Extreme chromosomal heterogeneity in a small-island population of Rumex acetosa

Chromosome analyses of 227 mature plants of the dioecious species Rumex acetosa collected on the small island of Skomer have revealed an extremely high level of unique and polymorphic variation. The three common polymorphisms in this species--supernumerary segments on chromosomes 1 and 6, and B-chromosomes--are widespread on the island and the frequency of supernumerary segment 1 is higher than in all 37 mainland populations previously studied. Novel variants, unknown elsewhere, occur in each polymorphism. Fourteen different chromosome rearrangements are unique to the island, and eleven of these were detected in 67 plants from a small area which had undergone a population crash in 1977. It is argued that the genome of R. acetosa is undergoing rapid reorganisation on this small island which may be associated with an enforced shift towards inbreeding in this dioecious species.