Master of Science
Thompson, Graham J
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.
Summary for Lay Audience
Termites are eusocial insects with reproductive and non-reproductive castes, the latter of which can only evolve via indirect selection. In this thesis I first look retrospectively at the role for genes and genetic variation on the origin and maintenance of caste differences in the social insects. My synthetic review concludes that additive genetic variants must have played an essential role in the origin of castes and that gene-by-environment interactions continue to govern the conditional development of caste differences for many ant, bee, wasp and termite species. From this broader standpoint, I launch my empirical study. Here, I test if genes associated with sterile workers and soldiers in a termite have similar patterns of molecular evolution to those of reproductive castes. The Eastern subterranean termite Reticulitermes flavipes showed patterns of nucleotide substitution at most caste-associated loci consistent with purifying selection. Some soldier-associated loci did, however, differ from this general pattern and appear to be evolving close to the neutral rate. A neutral pattern is rare for highly expressed protein coding genes but is consistent with the idea that genes evolving under indirect selection are buffered from the full strength of selection and thus may approach neutrality. These two genes, encoding a titan and a feruloyl esterase-like protein, together with others identified by my analysis suggests that caste can affect the strength of selection and in a manner consistent with some prior predictions. I discuss my novel results in the context of modern sociogenomic theory and offer my own ideas on ways to further test the relationship between caste-biased gene expression and patterns of molecular evolution.
Chernyshova, Anna M., "A genetic perspective on social insect castes: A synthetic review and empirical study" (2021). Electronic Thesis and Dissertation Repository. 7771.
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