The geography of sex: sexual conflict, environmental gradients and local loss of sex in facultatively parthenogenetic animals

Phenotypic specialization of the pea aphid in its southern limit of distribution

The success of biological invasions ultimately relies on phenotypic traits of the invasive species. Aphids, which include many important pests worldwide, may have been successful invading new environments partly because they can maximize reproductive output by becoming parthenogenetic and losing the sexual phase of their reproductive cycle. However, invasive populations of aphids invading wide ranges can face contrasting environmental conditions and requiring different phenotypic strategies. Besides transitions in their reproductive cycle, it is only partially known which phenotypic traits might be associated to the invasion success of aphid populations in extended novel ranges. Here, we used four genotypes of the pea aphid Acyrthosiphon pisum from two localities in Chile to test for phenotypic specialization that might explain their establishment and spread in habitats exhibiting contrasting environmental conditions. We show that lineages living at a higher latitude with low temperatures show, in addition to facultative sexual reproduction, smaller body sizes, lower metabolic rates and a higher tolerance to the cold than the obligate asexual lineages living in a mild weather, at the expense of fecundity. Conversely, at higher temperatures only asexual lineages were found, which exhibit larger body sizes, higher reproductive outputs and consequently enhanced demographic ability. As a result, in conjunction with the reproductive mode, lineage specialization in physiological and life-history traits could be taken into account as an important strategy for populations of pea aphid to effectively invade extended novel ranges comprising different climatic conditions.

Spermathecal variation in temperate Opiliones

Most arachnid fertilization occurs internally, allowing for a variety of post-copulatory mechanisms to take place. Females are expected to exert some level of control over sperm fate when 1) the point of gametic fusion is particularly distant from the point of oogenesis, 2) the time of syngamy is significantly later than the time of mating, 3) sperm are non-motile, and/or 4) the morphology of females allows for selective containment of sperm. Many of these conditions are met in Opiliones (a.k.a. "harvesters," "harvestmen," or "daddy-longlegs"), where we have evidence of sexual antagonism, multiple mating, and delayed oviposition for a number of species. We used confocal laser scanning microscopy to capture and analyze images of harvester spermathecae, structures within the genitalia of female arthropods that store and maintain sperm after copulation. Spermathecal morphology may have critical function in controlling seminal movement. We anticipated that species with previously identified traits associated with sexual antagonism would also have thicker and/or relatively more complex spermathecae. We examined spermathecal morphology in thirteen species of Leiobunum and one species of Hadrobunus, which were collected from North America and Japan. Our results show that eight species had structures consisting of a single chamber with no or partial invagination, and the remainder had multiple cuticular invaginations producing 2-3 lumina within the spermathecae. Using phylogenetic multivariate comparative methods, we estimated a trend towards cross-correlation between conflict and spermathecal traits. Some, but not all, of the species with thicker, more complex spermathecae had morphological traits associated with sexual conflict (larger body size, thicker genital muscle). In conclusion, we discuss methods to elucidate spermathecal mechanism and sperm precedence in these species. Confocal microscopy allowed us to visualize internal structures difficult to interpret with two-dimensional brightfield microscopy, a technique that could be applied to the characterization of internal reproductive structures in other arthropods.

Latitudinal trend in the reproductive mode of the pea aphid Acyrthosiphon pisum invading a wide climatic range

The maintenance of sexuality is a puzzling phenomenon in evolutionary biology. Many universal hypotheses have been proposed to explain the prevalence of sex despite its costs, but it has been hypothesized that sex could be also retained by lineage-specific mechanisms that would confer some short-term advantage. Aphids are good models to study the maintenance of sex because they exhibit coexistence of both sexual and asexual populations within the same species and because they invade a large variety of ecosystems. Sex in aphids is thought to be maintained because only sexually produced eggs can persist in cold climates, but whether sex is obligate or facultative depending on climatic conditions remains to be elucidated. In this study, we have inferred the reproductive mode of introduced populations of the pea aphid Acyrthosiphon pisum in Chile along a climatic gradient using phenotypic assays and genetic-based criteria to test the ecological short-term advantage of sex in cold environments. Our results showed a latitudinal trend in the reproductive mode of Chilean pea aphid population from obligate parthenogenesis in the north to an intermediate life cycle producing both parthenogenetic and sexual progeny in the southernmost locality, where harsh winters are usual. These findings are congruent with the hypothesis of the ecological short-term advantage of sex in aphids.

Linking local adaptation with the evolution of sex differences

Many conspicuous forms of evolutionary diversity occur within species. Two prominent examples include evolutionary divergence between populations differentially adapted to their local environments (local adaptation), and divergence between females and males in response to sex differences in selection (sexual dimorphism sensu lato). These two forms of diversity have inspired vibrant research programmes, yet these fields have largely developed in isolation from one another. Nevertheless, conceptual parallels between these research traditions are striking. Opportunities for local adaptation strike a balance between local selection, which promotes divergence, and gene flow-via dispersal and interbreeding between populations-which constrains it. Sex differences are similarly constrained by fundamental features of inheritance that mimic gene flow. Offspring of each sex inherit genes from same-sex and opposite-sex parents, leading to gene flow between each differentially selected half of the population, and raising the question of how sex differences arise and are maintained. This special issue synthesizes and extends emerging research at the interface between the research traditions of local adaptation and sex differences. Each field can promote understanding of the other, and interactions between local adaptation and sex differences can generate new empirical predictions about the evolutionary consequences of selection that varies across space, time, and between the sexes.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.