Kin-selection versus costs of inbreeding in the transition from subsocial to social spiders

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Sociality has evolved about 12 times independently in spiders, however resulting in less than 25 social species. Social spiders differ markedly from the eusocial insects in several aspects, for example are social spiders always characterised by extreme inbreeding. Inbreeding does result in very high degrees of relatedness and the evolution of sociality requires kin-associations since related individuals are much more likely to cooperate. However, inbreeding is also associated with several disadvantages and is therefore usually avoided. Hence, inbreeding may on one side facilitate cooperation while the associated fitness costs will determine the level of tolerance towards inbreeding and ultimately influence the potential for social evolution. The aim of this study is to identify the selection pressures and preconditions that have led to the evolution of extreme inbreeding in social spiders. In particular, we will focus on kin selection and the specific role of sexual selection through female choice, polyandry and sexual conflict in favouring or disfavouring the likelihood of inbreeding. To assess the relevance of genetic diversity and genetic distance for the evolution of cooperation and group performance, we will conduct experiments that test specific predictions derived from the hypothesis that kin selection strongly favours inbreeding in the evolution of communal tasks such as foraging in these spiders. Using experiments and molecular genetics we will assess the role of multiple mating in natural populations and its consequences for individual female fitness. The genus Stegodyphus with three independently evolved social species is ideal to address these questions. The proposed research is based on previous studies of subsocial Stegodyphus lineatus spiders that found (1) a high tolerance towards inbreeding and (2) an advantage of relatedness for communal feeding and growth. An additional study suggested short-time disadvantages of multiple paternity for groups of young.