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

The chromosome axis plays a crucial role in meiotic recombination. Here, we study the function of ASY1, the Arabidopsis homolog of the yeast chromosome axis-associated component Hop1. Specifically, 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 1,000 individual plants, we find that reduced ASY1 functionality leads to genomic instability and sometimes drastic genomic rearrangements. We further observed that COs are less frequent and appear in more distal chromosomal regions in plants with no or reduced ASY1 functionality, consistent with previous analyses. However, our sequencing approach revealed that the reduction in CO number is not as dramatic as suggested by cytological analyses. Analysis of double mutants of asy1 with mutants with three other CO factors, MUS81, MSH4, and MSH5, as well as the determination of foci number of the CO regulator MLH1 demonstrates that the majority of the COs in asy1, similar to the situation in the wildtype (WT), largely belong to the class I, which are subject to interference. However, these COs are redistributed in asy1 mutants and typically appear much closer than in the WT. Hence, ASY1 plays a key role in CO interference that spaces COs along a chromosome. Conversely, since a large proportion of chromosomes do not receive any CO, we conclude that CO assurance, the process that ensures the obligatory assignment of one CO per chromosome, is also affected in asy1 mutants. Significance statement: The regulation of the number and placement of cross-overs (COs) during meiosis is critical to ensure meiotic fidelity and promote new genetic combinations. Here, we investigated the function of ASY1, which resides on the chromosome axis and plays a key role in CO formation. Our results show that COs in asy1 mutants are positioned closer to each other than in the wildtype and that, despite a roughly similar number of COs, not every chromosome receives a CO. With this, our results shed light on the mechanisms regulating two important but still poorly understood aspects ofmeiosis: CO assurance, which safeguards at least one CO per chromosome pair, and CO interference, which prevents two COs from occurring close to each other.