GENETIC-STRUCTURE OF THE SEAGRASS POSIDONIA-OCEANICA IN THE WESTERN MEDITERRANEAN - ECOLOGICAL IMPLICATIONS

The seagrass Posidonia oceanica (L.) Delile plays a dominant role in c oastal ecosystem dynamics in the Mediterranean Sea. This species repro duces both sexually through germination and asexually through vegetati ve propagation of subterranean rhizomes. Though flowering and fruiting are common, seedlings rarely succeed. An extensive population of P. o ceanica on the coast of the Island of Ischia (Gulf oi Naples, Italy) w as found to be nearly clonal when analysed with 2 DNA fingerprinting t echniques, minisatellites and random amplified polymorphic DNA (RAPD); the results show similarity values of 1 and >0.91, respectively. Furt hermore, a comparably high genetic similarity was observed between thi s population and individuals from another Ischia population and from t he coast of Spain. These findings support clonal propagation as the do minant reproductive mode for this species. The presence of Limited num bers of genotypes in the Western Mediterranean basin coupled with a ve ry slow rhizome elongation rate and the significant anthropogenic stre ss on these populations is cause for concern over the long-term ecolog ical stability of this keystone species.