Articles
Information
The Canadian Entomologist , Volume 137 , Issue 5 , October 2005 , pp. 551 - 565
Copyright
Copyright © Entomological Society of Canada 2005

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