Intersexual Arms Race? Genital Coevolution in Nephilid Spiders (Araneae, Nephilidae)

Abstract

Genital morphology is informative phylogenetically and strongly selected sexually. We use a recent species-level phylogeny of nephilid spiders to synthesize phylogenetic patterns in nephilid genital evolution that document generalized conflict between male and female interests. Specifically, we test the intersexual coevolution hypothesis by defining gender-specific indices of genital complexity that summarize all relevant and phylogenetically informative traits. We then use independent contrasts to show that male and female genital complexity indices correlate significantly and positively across the phylogeny rather than among sympatric sister species, as predicted by reproductive character displacement. In effect, as females respond to selection for fecundity-driven fitness via giantism and polyandry (perhaps responding to male-biased effective sex ratios), male mechanisms evolve to monopolize females (male monogamy) via opportunistic mating, pre-and postcopulatory mate guarding, and/or plugging of female genitalia to exclude subsequent suitors. In males morphological symptoms of these phenomena range from self-mutilated genitalia to total castration. Although the results are compatible with both recently favored sexual selection hypotheses, sexually antagonistic coevolution, and cryptic female choice, the evidence of strong intersexual conflict and genitalic damage in both sexes is more easily explained as sexually antagonistic coevolution due to an evolutionary arms race.Cryptic female choice, monogamy, morphological complexity, polyandry, reproductive character displacement, sexual conflict, sexually antagonistic coevolution, sperm competition.

Remote copulation: male adaptation to female cannibalism

“…Partial palp breakage, common in spiders (here only in 12% of copulations), functions as mate plugging [6]. However, internal plugs do not necessarily prevent female remating [10][11][12]18], and cannot Table 1. Results from a generalized linear model testing the effects of five explorative factors (the time interval since palp breakage until its experimental removal from the epigynum, the sex initiated palp breakage, copulation duration, male size (carapace width: CW), and the number of sperm remaining in the palp) on the number of sperm transferred to the female's spermatheca.…”

Evolution of genital asymmetry, exaggerated eye stalks, and extreme palpal elongation in Panjange spiders (Araneae: Pholcidae)

“…This suggests that males and females may be engaged in an arms race (cf. Kuntner et al 2009), with ever longer scapes requiring ever longer palps. The fact that pholcid chelicerae contact the female during copulation in all cases studied (reviewed in Huber 1999) suggests that palpal elongation is not related to cannibalism avoidance by keeping the body far from the female.…”

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

“…For this reason alone, coevolution between genital morphologies of males and females is expected. Morphological and genetic components of male and female genitalia have been shown to covary in many taxa (Sota and Kubota 1998;Ilango and Lane 2000;Arnqvist and Rowe 2002;Brennan et al 2007;Rönn et al 2007;Kuntner et al 2009;Tatarnic and Cassis 2010;Cayetano et al 2011;Evans et al 2011Evans et al , 2013Simmons and García-González 2011;Yassin and Orgogozo 2013; and see examples in Table 1). The likely mechanism is that suggested by the authors, and it includes sexually antagonistic coevolution (SAC), natural selection, sexual selection, female choice, or none specified.…”