Relatedness and genetic structure in a socially polymorphic mixed population of the spider Anelosimus studiosus

Sociality is influenced by many internal mechanisms and external influences, it is often difficult to generalize across taxa as to how it has evolved and is maintained. A central consideration in this study is the social structure of the spider, Anelosimus studiosus, and the degree to which individuals that live in social groups are related. This comb-footed theridiid species exhibits phenotypic variation in social behavior, the extremes of which have been identified as 'asocial' and 'social' with the asocial phenotype associated with solitary nests and the social with multi-female/communal nests. Five microsatellite loci were used to analyze individuals from both communal and solitary nests within a population of A. studiosus. We found no evidence of genetic differentiation between social and solitary samples, implying high rates of interbreeding between phenotypes. This is consistent with the proposition that social and asocial phenotypes coexist as a behavioral polymorphism in A. studiosus populations. Pairwise relatedness coefficients were used to test the prediction made by kin selection that cooperating individuals are more closely related than expected by chance. We found significantly higher relatedness values for the communal pairs of adult females vs. the solitary pairs of adult females. Pairwise relatedness of females sharing communal webs averaged 0.26, or about the level expected for half-siblings. A simple randomization test showed that these social pairs were more closely related than random pairs from the local population. Solitary females not sharing webs but collected at similar spatial distances were also more closely related than expected by chance ( r = 0.18), but significantly less related, on average, than social pairs. These results imply that low dispersal rates contribute to an increased likelihood of interaction between kin, but relatedness between social pairs is not entirely explained by spatial structure.