Cytogenetic analysis and physical mapping of maize B chromosome deficiencies induced by the r-X1 deletion

The r-X1 deletion can induce nondisjunction and terminal deficiencies in maize B and normal A chromosomes during megaspore division; however, to date, only one case of B chromosome deficiency (Bdef) with a breakpoint in the third distal heterochromatin of the B chromosome has been analyzed. In this study, we examined r-X1-containing progeny from maternal r-X1 plants carrying two B chromosomes, and an additional 18 potential Bdefs of varying lengths smaller than the B chromosome were observed, suggesting that the r-X1 deletion leads to random breakages of the B chromosome. Three of these Bdefs (designated Bdef-2, Bdef-3, and Bdef-5) with breakpoints in the proximal euchromatin, the second heterochromatin and the fourth heterochromatin of the B chromosome, respectively, were successfully propagated and further analyzed. Transmission analysis revealed that all three Bdefs lost the ability to undergo nondisjunction during the second pollen mitosis, whereas the presence of the B chromosome restored this ability. Furthermore, it is hypothesized that specific regions of the B chromosome existed to suppress the meiotic loss of univalent B chromosomes in both female and male plants. Finally, using these three Bdefs, eight B chromosome-specific polymerase chain reaction markers were physically mapped more precisely onto the B chromosome. Our results suggest that Bdefs induced by the r-X1 deletion can serve as a new resource for studying structure and function of the maize B chromosome.