Cytomolecular characterization and origin of de novo formed maize B chromosome variants

B chromosomes are dispensable elements that occur in many species, including maize. The maize B chromosome is acrocentric and highly heterochromatic and undergoes nondisjunction during the second pollen mitosis. In this study, we determined the genetic behavior and organization of two naturally occurring B chromosome variants (designated B-ta and B-tb). The morphology and genetic behavior of the B-ta chromosome were similar to those of the typical B chromosome, but the B-ta chromosome contained a deletion in the first heterochromatin region and had higher transmission frequencies through both male and female parents. The B-tb chromosome was reduced in size, consisted primarily of heterochromatin, and had a lower transmission frequency. The B-tb chromosome lacked nondisjunctional behavior, which was restored by the presence of normal B chromosomes in the cell. Furthermore, the B-tb chromosome contained two centromeric regions, only one of which was active. The organization of these two naturally occurring B chromosome variants was also determined using fluorescence in situ hybridization with B-associated sequences and by amplification of B-specific molecular markers to create possible evolutionary models.