Bronwyn H. Bleakley

How and why cooperation evolves, particularly among nonrelatives, remains a major paradox for evolutionary biologists and behavioral ecologists. Although much attention has focused on fitness consequences associated with cooperating, relatively little is known about the second component of evolutionary change, the inheritance of cooperation or reciprocity. The genetics of behaviors that can only be expressed in the context of interactions are particularly difficult to describe because the relevant genes reside in multiple social partners. Indirect genetic effects (IGEs) describe the influence of genes carried in social partners on the phenotype of a focal individual and thus provide a novel approach to quantifying the genetics underlying interactions such as reciprocal cooperation. We used inbred lines of guppies and a novel application of IGE theory to describe the dual genetic control of predator inspection and social behavior, both classic models of reciprocity. We identified effects of focal strain, social group strain, and interactions between focal and group strains on variation in focal behavior. We measured psi, the coefficient of the interaction, which describes the degree to which an individual's phenotype is influenced by the phenotype of its social partners. The genetic identity of social partners substantially influences inspection behavior, measures of threat assessment, and schooling and does so in positively reinforcing manner. We therefore demonstrate strong IGEs for antipredator behavior that represent the genetic variation necessary for the evolution of reciprocity.

Inbred lines are an important tool of genetic studies of all traits, including behavior. Independently derived strains of ornamental "designer" guppies are readily available and predicted to be inbred; however, little is known about actual levels of inbreeding in any of these strains or whether these lines differ in genetic traits that have not been under strong directional artificial selection. We genotyped five designer strains of guppies known to vary in their responses to predator cues and a wild reference population to determine whether designer strains show evidence of inbreeding and whether the strains differed from each other at five microsatellite loci. The designer strains exhibited lower allelic diversity and observed heterozygosity than the wild population. Observed heterozygosity departed significantly from expected heterozygosity for most markers in all five strains of designer guppies. Inbreeding coefficient (f) comparisons between the wild reference population and the designer strains show considerable inbreeding in the designer strains. F(is) values for the designer strains also provide evidence of inbreeding. Finally, F(st) values indicate that the designer strains differ significantly from each other and the wild population. We therefore concluded that designer guppies are inbred compared to wild populations and differ among strains, making them useful tools for genetic studies of behavioral or life history traits.