Sexual conflict and genital evolution in moths

Abstract

Male genitalia are one of the fastest evolving and most divergent structures in sexually reproducing organisms. Theories behind genital evolution include the lock and key hypothesis (species isolation), pleiotropy and sexual selection, however, recent research has proposed sexual selection to be the most likely driver of rapid and divergent evolution of genitalia. One facet of such selection - sexual conflict - may drive genital evolution through opposing selection on male and female reproductive strategies as genital adaptations that allow males a competitive advantage may consequently reduce female fitness leading to an evolutionary arms race via sexually antagonistic coevolution. Lichen tuft moths of the genus Izatha (Lepidoptera: Xyloryctidae) and New Zealand Glaucocharis (Lepidoptera: Crambidae) were excellent candidates for examining the evolution of male genitalia and exploring sexual conflict driving genital evolution. By constructing a molecular phylogeny for both genera, the evolutionary relationships between species, patterns of male genital evolution and estimated ancestral state reconstructions of the male genitalia could be determined. Both New Zealand Glaucocharis and Izatha show variation in a number of structures associated with the genital capsule and phallus, and closely related species were more likely to share similar traits. In New Zealand Glaucocharis, the external apical spur on the phallus has the potential to cause harm to part of the female reproductive tract with which it most likely makes contact. In most species of Izatha, males also have sclerotised phallic teeth that make contact with the female ductus bursae during copulation and may cause scarring, while some Izatha males also possess sclerotised spines (deciduous cornuti) attached to the internal vesica wall of the phallus which are ejected into the female reproductive tract during mating. The coevolution of these male and female structures, controlling for phylogeny, was tested and revealed a significant positive correlation between the size of the external apical spur and the thickness of the female ductus bursae wall among species of New Zealand Glaucocharis, providing evidence of the potential sexually antagonistic coevolution of these two traits. In Izatha, there was a significant correlation between the size of the external phallic teeth and the thickness of the female ductus bursae wall, and between the shape of the phallus and area of scarring. A narrower phallus is associated with larger phallic teeth and larger teeth caused a larger area of scarring in females. The male deciduous cornuti and the female signum also revealed significant positive correlations between the presence, number, area and length of the deciduous cornuti and the area of the female signum. Previous research has suggested the signum evolved as a counter-adaptation to the hard spermatophores developed by males to increase the female refractory period to remating. In the present study it was found that females of Izatha species in which males possess deciduous cornuti have a larger signum, but females of some other species that do not possess these structures had a signum present, although reduced in size. Therefore, the evolution of a female trait, the signum, may be driving the evolution of deciduous cornuti in male Izatha via sexually antagonistic coevolution and deciduous cornuti may have evolved to counteract the effect of the signum. Therefore, sexually antagonistic coevolution appears to be occurring between these two sets of male and female traits in Izatha. In order to put these findings into context, background information about mating system dynamics help us to gain a better understanding of the type and level of sexual selection involved within a species. Three species of Izatha exhibited protandry, male-biased sex ratios and female-biased sexual size dimorphism. Male mating status showed a significant negative correlation with body size, with smaller males more likely to have mated. In one species, I. peroneanella, males present later in the breeding season were also more likely to have mated. However, these two factors may be strongly linked, as males that are smaller are also flying later in the season. The later emergence of females and potential scramble competition between males for mates may explain these patterns. These studies have contributed to the understanding of species relationships and the evolution of male genitalia in two genera of New Zealand moths. The comparison of key male and female traits among New Zealand Glaucocharis and Izatha have revealed the coevolution of structures, with sexually antagonistic coevolution appearing to be the driver in both sets of traits within Izatha. Examining the seasonal variation of three common species of Izatha has revealed primary characteristics within the mating system of these species and potential determinants of mating success. This provides the foundations for further investigation of the mating system and evolution of genitalia within Glaucocharis and Izatha, as well as other Lepidoptera.