Natural and sexual selection drive multivariate phenotypic divergence along climatic gradients in an invasive fish

Abstract

Invasive species that rapidly spread throughout novel distribution ranges are prime models to investigate climate-driven phenotypic diversification on a contemporary scale. Previous studies on adaptive diversification along latitudinal gradients in fish have mainly considered body size and reported either increased or decreased body size towards higher latitudes (i.e. Bergmann’s rule). Our study is the first to investigate phenotypic divergence in multiple traits, including sexually selected traits (size and shape of the male copulatory organ, the gonopodium) of invasive Gambusia affinis in China. We studied body size, life history traits and morphological variation across populations spanning 17 degrees of latitude and 16 degrees of longitude. Even though we found phenotypic variation along climatic gradients to be strongest in naturally selected traits, some sexually selected traits also showed systematic gradual divergence. For example, males from southern populations possessed wider gonopodia with increased armament. Generally, males and females diverged in response to different components of climatic gradients (latitudinal or longitudinal variation) and in different trait suites. We discuss that not only temperature regimes, but also indirect effects of increased resource and mate competition (as a function of different extrinsic overwinter mortality rates) alter the selective landscape along climatic gradients.

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

“…Likewise, increased reproductive skew in northern populations could indicate higher levels of female‐mediated sexual selection in these populations (Emlen & Oring, ), which could help females to mitigate the negative consequences of sexual harassment and to refine their mate choice (Hamilton, ; Pitcher et al, ), and produce higher‐quality offspring (Ojanguren et al, ). The idea that female‐mediated sexual selection (e.g., via cryptic female choice; Gasparini & Pilastro, ; Pilastro et al, ) increases towards northern population aligns with the results from another study on Chinese G. affinis , suggesting that male‐mediated forms of sexual selection on certain phenotypic traits increase towards southern populations, where males have a modified armament of their gonopodium tips that may improve sperm transfer during coercive mating (Ouyang et al, ).…”

“…At first sight, this result seems to provide support for direct climate‐effects, for instance, via female solicitation of MP to increase genetic variation and/or quality of their offspring under harsh (northern) conditions (Table ). However, path analysis suggested that climatic effects were mainly indirect, mediated by altered life‐history traits, both latitude‐dependent (Ouyang et al, ; Riesch et al, ) and ‐independent. Both types of analyses, however, received relatively low statistical support.…”

“…Northern populations are subject to higher overwinter mortality than southern ones (Cheng et al, ), leading to lower population densities and female‐biased sex ratios, at least at the beginning of the reproductive season (Zulian, Bisazza, & Marin, ). While mosquitofish males are, on average, larger at higher latitudes (Ouyang et al, ; Riesch et al, ), some males can achieve sexual maturity at particularly small body sizes under these conditions (Zulian et al, ; see also Borowsky, for Xiphophorus variatus ), and smaller males are more efficient in sneaky copulations, being more manoeuvrable and more difficult to spot by females (Pilastroet al, ).…”

“…We extracted the following site‐specific means: (a) annual temperature, (b) maximum temperature of the warmest month, (c) minimum temperature of the coldest month, (d) annual temperature difference (by subtracting the minimum monthly temperature from the maximum monthly temperature), and (e) annual precipitation. We obtained population‐specific information on (f) altitude and (g) distance to the sea from Google Earth (http://earth.google.com/; Table ; see Ouyang et al, ). We condensed all seven variables via principal component analysis (PCA) with Varimax rotation and retained three environmental principal components (EPCs) that described over 84% of the variance (Table a).…”

“…We obtained population/monthly means of life‐history traits for both our geographical and temporal samples using well‐established protocols (Ouyang et al, ; Riesch, Easter, Layman, & Langerhans, ; Riesch et al, ). For the geographical sample, we used a subset of 19 populations (leaving out Baoding and Shenzhen for lack of mature/pregnant individuals).…”