Reduced fertility caused by meiotic defects and micronuclei formation during microsporogenesis in x Brassicoraphanus

Background Hybridization and polyploidization events are important driving forces in plant evolution. Allopolyploids formed between different species can be naturally or artificially created but often suffer from genetic instability and infertility in successive generations. x Brassicoraphanus is an intergeneric allopolyploid obtained from a cross between Brassica rapa and Raphanus sativus , providing a useful resource for genetic and genomic study in hybrid species. Objective The current study aims to understand the cause of hybrid sterility and pollen abnormality in different lines of synthetic x Brassicoraphanus from the cytogenetic perspective. Methods Alexander staining was used to assess the pollen viability. Cytogenetic analysis was employed to monitor meiotic chromosome behaviors in pollen mother cells (PMCs). Origins of parental chromosomes in x Brassicoraphanus meiocytes were determined by genome in situ hybridization analysis. Results The x Brassicoraphanus lines BB#4 and BB#6 showed high rates of seed abortion and pollen deformation. Abnormal chromosome behaviors were observed in their PMCs, frequently forming univalents and inter-chromosomal bridges during meiosis. A positive correlation also exists between meiotic defects and the formation of micronuclei, which is conceivably responsible for unbalanced gamete production and pollen sterility. Conclusion These results suggest that unequal segregation of meiotic chromosomes, due in part to non-homologous interactions, is responsible for micronuclei and unbalanced gamete formation, eventually leading to pollen degeneration and inferior fertility in unstable x Brassicoraphanus lines.