Meetings of the Royal Botanical Society of The Netherlands

Twenty-four individuals were investigated over a geographic range of from < 1 m to 250 km using M-13 minisatellite DNA fingerprinting and RAPDs. Genetic relatedness within clusters (=coalesced holdfasts) was extremely high. Three of the tested clusters had at least two identical individuals. Similarity values among five of the six clusters (<1 m apart) examined were >90%. At distances of 10 m and 25 m, similarities dropped to 60% and 56% respectively. Among the three sites, similarity values dropped to about 50%. In contrast, RAPD data were unable to resolve any relationships within a site but were able to discriminate among the three sites corresponding to geographic distance. Results suggest that gene flow indeed occurs over distances of about 1 m. Clusters are most probably all siblings. Patches appear to be genetically discrete. As large patches of Postelsia are more likely to persist from year to year, patch size almost certainly influences the degree of intra- and inter-cluster relatedness.