Gross chromosome rearrangements are occurring in an accession of the grass Lolium rigidum.

Chromosome structure was analysed at mitosis in root tip meristems of eight genotypes of Lolium rigidum. FISH revealed changed positions in the rDNA sites indicating extensive chromosome rearrangements; indeed no two genotypes were the same. In one genotype, there were differences between cells within individual root tips. The changed positions of the rDNA sites appear to be reflections of chromosome translocations and this was confirmed by the presence of quadrivalents at metaphase I of meiosis. Possible mechanisms are discussed for this exceptional level of chromosome instability.