Contrasting levels of genetic diversity have also been observed within the Mediterranean basin for the congeneric species P. oceanica. Studies have been conducted on 33 meadows sampled along the entire distribution range of the species using microsatellite analysis. Overall genetic diversity seems to be low [average Dg = 0.727; (Procaccini et al., 2001; Procaccini et al., 2002)], with some identical genotypes present in different populations. Based on the results from published studies, P. oceanica appears as a genetically depauperate species as result of evolutionary processes or recent anthropogenic influence.
Populations from the western basin were genetically more polymorphic than those from the east ern Mediterranean Sea, where North Adriatic meadows were represented by only one clone (Ruggiero et al., 2002). Meadows are distinct genetically, with overall d = 0.451 (Nm d = 0.304). A clear genetic sub-division seems to exist within the basin, with the presence of three main groups representing north-western, south-western, and eastern populations (Fig. 6). The existence of defined population groups may result from processes acting on different time scales. Present surface circulation patterns of surface currents may affect gene flow and/or colonization of different areas and could account for the existence of genetic structure over the Mediterranean Sea as a whole. The genetic structure existing in the Tyrrhenian Sea, with the presence of a clear distinction between northern and southern populations (Ruggiero et al., 2002) (Fig. 6), reflects the
presence of seasonal circulation gyres in this area (Astraldi and Gasparini, 1994). The Mediterranean Sea has experienced dramatic changes of environmental parameters related to glaciations and hyper-salinity crises (Maldonado, 1985; Thiede, 1978). The evolution of the basin was controlled by the interplay between geological processes and environmental parameters determining species loss and recolonisation in particular areas. Dramatic fluctuations of more than 100 m of sea water level during last glaciation surely have caused the disappearance ofPosidonia from some areas. Thus, the actual distribution could have originated by recolonisation from populations persisting in relict zones and the low genetic variability detected in some areas could have originated from a founder effect. The latter possibility seems to be the case for the uniclonal patches of the North-Adriatic, where locally adapted genotypes are now present in the area (Ruggiero et al., 2002).
New polymorphic di-nucleotide microsatellite loci recently selected (Alberto et al., 2003a) are showing higher polymorphism levels. Preliminary analysis at basin level is confirming at broad scale the separation between eastern and western Mediterranean populations. Intra-population analyses instead are showing higher heterozygosity and clonal diversity values (Alberto et al., 2003a; Arnaud-Haond personal communication).
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