Geographical and temporal variation of multiple paternity in invasive mosquitofish (Gambusia holbrooki, Gambusia affinis)

Paternity share predicts sons' fetal testosterone

Multiple paternity is common in many species. While its benefits for males are obvious, for females they are less clear. Female indirect benefits may include acquiring 'good genes' for offspring or increasing litter genetic diversity. The nutria (Myocastor coypus) is a successful invasive species. In its native habitat, it is polygynous, with larger and more aggressive males monopolizing paternity. Here, using culled nutria we genetically examined multiple paternity in-utero and found a high incidence of multiple paternity and maintenance of the number of fathers throughout gestation. Moreover, male fetuses sired by the prominent male have higher testosterone levels. Despite being retained, male fetuses of 'rare' fathers, siring commonly only one of the fetuses in the litter, have lower testosterone levels. Considering the reproductive skew of nutria males, if females are selected for sons with higher future reproductive success, low testosterone male fetuses are expected to be selected against. A possible ultimate explanation for maintaining multiple paternity could be that nutria females select for litter genetic diversity e.g., a bet-hedging strategy, even at the possible cost of reducing the reproductive success of some of their sons. Reproductive strategies that maintain genetic diversity may be especially beneficial for invasive species, as they often invade through a genetic bottleneck.

Invasive fish retain plasticity of naturally selected, but diverge in sexually selected traits

Invasive alien species (IAS) have become a major threat to ecosystems worldwide. From an evolutionary ecological perspective, they allow teasing apart the relative contributions of plasticity and evolutionary divergence in driving rapid phenotypic diversification. When IAS spread across extensive geographic ranges, climatic variation may represent a source of strong natural selection through overwinter mortality and summer heat stress. This could favour local adaptation, i.e., evolutionary divergence of certain traits. IAS, however, are likely to show plasticity in survival-related traits, and environmental fluctuation in their new distribution range could favour the maintenance of this pre-existing phenotypic plasticity. By contrast, sexually selected traits are more likely to undergo evolutionary divergence when components of sexual selection differ geographically. Here, using data from a common-garden rearing experiment of Western mosquitofish (Gambusia affinis Baird and Girard, 1853) from five populations across the species' invasive range in China, we show that invasive mosquitofish have retained plasticity in key physiological (thermal tolerances), morphological and life-history traits even 100 years after their introduction to China, but exhibit heritable population differences in several sexually selected traits, including the shape of the male copulatory organ. Adaptive plasticity of traits linked to immediate survival in different thermal environments-while likely responsible for the species' extraordinary invasion success-could slow down genetic evolution. Several sexually selected traits could diverge geographically and show rapid evolutionary change, e.g., because climate alters selective landscapes arising from mate competition as an indirect consequence of variation in overwinter mortality.

Quantifying the costs of pre- and postcopulatory traits for males: Evidence that costs of ejaculation are minor relative to mating effort

Although it is widely stated that both mating behavior and sperm traits are energetically costly for males, we currently lack empirical estimates of the relative costs to males of pre‐ versus postcopulatory investments. Such estimates require the experimental separation of the act of mating from that of ejaculation, which is a nontrivial logistical challenge. Here, we overcome this challenge using a novel morphological manipulation (gonopodium tip ablation) in the eastern mosquitofish (Gambusia holbrooki) to tease apart investment in mating effort from that in sperm replenishment following ejaculation. We quantified the relative cumulative costs of investing in mating effort and ejaculation by comparing somatic traits and reproductive performance among three types of males: ablated males that could attempt to mate but not ejaculate; unablated males that could both mate and ejaculate; and control males that had no access to females. We show that, after eight weeks, mating investment significantly reduces both body growth and immunocompetence and results in a significant decline in mating effort. In contrast, cumulative investment into sperm replenishment following ejaculation has few detectable effects that are only apparent in smaller males. These minor costs occur despite the fact that G. holbrooki has very high levels of sperm competition and multiple mating by both sexes, which is usually associated with elevated levels of sperm production. Crucially, our study is the first, to our knowledge, to experimentally compare the relative costs of pre‐ and postcopulatory investment on components of male fitness in a vertebrate.

