The Arabidopsis Hop1 homolog ASY1 mediates cross-over assurance and interference

We investigated the function of ASY1, the Arabidopsis homolog of the yeast chromosome axis associated component Hop1. We characterized cross-over (CO) distribution in female and male meiosis by deep sequencing of the progeny of an allelic series of asy1 mutants. Combining data from nearly 1000 individual plants, we find that reduced ASY1 activity leads to genomic instability. We further observed that COs are less frequent and appear in more distal chromosomal regions in plants with no or reduced ASY1 activity. Importantly, COs are redistributed in asy1 mutants, as clusters of closely positioned COs occur frequently. Since the number of foci of the recombination factor MLH1 is not significantly reduced in asy1 mutants, we conclude that ASY1 plays a key role in CO interference that spaces COs along a chromosome. Conversely, a large proportion of chromosomes do not receive any CO, suggesting that ASY1 also influences CO assurance, the process that insures the obligatory assignment of one CO per chromosome.