Latitudinal variation and coevolutionary diversification of sexually dimorphic traits in the false blister beetle Oedemera sexualis

Abstract

Sexual traits are subject to evolutionary forces that maximize reproductive benefits and minimize survival costs, both of which can depend on environmental conditions. Latitude explains substantial variation in environmental conditions. However, little is known about the relationship between sexual trait variation and latitude, although body size often correlates with latitude. We examined latitudinal variation in male and female sexual traits in 22 populations of the false blister beetle Oedemera sexualis in the Japanese Archipelago. Males possess massive hind legs that function as a female-grasping apparatus, while females possess slender hind legs that are used to dislodge mounting males. Morphometric analyses revealed that male and female body size (elytron length), length and width of the hind femur and tibia, and allometric slopes of these four hind leg dimensions differed significantly among populations. Of these, three traits showed latitudinal variation, namely, male hind femur was stouter; female hind tibia was slenderer, and female body was smaller at lower latitudes than at higher latitudes. Hind leg sizes and shapes, as measured by principal component analysis of these four hind leg dimensions in each sex, covaried significantly between sexes, suggesting coevolutionary diversification in sexual traits. Covariation between sexes was weaker when variation in these traits with latitude was removed. These results suggest that coevolutionary diversification between male and female sexual traits is mediated by environmental conditions that vary with latitude.

Keywords: antagonistic coevolution; body size; latitudinal cline; sexual conflict; sexual selection.

Figure 1

(a) A male Oedemera sexualis struggles to copulate with a female; the massive hind legs function as a female‐grasping apparatus. The female tries to kick‐off the mounting male using her hind legs. (b) Map showing the locations of the 22 sample populations of O. sexualis. Shading indicates the species distributional range. (c) Measurements of five morphological traits. EL: Elytron length (body size); FL: femur length; FW: femur width; TL: tibia length; TW: tibia width.

Figure 2

Latitudinal variation in (a) male and (b) female body sizes, and (c, e) male and (d, f) female hind leg shapes. Linear regressions show significant associations with latitude (p < 0.05)

Figure 3

Covariation in hind leg morphology between sexes. (a) Male PC1 versus female PC1, (b) male PC1 versus female PC2, (c) male PC2 versus female PC1, (d) male PC2 versus female PC2. The fitted line shows significant correlations (p < 0.05).