The world is changing rapidly and organisms need to adapt to those changes. Animal personality describes how individuals differ in how well they cope with such challenges and is an important factor for explaining fitness of individuals and viability of populations. The search for hereditary mechanisms underlying these animal personality traits has focussed on the identification of underlying genomic polymorphisms, but especially for natural populations this has not been fruitful. It has become clear that hereditary variation is more than genomic variation alone. Also epigenetic mechanisms, such as DNA methylation, can alter gene expression over multiple generations, while this epigenetic variation may originate from both genetic and environmental factors. This raises the question whether there is an epigenetic basis for hereditary variation in personality traits, and how genetic variation interacts with epigenetic variation?
Here we combine whole-genome sequencing with whole-methylome (bisulfite) sequencing to study the influence of DNA methylation on the variation and evolution of personality traits in both natural populations and lines selected for extreme personalities in the great tit (Parus major). We integrate long-term behavioural data, whole-genome sequence and RNAseq data with whole-methylome, RRBS and candidate gene methylation data. We show that gene expression is a complex interaction between genomic and epigenetic information. The epigenome, as a regulator of the expression of the genome, thereby is an important factor explaining the heritable-but-plastic character of behavioural traits. Moreover, since epigenetic changes are more likely to interact with environmental variation, epigenetic mechanisms may react faster to selection pressusres compared to genetic polymorphisms. This is important for estimating the potential for species to adapt to environmental change such as global warming and urbanization.