Pre and Post-copulatory Selection Favor Similar Genital Phenotypes in the Male Broad Horned Beetle

Abstract

Sexual selection can operate before and after copulation and the same or different trait(s) can be targeted during these episodes of selection. The direction and form of sexual selection imposed on characters prior to mating has been relatively well described, but the same is not true after copulation. In general, when male–male competition and female choice favor the same traits then there is the expectation of reinforcing selection on male sexual traits that improve competitiveness before and after copulation. However, when male–male competition overrides pre-copulatory choice then the opposite could be true. With respect to studies of selection on genitalia there is good evidence that male genital morphology influences mating and fertilization success. However, whether genital morphology affects reproductive success in more than one context (i.e., mating versus fertilization success) is largely unknown. Here we use multivariate analysis to estimate linear and nonlinear selection on male body size and genital morphology in the flour beetle Gnatocerus cornutus, simulated in a non-competitive (i.e., monogamous) setting. This analysis estimates the form of selection on multiple traits and typically, linear (directional) selection is easiest to detect, while nonlinear selection is more complex and can be stabilizing, disruptive, or correlational. We find that mating generates stabilizing selection on male body size and genitalia, and fertilization causes a blend of directional and stabilizing selection. Differences in the form of selection across these bouts of selection result from a significant alteration of nonlinear selection on body size and a marginally significant difference in nonlinear selection on a component of genital shape. This suggests that both bouts of selection favor similar genital phenotypes, whereas the strong stabilizing selection imposed on male body size during mate acquisition is weak during fertilization.

Multivariate stabilizing sexual selection and the evolution of male and female genital morphology in the red flour beetle*

“…Indeed, where stabilizing selection has been demonstrated, it appears equally as strong, at least for insects ( γ = –0.28, Tadler ; γ = –0.19 and –0.34, House et al. ; γ = –0.38, Dougherty and Shuker ). Yet, genital morphology is known to evolve rapidly (e.g., Simmons et al.…”

“…; Wojcieszek and Simmons ; House et al. ; Dougherty and Shuker ). This pattern also appears true more generally for selection on male sexual traits, although it should be noted that most experimental designs have more power to detect linear than nonlinear forms of selection (Higgins et al.…”

“…; millipede: Wojcieszek and Simmons ; broad horned beetle: House et al. ; water strider: Bertin and Fairbairn ). Although this finding appears to support the sexual selection hypothesis, it is important to note that it is easier to statistically detect linear than stabilizing selection (Lande and Arnold ), and that there are also many empirical examples showing that sexual selection is not always linear in form (e.g., Blows et al.…”

Sexual selection on the genital lobes of male Drosophila simulans

“…2009; millipede: Wojcieszek and Simmons 2011a,b; broad horned beetle: House et al. 2016). Evidence for sexual selection acting on genitalia is rarer in vertebrates, but studies have been undertaken in reptiles (King et al.…”

Condition-dependent female preference for male genitalia length is based on male reproductive tactics

“…For instance, several morphological characteristics in male genitalia have been attributed to processes of male-male competition because they serve as adaptations to sperm removal and repositioning during copulations, thus influencing the outcome of sperm competition [4]. Comparisons between the effect of pre-and postcopulatory sexual selection on male genitalia have been gaining attention in the last years [5,6]. However, only a few studies have focused on the pre-copula process of female mate choice (i.e.…”