Cytomolecular characterization of de novo formed rye B chromosome variants

Background: B chromosomes (Bs) are dispensable elements which occur in many species including rye (Secale cereale). We determined the organization of B variants to obtain insights into the origin of B polymorphisms in rye. Results: The observed B variants were classified according to their morphology and in situ hybridization patterns with the B-specific repeats D1100 and CL11 into (I) long arm iso B, (II) D1100-deficient B and (III) small metacentric B variants. Long arm iso Bs are likely products of a meiotic centromere misdivision and subsequent duplication of the long arm, whereas small B variants are probably generated by chromosome breakage. Some deficient Bs experienced extensive amplification of CL11 repeats. Conclusions: Both the pericentromere and the nondisjunction control region seem to be involved in the generation of rye B chromosome variants. However, due to the loss of the B-specific nondisjuction control region most of the variants generated are not capable to accumulate in a population.