Cooperative social clusters are not destroyed by dispersal in a ciliate.
BACKGROUND
The evolution of social cooperation is favored by aggregative behavior to facilitate stable social structure and proximity among kin. High dispersal rates reduce group stability and kin cohesion, so it is generally assumed that there is a fundamental trade-off between cooperation and dispersal. However, empirical tests of this relationship are rare. We tested this assumption experimentally using ten genetically isolated strains of a ciliate, Tetrahymena thermophila.


RESULTS
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.


CONCLUSIONS
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.
Schtickzelle
Nicolas
N
Biodiversity Research Centre, Université catholique de Louvain, Croix du Sud 4, 1348 Louvain-la-Neuve, Belgium. nicolas.schtickzelle@uclouvain.be
Fjerdingstad
Else J
EJ
Chaine
Alexis
A
Clobert
Jean
J
D000818:Animals; D005075:Biological Evolution; D002465:Cell Movement; D014644:Genetic Variation; D010641:Phenotype; D011157:Population Dynamics; D012919:Social Behavior; D016808:Tetrahymena thermophila / Q000235:genetics / Q000254:growth & development / Q000502:physiology*