Epigenetic differentiation and relationship to adaptive genetic divergence in discrete populations of the violet Viola cazorlensis

In plants, epigenetic variations based on DNA methylation are often heritable and could influence the course of evolution. Before this hypothesis can be assessed, fundamental questions about epigenetic variation remain to be addressed in a real-world context, including its magnitude, structuring within and among natural populations, and autonomy in relation to the genetic context. Extent and patterns of cytosine methylation, and the relationship to adaptive genetic divergence between populations, were investigated for wild populations of the southern Spanish violet Viola cazorlensis (Violaceae) using the methylation-sensitive amplified polymorphism (MSAP) technique, a modification of the amplified fragment length polymorphism method (AFLP) based on the differential sensitivity of isoschizomeric restriction enzymes to site-specific cytosine methylation. The genome of V. cazorlensis plants exhibited extensive levels of methylation, and methylation-based epigenetic variation was structured into distinct between- and within-population components. Epigenetic differentiation of populations was correlated with adaptive genetic divergence revealed by a Bayesian population-genomic analysis of AFLP data. Significant associations existed at the individual genome level between adaptive AFLP loci and the methylation state of methylation-susceptible-MSAP loci. Population-specific, divergent patterns of correlated selection on epigenetic and genetic individual variation could account for the coordinated epigenetic-genetic adaptive population differentiation revealed by this study.