SOCIAL EVOLUTION IN ANTS: DIRECT AND INDIRECT GENETIC EFFECTS

Abstract

When social interactions occur, the phenotype of an individual can be influenced both directly by its own genes and indirectly by genes expressed in social partners. Eusocial insect colonies are notable for extensive behavioral interactions among larval, worker, and queen nestmates. In particular, developing larvae are reliant on care provided by workers and queens. Social insect phenotypes are thus affected by zygotic genes expressed during development (direct genetic effects), genes expressed in care-giving adult workers (sibsocial genetic effects), and genes expressed in queens (maternal genetic effects). The purpose of this dissertation was to incorporate this complexity into models of social insect evolution and to empirically study the evolutionary importance of direct and indirect genetic effects on ant phenotypes.

The first chapter reviews existing models for the evolutionary origin and maintenance of eusociality in social insects and proposes a new model incorporating both direct and indirect genetic effects. The second chapter uses a quantitative genetic approach to estimate variation for direct and indirect genetic effects on worker, gyne, and male mass, caste ratio, and sex ratio within a population of the ant Temnothorax curvispinosus. There was genetic variation for direct, maternal, and sibsocial effects for all traits, suggesting that larval, queen, and worker influences on mass, caste ratio, and sex ratio can respond to selection. However, there was also evidence for negative genetic correlations between each effect, demonstrating a potential constraint to the independent evolution of these effects. The third chapter uses the same ant colonies to study the effects of experimental queen removal and manipulation of relatedness among workers and larvae on mean colony worker, gyne, and male mass, caste ratio, and sex ratio. The fourth chapter examines the contribution of direct and sibsocial genetic effects to phenotypic differences between three Temnothorax species. The results demonstrate that among-species differences are influenced by the interaction of genes expressed in developing larvae and care-giving workers. Overall, this dissertation demonstrates the evolutionary importance of both direct and indirect genetic effects to social insect phenotypes.