The role of the meiotic chromosome axis in the formation of crossovers in brassicas

Meiosis is a specialised cell division, which halves the chromosome number, giving rise to the formation of four haploid products, which in plants give rise to the formation of spores. Male meiosis results in the formation of four microspores, which after various rounds of cell division form pollen grains, the male gametophytes. Similarly, female meiosis results in the formation of four haploid products of which only one survives forming the megaspore that develops into the female gametophyte after several mitotic divisions. During meiosis, the homologous chromosomes undergo homologous recombination which leads to the formation of crossovers (COs). These COs can be visualised as chiasmata during metaphase I and they allow the correct segregation of the chromosomes in anaphase I. Moreover, as an outcome of meiotic recombination, new allelic combinations arise, being a major source of genetic variation for both: natural and artificial selection. In this way, CO formation constitutes a limiting factor in plant breeding programmes as CO formation is limited in terms of their number and distribution through the genome. One of the key characteristics of meiotic chromosomes is the formation of the axial elements (AEs). These are proteinaceous structures assembled between the sister chromatids during the leptotene stage and are composed of cohesin and condensin complexes among other proteins. In plant meiosis, proteins such as ASY1, ASY3, and ASY4, are associated with these AEs. In addition, the AEs undergo a remodelling process, mainly directed by PCH2, concomitant with the polymerisation of the synaptonemal complex (SC). The meiotic chromosome axis and its remodelling is essential for synapsis and recombination between the homologous chromosomes. In this way, asy1/3/4 and pch2 mutants show major defects in the polymerisation of the SC and CO formation, which lead to chromosome missegregation and reduced fertility. This study aims to translate the knowledge of meiosis accumulated from the plant model species Arabidopsis thaliana to crops of the Brassica genus, focusing on the role of the coiled-coil domain protein ASY3 and the axis remodelling protein PCH2 in CO formation. In this way, by means of a reverse genetic approach the ASY3 and PCH2 orthologs of B. rapa and B. oleracea have been studied through the cytological analysis of meiosis in TILLING and CRISPR mutants. In a complementary way, the relation between the AEs and their remodelling with CO formation ha