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, 10 (86), 20130409

Male Clasping Ability, Female Polymorphism and Sexual Conflict: Fine-Scale Elytral Morphology as a Sexually Antagonistic Adaptation in Female Diving Beetles

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Male Clasping Ability, Female Polymorphism and Sexual Conflict: Fine-Scale Elytral Morphology as a Sexually Antagonistic Adaptation in Female Diving Beetles

Kristina Karlsson Green et al. J R Soc Interface.

Abstract

During sexual conflict, males and females are expected to evolve traits and behaviours with a sexually antagonistic function. Recently, sexually antagonistic coevolution was proposed to occur between male and female diving beetles (Dytiscidae). Male diving beetles possess numerous suction cups on their forelegs whereas females commonly have rough structures on their elytra. These rough structures have been suggested to obstruct adhesion from male suction cups during mating attempts. However, some diving beetle species are dimorphic, where one female morph has a rough elytra and the other has a smooth elytra. Here, we used biomechanics to study the adhesive performance of male suction cups on the female morphs in two diving beetle species: Dytiscus lapponicus and Graphoderus zonatus. We compared adhesion on the rough and the smooth female morphs to infer the function of the rough elytral modifications. We found that the adhesive force on the rough structures was much lower than on other surfaces. These findings support the suggestion of sexual conflict in diving beetles and a sexually antagonistic function of the rough female structures. In addition, males differed in their adhesive capacity on different female surfaces, indicating a male trade-off between adhering to smooth and rough female morphs.

Keywords: Dytiscidae; adhesion; biomechanics; polymorphism; sexually antagonistic coevolution; sexually antagonistic trait.

Figures

Figure 1.
Figure 1.
Male protarsal palette with suction cups. (a) Dytiscus lapponicus (light microscopy), (b) Graphoderus zonatus (light microscopy), (c) detail of suction cups in D. lapponicus (scanning electron microscopy). Scale bars, (a,b) 1 mm. Scale bar in the bottom right corner in (c) denotes 500 µm.
Figure 2.
Figure 2.
Species used in the study. From left to right in each panel: male, smooth female morph, rough female morph. Scale bars, 10 mm. (a) Dytiscus lapponicus and (b) Graphoderus zonatus. Photographs courtesy of Johannes Bergsten.
Figure 3.
Figure 3.
Frequencies of the smooth and the rough morphs in natural populations. Black bar indicates frequency of smooth morph and white bar indicates frequency of rough morph. Populations used in the current study are indicated with an asterisk above the bars. (a) Dytiscus lapponicus populations and (b) Graphoderus zonatus populations.
Figure 4.
Figure 4.
Schematic drawing of the set-up for adhesion measurements. (a) Experimental set-up. The male leg bearing suction cups was mounted on a plastic cube attached to the force sensor. Beetles, on which dorsal surface adhesion was measured, were mounted under water. The force sensor was moved downwards to the dorsal side of the beetle until the suction cups came into contact with the dorsal surface and adhered. (b) Close up of the mounted male foreleg. (c) Dorsal side of the beetle, the dorsal surface of which was probed. Measurements were taken approximately on the shadowed areas of the pronotum (pn) and the elytra (el). The figure also shows: plastic tube (pt), force sensor (fs), cable connection to the amplifier (cb), male foreleg (ml), Petri dish filled with water (wt), beetle body firmly attached to the bottom of the Petri dish (bt).
Figure 5.
Figure 5.
SEM images of Dytiscus lapponicus. Elytral surfaces (a,c,e) and pronotal surfaces (b,d,f). Scale bars are given in the bottom right corner of each micrograph. (a) Male elytron, (b) male pronotum, (c) smooth female elytron, (d) smooth female pronotum, (e) rough female elytron and (f) rough female pronotum.
Figure 6.
Figure 6.
SEM images on Graphoderus zonatus. Elytral surfaces (a,c,e) and pronotal surfaces (b,d,f). Scale bars are given in the bottom right corner of each surface figure. (a) Male elytron, (b) male pronotum, (c) smooth female elytron, (d) smooth female pronotum, (e) rough female elytron and (f) rough female pronotum.
Figure 7.
Figure 7.
Average adhesion (pull-off force) on the different beetle surfaces for the three populations. Different letters indicate statistically significant differences in adhesion force between treatments (ANCOVA, Tukey's post hoc test; see also table 1). Note the different scales on the y-axes in (ac). (a) D. lapponicus, (b) G. zonatus (Lomtjärn population) and (c) G. zonatus (Öster-Skivsjön population).
Figure 8.
Figure 8.
Adhesion (pull-off force) of individual males on the rough and smooth elytra (connected by lines), respectively. (ac) Illustrate the interaction male ID×surface, which was significant according to the ANCOVAs in all the populations (see also table 1). Note the different scales on the y-axes in the three panels. (a) D. lapponicus, (b) G. zonatus (Lomtjärn population) and (c) G. zonatus (Öster-Skivsjön population).

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