Epigenetic Potential and DNA Methylation in an Ongoing House Sparrow (Passer domesticus) Range Expansion

During range expansions, epigenetic mechanisms may mediate phenotypic responses to environmental cues, enabling organisms to adjust to novel conditions at novel sites. Here, we predicted that the number of CpG sites within the genome, one form of epigenetic potential, would be important for success during range expansions because DNA methylation can modulate gene expression and hence facilitate adaptive plasticity. Previously, we found that this same form of epigenetic potential was higher in introduced compared to native populations of house sparrows (Passer domesticus) for two immune genes (Toll-like receptors 2A and 4). Here, we took a reduced-representation sequencing approach (ddRadSeq and EpiRadSeq) to investigate how CpG site number, as well as resultant DNA methylation, varied across five sites in the ~70 year-old Kenyan house sparrow range expansion. We found that the number of CpG sites increased towards the vanguard of the invasion, even when accounting for variation in genetic diversity among sites. This pattern was driven by more losses of CpG sites towards the core of the invasion (the initial site of introduction). Across all sequenced loci, DNA methylation decreased but became more variable towards the range-edge. However, in the subset CpG sites proximal to mutated CpG sites, DNA methylation increased and variation declined. These results indicate that epigenetic potential influenced the Kenyan house sparrow range expansion, likely by providing greater phenotypic plasticity which is genetically assimilated as populations adapt to local conditions. Similar mechanisms might underlie the successes and failures of other natural and anthropogenic range expansions.