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.

Limits to the exaggeration and diversification of a male sexual trait in the false blister beetle Oedemera sexualis

“…In accordance with analyses of geographical variation in external morphology (Satomi et al 2019), we found that both male and female hindleg sizes and the degree of sexual dimorphism differ significantly among populations. The EF analysis also revealed that interpopulation variation in male hind femur shape can be largely attributed to variation in width, as captured by PC1, and is slightly related to variation in the shape of the proximal end of hind femur, as captured by PC3.…”

“…Traits were measured bilaterally, and two measurements of the left and right parts were averaged prior to subsequent analyses. These measurements are highly repeatable (Satomi et al 2019).…”

“…The Naruto and Yonaguni populations are situated near the northeasternmost and southwesternmost regions within the range of the species, respectively. The Amami population is situated at an intermediate latitude but has an exceptionally narrow male hind femur (Satomi et al 2019), and male mating behavior differs from that in other populations (C. Koshio, unpubl. data, 2012).…”

“…S1). Live beetles were transferred to the laboratory and then frozen at −20 C. These three populations were representative of the 22 populations examined by Satomi et al (2019). The Naruto and Yonaguni populations are situated near the northeasternmost and southwesternmost regions within the range of the species, respectively.…”

“…These facts suggest that the sexually dimorphic hindlegs of this species can be explained by intersexual selection or sexual conflict. More interestingly, the male hind femur is thicker in populations at more southern latitudes, suggesting that the evolution of the sexual trait is constrained or modified by environmental factors that vary with latitude (Satomi et al 2019).…”