High-resolution mapping of meiotic crossovers and non-crossovers in yeast

Abstract

Meiotic recombination has a central role in the evolution of sexually reproducing organisms. The two recombination outcomes, crossover and non-crossover, increase genetic diversity, but have the potential to homogenize alleles by gene conversion. Whereas crossover rates vary considerably across the genome, non-crossovers and gene conversions have only been identified in a handful of loci. To examine recombination genome wide and at high spatial resolution, we generated maps of crossovers, crossover-associated gene conversion and non-crossover gene conversion using dense genetic marker data collected from all four products of fifty-six yeast (Saccharomyces cerevisiae) meioses. Our maps reveal differences in the distributions of crossovers and non-crossovers, showing more regions where either crossovers or non-crossovers are favoured than expected by chance. Furthermore, we detect evidence for interference between crossovers and non-crossovers, a phenomenon previously only known to occur between crossovers. Up to 1% of the genome of each meiotic product is subject to gene conversion in a single meiosis, with detectable bias towards GC nucleotides. To our knowledge the maps represent the first high-resolution, genome-wide characterization of the multiple outcomes of recombination in any organism. In addition, because non-crossover hotspots create holes of reduced linkage within haplotype blocks, our results stress the need to incorporate non-crossovers into genetic linkage analysis. During meiosis, the cell division that forms reproductive gametes, DNA strands from each sister chromatid swap partners. There are two possible genetic outcomes of this recombination: crossovers and non-crossovers. The genome-wide distribution of crossovers was measured previously, but data on non-cross-over distribution are lacking due to the difficulty of identifying recombined DNA when flanking markers are not exchanged. Now Mancera et al. present a landmark high-resolution map of all recombination products in the yeast Saccharomyces cerevisiae. The map reveals the existence of crossover- and non-crossover-specific hot spots.