Reproductive Mode and Conflict Shape the Evolution of Male Attributes and Rate of Speciation in the Fish Family Poeciliidae

Sexual conflict is caused by differences between the sexes in how fitness is maximized. These differences are shaped by the discrepancy in the investment in gametes, how mates are chosen and how embryos and young are provided for. Fish in the family Poeciliidae vary from completely provisioning eggs before they are fertilized to providing virtually all resources after fertilization via the functional equivalent of a mammalian placenta. This shift in when females provision their young relative to when an egg is fertilized is predicted to cause a fundamental change in when and how sexual conflict is manifested. If eggs are provisioned before fertilization, there should be strong selection for females to choose with whom they mate. Maternal provisioning after fertilization should promote a shift to post-copulatory mate choice. The evolution of maternal provisioning may in turn have cascading effects on the evolution of diverse features of the biology of these fish because of this shift in when mates are chosen. Here we summarize what these consequences are and show that the evolution of maternal provisioning is indeed associated with and appears to govern the evolution of male traits associated with sexual selection. The evolution of placentas and associated conflict does not cause accelerated speciation, contrary to predictions. Accelerated speciation rate is instead correlated with the evolution of male traits associated with sexual selection, which implies a more prominent role of pre-copulatory reproductive isolation in causing speciation in this family.

Comparative gut content analysis of invasive mosquitofish from Italy and Spain

Eastern mosquitofish (Gambusia holbrooki) are among the most widely introduced freshwater species globally. To gain a better understanding of feeding patterns in non-native populations, and which local factors may influence them at the population level, we carried out gut content analysis on 163 specimens from nine invasive populations in Italy and Spain. Based on previous studies, we predicted that (a) mosquitofish are omnivores with a preference for detritus and cladocerans; (b) they display size- and population-specific differences in gut morphologies and diet, with larger fish feeding more intensively over a wider range of prey items; and (c) some of the variation would be associated with differences in local environmental and climatic factors. Our results confirmed our first prediction, because mosquitofish fed on a variety of diet items, among which detritus and Cladocera dominated. However, not a single diet item was shared among all populations. Congruent with our second prediction, we further identified size- and population-specific differences in the occurrence of some diet items and gut morphologies. However, observed patterns in dietary habits did not seem to be driven by the environmental and climatic variables we had quantified. The fairly variable diet likely aids invasion success and helps explain the ubiquity of invasive mosquitofish across Italy and Spain, as mosquitofish seem to be able to rely on whatever a local habitat provides. We further propose that size-specific differences likely capture the substantial sexual size dimorphism (males are smaller than females), while population-specific differences are likely the result of differences in local prey abundance. The lack of an influence of temperature on dietary habits suggests that mosquitofish feeding ecology may be less impacted by rising temperatures than other freshwater fish species. If true, then this suggests climate change-induced effects may further exacerbate the competitive superiority of mosquitofish over native species in the future.

Integrative demographic study of the Iberian painted frog (Discoglossus galganoi): inter-annual variation in the effective to census population size ratio, with insights on mating system and breeding success

In the face of worldwide amphibian declines, integrative studies combining individual-based information and genetic data represent a powerful approach to produce robust, reliable, and comparable assessments of demographic dynamics. The Iberian painted frog (Discoglossus galganoi) is endemic to Spain and Portugal and shows decreasing population trends across its range, but few studies have attempted to estimate census sizes or assess genetic diversity in wild populations, and little is known about their reproductive biology. We applied an integrative approach based on the combination of capture-mark-recapture data and multilocus genotypes to monitor a breeding population of D. galganoi in central Spain during two consecutive breeding seasons, focusing on the estimation of demographic parameters and their temporal variation. Specifically, we estimated the number of adults (N_a ), the effective population size (N_e ), and the effective number of breeders (N_b ), as well as survival and migration rates. We documented a >50% decrease in the estimated number of adults of both sexes between the breeding seasons of 2018 and 2019, probably associated with reduced rainfall in the latter. Estimates of N_b and the N_b /N_a ratio were low in both seasons, with a 20-30% decrease in N_b and a 47% increase in the N_b /N_a ratio in 2019. Based on the reconstruction of pedigrees from larval and adult genotypes, we provide the first genetic evidence of polygamy in males and females of D. galganoi and the first estimates of breeding success in the species.