Alteration of basic chromosome number by fusion-fission cycles.

A complete chromosomal fusion-fission cycle is described for the first time. In the field bean, Vicia faba, this cycle probably started with a reversible fusion of two telocentrics giving rise to the standard metacentric chromosome I. The next step was a recent fission of this chromosome into two stable telocentrics eventually followed by a new fusion reconstituting the metacentric chromosome.