# The recombination landscape of introgression in yeast

## TL;DR

This study shows that genetic variation from hybridization in yeast reduces genetic shuffling during reproduction, which may help eliminate hybrid genes over time.

## Contribution

The study reveals that introgression from hybridization alters the recombination landscape in yeast, reducing crossovers and increasing non-crossovers.

## Key findings

- Crossover rates are significantly reduced in heterozygous introgression regions compared to syntenic regions.
- Introgression leads to reduced allele shuffling within and across chromosomes.
- Hybridization influences the recombination landscape and may facilitate the purging of introgression.

## Abstract

Meiotic recombination is an evolutionary force that acts by breaking up genomic linkage, increasing the efficacy of selection. Recombination is initiated with a double-strand break which is resolved via a crossover, which involves the reciprocal exchange of genetic material between homologous chromosomes, or a non-crossover, which results in small tracts of non-reciprocal exchange of genetic material. Crossover and non-crossover rates vary between species, populations, individuals, and across the genome. In recent years, recombination rate has been associated with the distribution of ancestry derived from past interspecific hybridization (introgression) in a variety of species. We explore this interaction of recombination and introgression by sequencing spores and detecting crossovers and non-crossovers from two crosses of the yeast Saccharomyces uvarum. One cross is between strains which each contain introgression from their sister species, S. eubayanus, while the other cross has no introgression present. We find that the recombination landscape is significantly different between S. uvarum crosses, and that some of these differences can be explained by the presence of introgression in one cross. Crossovers are significantly reduced in heterozygous introgression compared to syntenic regions in the cross without introgression. This translates to reduced allele shuffling within introgressed regions, and an overall reduction of shuffling on most chromosomes with introgression compared to the syntenic regions and chromosomes without introgression. Our results suggest that hybridization can significantly influence the recombination landscape, and that the reduction in allele shuffling contributes to the initial purging of introgression in the generations following a hybridization event.

Mating between different species (i.e., hybridization) can introduce novel and distinct genetic variation in a genome. The persistence and distribution of this variation introduced from hybridization is influenced by many processes, including recombination, which occurs during gamete development in all sexually reproducing organisms and shuffles alleles between homologous chromosomes. Here, we set up crosses between 1) strains of yeast without variation from hybridization, and 2) strains of yeast with variation introduced from hybridization, in order to understand the interaction between recombination and variation introduced from hybridization. We observe that DNA sequence differences in regions derived from hybridization suppress the reciprocal exchange of genetic material (known as crossovers), and most recombination events are repaired through an alternate resolution known as non-crossovers. Variation from hybridization changes the amount of allele shuffling locally and across chromosomes, likely helping to facilitate the elimination of genetic variation from hybridization after hybridization occurs.