Plant breeding - which harnesses the natural genetic variation that arises during meiosis - will have a key role for Food Security in the future by improving crop varieties. During meiosis, diploid cells undergo one round of DNA replication followed by two successive chromosome segregation rounds, producing haploid gametes to maintain somatic diploidy following their fusion (Fig 1).

Figure 1. Chromosomes during male meiosis in Hordeum vulgare (a) and in Arabidopsis thaliana (b). Meiosis consists of an extended prophase I (leptotene, zygotene, pachytene, diplotene and diakinesis) during which meiotic chromosome axis, synaptonemal complex (SC) and crossover (CO)-formation take place. These events culminate in the formation of so-called bivalents representing linked homologous chromosomes (seven homologous chromosome pairs in Hordeum vulgare and five in Arabidopsis thaliana) physically connected based on CO events (cytological manifested as chiasmata visible by diakinesis until anaphase I). During the first division homologous chromosomes segregate and during the second division sister chromatids are separated resulting in four haploid gametes (tetrads) that will finally give rise to male and female gametes.



Meiosis assures genetic variation by regulated genetic exchange through homologous recombination (HR). HR is initiated by formation of numerous DNA double strand breaks (DSBs). These DSBs are processed either as crossover (CO), recombining homologous parental chromosomes, or as non-CO, exchanging only short DNA stretches. HR occurs when meiotic chromosomes are organised around protein axes, which progressively juxtapose forming the synaptonemal complex (SC) (Fig 2). Which DSB matures into a CO depends on the interplay between the recombination machinery and these meiotic chromatin structures. In addition, epigenetic chromatin modifications, posttranslational protein modifications, cytoskeletal forces and environmental factors further influence CO patterning. Together, CO formation is subject to stringent controls regulating CO frequency and distribution.                                              

Figure 2. Immunolocalization of the meiotic chromosome axis component ASY1 and the synaptonemal complex component ZYP1 in male meiotic cells of Brassica oleracea. Chromosomes (blue) are organised around protein axes (ASY1, green), which juxtapose together as the synaptonemal complex (ZYP1, red) polymerises between them.



In barley (Hordeum vulgare) CO are restricted both in terms of their total number and in their distribution to distal chromosome regions (Fig 3). Therefore, barley breeding programs do not fully exploit a large pool of the potential genetic variation (~30% of the genes lie within interstitial chromosome regions that are recalcitrant to recombination) that is naturally available for the development of new varieties. This restricts both introgression of new genetic traits as well as gene isolation, creates linkage drag, and thus hampers map-based cloning and marker-assisted selection of agronomical important phenotypes. In a nutshell, modifying the number and distribution of CO has great potential to improve and accelerate barley breeding processes.                                                                                                                                    





Figure 3. Barley meiotic chromosomes at metaphase I. Homologous chromosomes typically form rod (one distal recombination event; asterisk) or ring (two distal recombination events in opposite chromosomes arms; arrow) bivalents revealing cytological the phenomenon of CO-heterogeneity, i.e. recombination events are limited to distal chromosome parts.