Data from: Higher relatedness mitigates mortality in a nematode with lethal male fighting

According to kin selection theory, individuals show less aggression towards their relatives. Limited dispersal promotes interactions among relatives but also increases competition among them. The evolution of cooperation in viscous populations has been subject of mainly theoretical exploration. We investigated the influence of relatedness on aggression in males of entomopathogenic nematode Steinernema longicaudum that engage in lethal fighting. In a series of in vitro experiments, we found that both competitor male group size and relatedness influence male mortality rates. Higher relatedness led to progressively lower rates of male mortality. In experimentally infected insects, wherein large numbers of males and females interact, the proportion of dead and paralysed (= terminally injured) males was higher when infection was established by infective juveniles originating from a mixture of three lines than in those infected by a single line. The results collectively show that Steinernema longicaudum males recognize their kin and consequently male mortality rates are lower in groups consisting of more related males. Furthermore, this monotonic negative relationship between aggression and relatedness suggests that kin selection benefits are still substantial even under extreme competition. Our experiments also suggest that kin recognition in entomopathogenic nematodes has a genetic basis rather than being strictly based on environmental cues. We discuss our findings within the theoretical context of the evolution of altruistic/cooperative behaviour in structured populations.根据亲缘选择理论（kin selection theory），个体对亲属的攻击性普遍更低。有限扩散（limited dispersal）虽能促进亲属间的互动，却也会加剧彼此间的竞争。此前针对粘性种群（viscous populations）中合作行为演化的研究，多以理论探索为主。我们针对可发生致命争斗的昆虫病原线虫（entomopathogenic nematode）长尾斯氏线虫（Steinernema longicaudum）的雄性个体，探究了亲缘关系对其攻击性的影响。在一系列体外（in vitro）实验中，我们发现雄性竞争者的群体规模与亲缘关系均会影响雄性死亡率：亲缘关系越近，雄性死亡率逐步降低。在实验感染的昆虫宿主内，大量雌雄线虫相互作用，当侵染源为三个品系混合的侵染期幼虫（infective juveniles）时，死亡与瘫痪（即致命受伤）的雄性个体比例，显著高于侵染源为单一品系的实验组。综合所有实验结果可见，长尾斯氏线虫的雄性个体能够识别亲属，因此由亲缘关系更近的雄性组成的群体中，雄性死亡率更低。此外，攻击性与亲缘关系之间呈现出单调负相关关系，这表明即便在极端竞争环境下，亲缘选择的收益依然可观。本实验还表明，昆虫病原线虫的亲缘识别机制具有遗传基础，而非完全依赖环境线索。最后，我们结合结构化种群（structured populations）中利他/合作行为（altruistic/cooperative behaviour）演化的理论框架，对本研究的发现展开了讨论。