Spatial patterns of genetic diversity in Posidonia oceanica, an endemic Mediterranean seagrass

Posidonia oceanica is an endemic seagrass species in the Mediterranean Sea. In order to assess levels of genetic structure in this species, the micros atellite polymorphism was analysed from meadows collected in several locali ties, along the coasts of the Tyrrhenian Sea (Mediterranean Sea). The exist ence of single population units and the recruitment of seedlings collected in some localities were investigated. Moreover, genetic structure at differ ent spatial scales and biogeographic relationships among populations were a lso assessed. Our analysis showed the existence of clear patterns of geneti c structure in P. oceanica in the area considered in the analysis. P. ocean ica, in fact, is present in separate meadows that represent discrete popula tions, characterized by low genetic diversity. Comparable levels of genetic variability between mature meadows and seedlings were found. Patterns of g enetic relatedness among populations seem to be in accord with direction of dominant current flux in the whole area, separating South Tyrrhenian from North Tyrrhenian populations. Moderate levels of gene flow between populati ons and genetic substructure within populations, together with the finding of the limited role of sexual reproduction in increasing genetic variabilit y, should be a cause for concern for the persistence of this essential reso urce in the Mediterranean basin.