Detecting Selection Signature After Meiotic Recombination In A Yeast Population

Meiotic recombination is one of the most important fundamental biological processes. It not only influences the variation of genomes but also mediate the effects of natural selection on the genome evolution. Meiotic recombination usually has two possible outcomes-crossover (CO) and gene conversion (GC). High resolution maps of meiotic recombination have been reported in several model organisms. However, few studies directly studied how selection quickly affects of the products of meiotic recombination in lab conditions.Here we constructed a simple yeast population that contains38randomly selected haploid strains derived from meiosis of hybridizations of parental strains S288c and YJM789. Every strain in this population was sequenced independently twice with high coverage. We obtained high quality reads to do mapping and SNP calling. In this way, about50,000SNP markers were identified, which enabled us to identify every block of crossover and even small gene conversion tracts in each sample. On average, we found66.8crossovers and75.9gene conversions per meiosis in yeast. Interestingly, the distribution pattern of crossover and gene conversion is distinct. Crossover distributes randomly but gene conversion has obvious hotspots across the genome. We also discovered the dorminance of short gene conversion tracts that are smaller than100bp. More important, we identified20regions with strong biased allele frequency across the genome, revealing signatures of selection in a rather short period. These regions harbor ample COs and GCs, which enable us to trace how selection works on the genomic fragments after meiosis. We quantatatively estimated the contribution of CO and GC to the shaping of selected regions. The contribution of CO to long regions under selection is dominant while GC has its important role in small selected regions. The total length of such regions under selection accounts for5%of the entire genome, and those regions contain many genes with shared functions. In addition, recombination breaks down linkage disequilibrium (LD) to half of its maximum within42kb and reduces nucleotide diversity significantly in selected regions. Our study directly inspects the details of directional selection on the outcomes of meiotic recombination using experimental approaches, and will shed light on our understanding of the fast reshaping of population structure by selection, as well as the important roles of CO and GC.