Effects of DNA Methylation and Chromatin State on Rates of Molecular Evolution in Insects

Epigenetic information is widely appreciated for its role in gene regulation in eukaryotic organisms. However, epigenetic information can also influence genome evolution. Here, we investigate the effects of epigenetic information on gene sequence evolution in two disparate insects: the fly Drosophila melanogaster, which lacks substantial DNA methylation, and the ant Camponotus floridanus, which possesses a functional DNA methylation system. We found that DNA methylation was positively correlated with the synonymous substitution rate in C. floridanus, suggesting a key effect of DNA methylation on patterns of gene evolution. However, our data suggest the link between DNA methylation and elevated rates of synonymous substitution was explained, in large part, by the targeting of DNA methylation to genes with signatures of transcriptionally active chromatin, rather than the mutational effect of DNA methylation itself. This phenomenon may be explained by an elevated mutation rate for genes residing in transcriptionally active chromatin, or by increased structural constraints on genes in inactive chromatin. This result highlights the importance of chromatin structure as the primary epigenetic driver of genome evolution in insects. Overall, our study demonstrates how different epigenetic systems contribute to variation in the rates of coding sequence evolution.