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Functional significance of elaborate secondary sexual traits and their evolution in the water strider genus Rheumatobates1

  • Locke Rowe (a1), Kathleen P. Westlake (a2) and Douglas C. Currie (a3)
Abstract
Abstract

Sexual conflict may drive the evolutionary elaboration of sexually antagonistic traits that function in mating interactions. One of the most striking cases of elaboration of male morphology occurs in the water strider genus Rheumatobates Bergroth (Hemiptera: Gerridae). The functional significance of the bizarre modifications of appendages in this group is not known. Here we focus on one of the more elaborate of the species, R. rileyi Bergroth. We conduct observational and experimental studies aimed at determining the general sequence of mating behaviour, the role of females in the outcome of sexual interactions, and the functional significance of the highly modified appendages of males. We also map these traits on a known phylogeny of the genus to determine their pattern of evolution. Males repeatedly harass females and females respond with evasive skating or, if the male successfully grasps her, with a premating struggle. The dynamics of the struggle determine the success of mating attempts. Short struggles typically lead to mating, and long struggles typically result in disengagement of the pair. Following a short period of copulation, males withdraw their genitalia and dismount. Females that have been isolated from males for a period of time become less reluctant to mate. During the premating struggle, the antennae of males are used to grasp the females around the head, and the rear legs are used to lift the females' rear legs off the water surface. Neither antennae nor rear legs are used during copulation, thus they are not used for copulatory courtship. Mapping of these traits on the phylogeny indicates multiple independent origins and a pattern of escalation (16 origins, 7 losses). We conclude that these bizarre traits of males are sexually antagonistic and have evolved repeatedly in the genus.

Résumé

Les conflits sexuels peuvent favoriser le développement au cours de l'évolution de caractères sexuels antagonistes qui servent durant les interactions de l'accouplement. Un des cas les plus remarques de l'élaboration de caractères morphologiques chez le mâle s'observe chez les patineurs du genre Rheumatobates Bergroth (Hemiptera : Gerridae). La signification fonctionnelle des modifications bizarres des appendices dans ce groupe reste inconnue. Nous traitons ici principalement d'une des espèces les plus développés à ce titre, R. rileyi Bergroth. Des observations et des expériences nous servent à déterminer la séquence générale du comportement d'accouplement, le rôle des femelles dans l'issue des interactions sexuelles et la signification fonctionnelle des appendices fortement modifiés du mâle. Nous plaçons ces caractères sur un arbre phylétique reconnu du genre afin de déterminer leur patron d'évolution. Les mâles harcellent les femelles à répétition et les femelles réagissent en s'éloignant en patinant; cependant, si le mâle réussit à l'attraper, il se produit une échauffourée pré-copulatoire. La dynamique de l'échauffourée détermine le succès de la tentative d'accouplement. Les échauffourées courtes mènent normalement à l'accouplement, alors que les échauffourées prolongées se terminent généralement par la séparation du couple. Après une courte période de copulation, le mâle retire ses organes génitaux et débarque de la femelle. Les femelles qui ont été tenues à l'écart des mâles pendant un certain temps sont moins réticentes à l'accouplement. Durant l'échauffourée pré-copulatoire, les antennes des mâles servent à entourer la tête de la femelle et les pattes postérieures sont utilisées pour soulever les pattes postérieures de la femelles au-dessus de la surface de l'eau. Ni les antennes, ni les pattes postérieures ne servent durant la copulation; elles ne sont donc pas impliquées dans la cour associée à l'accouplement. En reliant ces caractéristiques à la phylogénie, on observe de multiples origines indépendantes et un pattern d'escalade (16 origines et 7 pertes). Nous concluons que ces caractères bizarres des mâles sont des caractères sexuels antagonistes et qu'ils sont apparus à plusieurs reprises chez ce genre.

[Traduit par la Rédaction]

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2 Corresponding author (e-mail: lrowe@zoo.utoronto.ca).
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1

This paper is part of a special issue honouring Geoffrey G.E. Scudder for his significant contributions to entomology in Canada.

Footnotes
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The Canadian Entomologist
  • ISSN: 0008-347X
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