Aggregation of chromosome axis proteins on the chromatin and in the nucleoplasm of Brassica oleracea meiocytes

Meiotic recombination is essential for the generation of genetic diversity in natural and breeding context. The chromosome axis comprises cohesin, HORMA-domain containing proteins and coiled coil proteins and is crucial for the establishment of meiotic recombination. These proteins form a complex during meiosis of Brassica oleracea but information about their respective localisation and dynamic on meiotic chromosomes remain sparse. Our study reveals that the HORMA-protein ASY1 aggregates on the chromatin and forms domains of high and low abundances. The regions enriched for ASY1 are also highly enriched for the axis proteins ASY3, SMC3 and SCC3, although to varying degrees between leptotene and pachytene stages. At later stages, when most DNA double strand breaks are repaired and the chromosome axis disassemble, ASY1, ASY3, SCC3 and SMC3 co-localise and form large aggregates on the discontinuous axis structures. As the axis structures reduce in length, we found that all four axis proteins relocalise in the nucleoplasm and further aggregates. Moreover, we found that ZYP1, the transverse filament of the synaptonemal complex, forms numerous chromosomic aggregates that are sometimes associated with MLH1 and can form ectopic synaptic interactions. Overall, our study indicates that axis proteins have a high propensity to aggregate. This property is important for assembling the chromosome axis but the association of axis proteins with the chromatin must be tightly regulated to limit polycomplex formation.