Evolutionary responses and condition dependence of male and female genital traits in the seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae)

Abstract

This investigation is comprised of two detailed studies into how various components of the genitalia of Callosobruchus maculatus (commonly known as the seed beetle) respond to manipulations of sexual and life history selection regimes and diet quality. Experimental evolution is used in the first study, with evolutionary responses being tested for, while diet manipulations are used in the second study to expose plastic responses. A central focus throughout is sexual conflict, a phenomenon thought to have great importance for the evolution of sexual characteristics where the interests of the sexes diverge with respect to reproductive outcomes. Responses in allometry (trait size scaled against body size) and mean trait size in both sexes were tested for. The selection experiment is the first involving genitalia in C. maculatus and the first to look at the simultaneous effects of variation in sexual and life history selection on genital evolution; the condition study is the first to look at diet manipulation responses involving these traits in this species. Effects were found for both types of experimental manipulation. Of particular note was the response for the male genital spines on the intromittent organ: they evolved depressed allometric slope and reduced mean size under relaxed sexual selection (monogamous mating) relative to polygamous mating. The results corroborate their status as being harmful to females. No female traits showed responses of any kind to selection. This is consistent with a time-lag effect wherein females reduce their investment in defensive traits that mitigate harm only when males have evolved to significantly reduce this harm. In the diet manipulation experiment, females exhibited an interesting response: spines located in the reproductive tract were larger in individuals grown in low quality diet than in high quality diet. Male spines did not show diet treatment effects. The study has cast light on a number of important factors in the evolution of genitalia, including the rapidity with which it can take place, the relevance of life history selection, the potential for time-lag effects to come into play when comparing the evolutionary responses of the sexes, and the apparent irrelevance of diet quality to the expression of most genital traits in this species.