Meiosis in the Allodiploid Plant xBrassicoraphanus Derived from the Intergeneric Hybridization Between Brassica rapa and Raphanus sativus

Hallmark features of meiosis include the formation of synapsis between the homologous chromosomes and the production of haploid spores with unique genetic compositions. Several studies reported meiotic behaviors in the haploid plant cells, which often displayed very limited synapsis and chiasma formation during meiotic prophase I compared to the diploid cells, resulting in many abnormal chromosome behaviors in the subsequent stages. This is most likely due to the absence of homologous chromosome pairing required for the success of meiosis. However, it is largely unknown how meiosis takes place in the allodiploid plants, whose haploid chromosome sets are derived during hybridization from the parents sharing a significant level of homology between the two genomes. This exerts a very unprecedented environment to the allodiploid meiocyte which goes through differentiation into gametes through reducing meiotic cell division. In this study, we observed the chromosome behavior of a newly synthesized allodiploid xBrassicoraphanus derived from a cross between Brassica rapa (2n = 20) and Raphanus sativus (2n = 18). The resulting allodiploid plant was found to retain a total of 19 chromosomes (n = 19). Meiosis in the pollen mother cell displayed severe defects in chromosome pairing and synaptonemal complex formation, eventually leading to the production of gametes with unbalanced chromosome sets such as diads and triads instead of normal haploid tetrads. This study should provide interesting mechanistic insights into the meiotic behavior in intergeneric allodiploid plants that retain a substantial degree of homology and distinction at the same time between the two genomes.