A novel allele of ASY3 promotes meiotic stability in autotetraploid Arabidopsis lyrata

In this study we performed a genotype-phenotype association analysis of meiotic stability in ten autotetraploid Arabidopsis lyrata and A. lyrata/A. arenosa hybrid populations collected from the Wachau region and East Austrian Forealps. The aim was to determine the effect of eight meiosis genes under extreme selection upon adaptation to whole genome duplication. Individual plants were genotyped by high-throughput sequencing of the eight meiosis genes (ASY1, ASY3, PDS5b, PRD3, REC8, SMC3, ZYP1a/b) implicated in synaptonemal complex formation and phenotyped by assessing meiotic metaphase I chromosome configurations. Our results reveal that meiotic stability varied greatly (20-100%) between individual tetraploid plants and was associated with segregation of a novel allele orthologous to the budding yeast RED1 chromosome axis protein, Asynapsis3 (ASY3), derived from A. lyrata. The adaptive ASY3 protein possesses a putative in-frame tandem duplication (TD) of a serine-rich region upstream of the coiled-coil domain that has arisen at sites of DNA microhomology. The frequency of multivalents observed in plants homozygous for the ASY3 TD haplotype was significantly lower than plants heterozygous for TD/ND (non-duplicated) ASY3 haplotypes. Chiasma distribution was significantly altered in the stable plants compared to the unstable plants with a shift from proximal and interstitial to predominantly distal locations. The number of HEI10 foci at pachtyene that mark class I crossovers was significantly reduced in meiotic nuclei from ASY3 TD homozygous plants compared to ASY3 ND/TD heterozygotes, indicating an adaptive consequence of the ASY3 TD allele. From the ten populations, fifty-eight alleles of these 8 meiosis genes were identified, demonstrating dynamic population variability at these loci which nevertheless exhibit signatures of strong hard selective sweeps. Widespread chimerism between alleles originating from A. lyrata/A. arenosa and diploid/tetraploids indicates that this group of rapidly evolving genes provide precise adaptive control over meiotic recombination in the tetraploids, the very process that gave rise to them