Predominantly genetic determination and stable transmission of DNA methylation in an avian hybrid zone

The reshuffling of divergent genomes upon hybridization may disrupt co-evolved regulatory systems and contribute to epigenetic instability and, ultimately, reproductive isolation. While the genetic consequences of hybridization are well documented, insights into the consequences of hybridization for DNA methylation are currently limited. To obtain insights into the regulation of methylation and its transmission under hybridization, we here investigated genome-wide methylation in a natural hybrid zone of songbirds (wheatears of the Oenanthe hispanica complex) by integrating nearly 100 methylomes with population genomic data. Across 436,762 CpG sites, the population structure of methylation closely mirrors genetic population structure. Methylation quantitative trait locus analyses identify widespread associations of genetic with methylation variation, predominantly in trans, consistent with a regulatory architecture in which the genetic background determines methylation variation. Between species, methylation divergence is limited, with only 0.31% of CpGs differentially methylated. While at the level of chromosomes methylation divergence strongly correlates with genetic differentiation, the extent to which differentially methylated loci coincide with high genetic differentiation differs among chromosomes. A close-to-absent methylation divergence from promoters and coding regions indicates conservation of core regulatory architectures. Finally, CpGs with highest methylation divergence exhibit predominantly additive or dominant transition patterns in hybrids. In contrast, transgressive methylation is exceedingly rare, and we find no evidence for widespread hybrid-induced demethylation. Or results corroborate that DNA methylation primarily reflects underlying genetic variation in birds and remains robust to genome reshuffling, and at least for wheatears suggest a limited role for methylation divergence in hybrid dysfunction and reproductive isolation.