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Hybridization and polyploidization are major driving forces in plant evolution. Allopolyploids can be occasionally formed from a cross between distantly related species but often suffer from chromosome instability and infertility. x Brassicoraphanus  is an intergeneric allotetraploid (AARR; 2n = 38) derived from a cross between  Brassica rapa  (AA; 2n = 20) and  Raphanus sativus  (RR; 2n = 18). x Brassicoraphanus  is fertile and genetically stable, while retaining complete sets of both  B. rapa  and  R. sativus  chromosomes. Precise control of meiotic recombination is essential for the production of balanced gametes, and crossovers (COs) must occur exclusively between homologous chromosomes. Many interspecific hybrids have problems with meiotic division at early generations, in which interactions between non-homologous chromosomes often bring about aneuploidy and unbalanced gamete formation. We analyzed meiotic chromosome behaviors in pollen mother cells (PMCs) of allotetraploid and allodiploid F1 individuals of newly synthesized x Brassicoraphanus . Allotetraploid x Brassicoraphanus  PMCs showed a normal diploid-like meiotic behavior. By contrast, allodiploid x Brassicoraphanus  PMCs displayed abnormal segregation of chromosomes mainly due to the absence of homologous pairs. Notably, during early stages of meiosis I many of allodiploid x Brassicoraphanus  chromosomes behave independently with few interactions between  B. rapa  and  R. sativus  chromosomes, forming many univalent chromosomes before segregation. Chromosomes were randomly assorted at later stages of meiosis, and tetrads with unequal numbers of chromosomes were formed at completion of meiosis. Immunolocalization of HEI10 protein mediating meiotic recombination revealed that COs were more frequent in synthetic allotetraploid x Brassicoraphanus  than in allodiploid, but less than in the stabilized line. These findings suggest that structural dissimilarity between  B. rapa  and  R. sativus  chromosomes prevents non-homologous interactions between the parental chromosomes in allotetraploid x Brassicoraphanus , allowing normal diploid-like meiosis when homologous pairing partners are present. This study also suggests that CO suppression between non-homologous chromosomes is required for correct meiotic progression in newly synthesized allopolyploids, which is important for the formation of viable gametes and reproductive success in the hybrid progeny.

frontiers in plant science

Meiotic Chromosome Stability and Suppression of Crossover Between Non-homologous Chromosomes in xBrassicoraphanus, an Intergeneric Allotetraploid Derived From a Cross Between Brassica rapa and Raphanus sativus