Evolutionary Mechanisms Of Life-History Trait (Reproductive Biology Traits) Variationof Invasive Western Mosquitofish (Gambusia Affinis) And Predator-Prey Relationships With Native Diplonychus Esakii

The drastic changes of biotic and abiotic environmental conditions accompanying biological invasions impose challenges on invasive species.Western mosquitofish?Gambusia affinis?,which belongs to the order of toothcarps?Cyprinodontiformes?,wherein it belongs to the family of livebearers?Poeciliidae?,and the genus of mosquitofishes?Gambusia?,originates from North America.Due to its consumption of mosquito larvae,G.affinis has been introduced to various temperate to tropical regions around the world in an attempt to control mosquito-borne diseases such as malaria.Following repeated introduction campaigns,feral G.affinis populations are now flourishing in more than sixty countries.However,G.affinis prey heavily on the larvae of native aquatic species?e.g.,amphibians?,threatening local ecological communities and ecosystem stability.Still,little is known about the evolutionary mechanisms underlying the species'high invasiveness.This study represents a first step towards vertical and horizontal integration of evolutionary analyses by focusing on phenotypic variation and evolutionary mechanisms across a replicated latitudinal gradient inhabited by populations of invasive G.affinis in China.I first conducted life-history analyses on invasive G.affinis populations across a wide distribution range in mainland China.To investigate life-history trait variation between seasons,I furthermore sampled G.affinis on a monthly basis from three sites for a year.Moreover,a common garden rearing experiment was conducted to unveil the underlying mechanism of phenotypic variation observed in wild populations?i.e.,heritability versus phenotypic plasticity?.I also manipulated different environmental factors to understand potential adaptive plasticity of G.affinis life histories in a fluctuating environment.Lastly,several life-history traits in poeciliid fishes have been shown to be affected by predation and so I conducted a semi-natural predation experiment to investigate interactions between invasive G.affinis and a native aquatic heteropteran,Diplonychus esakii.My main results can be summarized as follows:?1?I used invasive populations of G.affinis to investigate potential divergence of phenotypic traits known to be under different forms of selection.I condensed?mainly climate-related?environmental information using a factor reduction procedure?principal component analysis,PCA?.Male?but not female?body size showed a weak?but significant?signal congruent with‘Bergman's rule';i.e.,males became larger towards higher latitudes?i.e.,towards the north?and towards more coastal sites.Overall,males and females showed phenotypic divergence in response to different components of climatic variation,suggesting different evolutionary trajectories for both sexes:males diverged especially along the latitudinal gradient?climatic PC1?,while females diverged primarily along the longitudinal/continental gradient?climatic PC2?.I thus demonstrate that both climatic gradients,probably along with unquantified environmental factors—such as predation—drive the observed pattern.?2?I tracked seasonal life-history variation in three invasive G.affinis populations over the course of a year.Both male and female G.affinis increased their body size during the reproductive season.Male body size was influenced by intra-sexual competition during the reproductive months and increased with increasingly male-biased sex ratios.Female fat content increased along with fecundity and embryo fat content during the reproductive season,suggesting that resource availability and temperature variation were the main factors driving the observed temporal life-history variation.?3?In the common-garden rearing experiment,I found life-history traits of the F₂ filial laboratory-reared generation to show an intermediate phenotype.Hence,two generations of exposure to uniform and stable experimental conditions result in trait homogenenization,suggesting that phenotypic plasticity was the main factor driving life-history variation among wild G.affinis populations.?4?When rearing G.affinis offspring under different temperature regimes,adults reared thusly showed optimal growth at 25°C.Rearing offspring at 20°C and 30°C appears to have imposed thermal stress and thus impaired the onset and frequency of reproductive maturity in the studied populations.When rearing offspring instead under different forms of food stress?ad libitum feeding every two days?,I found only body condition and embryo fat content showed significant variation.Moreover,I reared offspring under increasing salinities?10,20,and 30‰?and found that all populations could endure salinities as high as 20‰,whereby offspring survived for at least 30 days?i.e.,the time of experimental exposure?.?5?I conducted 48-h prey choice experiments under semi-natural conditions.In a‘no-choice'experiment,I found the heteropteran Diplonychus esakii to prey more on large-bodied fish,a pattern that was also described for other belostomatids,while prey sex had no effect on capture rates.However,subsequent binary prey choice experiments using one predator and two prey—either large and small females or male and female—did not confirm the results of our first experiment.I argue that development stages and senescense of the heteropteran predators are the main factors underlying the observed pattern.With increasing global commerce and global climate change,biological invasions are becoming more and more frequent.Yet,we currently have a poor understanding of the evolutionary mechanisms by which invasive organisms successfully colonize new environments and expand their distribution ranges.Using invasive G.affinis in China as a model species,I was able to add to the understanding of the mechanisms underlying range expansions and the colonization of novel environments.In this study,I found that invasive G.affinis could endure harsh temperature differences and drastic fluctuations of various other environmental factors and respond mainly via?adaptive?phenotypic plasticity.Phenotypic plasticity may also facilitate the species'further range expansion in the near future.It is recommended to construct species distribution models in order to forecast the future spread of this species and to warn local authorities of its advent,which can have devastating consequences for native aquatic ressources.