A genetic perspective on social insect castes: A synthetic review and empirical study
Abstract
The process of caste differentiation is central to understanding insect sociality because it is castes that enable division of labor. Presumably selection favors colonies that can divide labor in response to environmental demands, and for many taxa genetic factors are an important part of this equation. In my thesis, I first provide a framework for understanding genetic and epigenetic effects on caste. From mostly ant, bee and termite examples, I make clear that genotype-caste associations can evolve in different and sometimes complex ways and can involve additive or non-additive genetic effects that, in turn, may arise directly from focal individuals or indirectly via their social partners. I use this framework to launch my empirical analysis of my own. In my second chapter, I test alternative hypotheses that describe how genes evolve under direct versus indirect selection. I predict that genes associated with reproductive castes will evolve mostly under direct selection and show patterns of nucleotide substitution that differ from those associated with non-reproductive helper castes and thus evolving under indirect selection. Using an RNA-Seq dataset for the Eastern subterranean termite, I found that caste-biased and un-biased genes evolve at similar rates, most consistent with purifying selection. I therefore did not detect an obvious pattern of molecular evolution that is diagnostic of indirect or 'kin' selection. I did discover other, more subtle patterns of nucleotide substitution that I discuss in the context of termite social biology.