Cooperative social clusters are not destroyed by dispersal in a ciliate

The propensity for social aggregation was greater in strains with reduced cell quality and lower growth performance. While we found a trade-off between costly aggregation and local dispersal in phenotypic analyses, aggregative strains showed a dispersal polymorphism by producing either highly sedentary or long-distance dispersive cells, in contrast to less aggregative strains whose cells were monomorphic local dispersers.High dispersal among aggregative strains may not destroy group stability in T. thermophila because the dispersal polymorphism allows social strains to more readily escape kin groups than less aggregative strains, yet still benefit from stable group membership among sedentary morphs. Such dispersal polymorphisms should be common in other social organisms, serving to alter the nature of the negative impact of dispersal on social evolution.Aggregative and dispersive behaviors could be antagonistic in many systems since high mobility should reduce the formation of aggregative associations. If aggregative behavior confers substantial benefits (e.g. via cooperation among kin), high dispersal destroys these benefits by reducing group stability [1]. Under these conditions, one should expect social species to have much lower dispersal than solitary species producing an aggregation-dispersal trade-off. The connection between dispersal and aggregation or cooperation, however, is far from straightforward. In a characteristically groundbreaking discussion of the issue, Hamilton and May [1] suggested that while aggregation among kin can indeed drive kin cooperation, as aggregations grow so does competition among related individuals (kin competition). Such strong competition among kin hence engenders inclusive fitness costs of aggregation that could be ameliorated by dispersal [2,3], but dispersal reduces group stability, and stability should promote cooperation [4-6]. Because aggregation can lead to more advanced forms of cooperation, including altruism [7-10], u