The role of methylation and structural variants in shaping the recombination landscape of barley

Meiotic recombination is not only a key mechanism for sexual adaptation in eukaryotes but crucial for the accumulation of beneficial alleles in breeding populations. The effective manipulation of recombination requires, however, a better understanding of the mechanisms regulating the rate and distribution of recombination events in genomes. Here, we identified the genomic features that best explain the recombination variation among a diverse set of segregating populations of barley at a resolution of 1 Mbp and investigated how methylation and structural variants determine recombination hotspots and coldspots at a high-resolution of 10 kb. Hotspots were found to be in proximity to genes and the genetic effects not assigned to methylation were found to be the most important factor explaining differences in recombination rates among populations along with the methylation and the parental sequence divergence. Interestingly, the inheritance of a highly-methylated genomic fragment from one parent only was enough to generate a coldspot, but both parents must be equally low methylated at a genomic segment to allow a hotspot. The parental sequence divergence was shown to have a sigmoidal correlation with recombination indicating an upper limit of mismatch among homologous chromosomes for CO formation. Structural variants (SVs) were shown to suppress COs, and their type and size were not found to influence that effect. Methylation and SVs act jointly determining the location of coldspots in barley and the weight of their relative effect depends on the genomic region. Our findings suggest that recombination in barley is highly predictable, occurring mostly in multiple short sections located in the proximity to genes and being modulated by local levels of methylation and SV load.