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across the OAF as well as along the north-African shores (Espinosa and Ozawa 2006,
Casu et al. accepted). These results agree with the growing evidence that potential for
dispersa! alone may be a poor predictor of population differentiation in coastal
invertebrates, as marine currents, habitat preferences and historical processes play an
important role (Ayre and Hughes 2009, Kelly and Palumbi 2010). These processes are
not uncommon in intertidallimpets; for instance, in three hawayan species of the genus
Cellana with similar !ife history traits contrasting phylogeographic patterns were mainly
related to biogeographical range and habitat specificity (Bird et al. 2007). As well,
species with different dispersa! capabilities may show similar pattems of genetic
variation, albeit the causai factors may be different as shown in Syphonaria concinna
and S. nigerrima (Teske et al. 2011).
Similarly, the lack of divergence between Atlantic and Mediterranean populations of
many species has been related to a recent expansion and /or (re-)colonisation of one
area rather than differences in the dispersa! potential (Patamello et al. 2007). Though
ISSR markers do not allow inferences on population demographic history, the
decreasing average membership to cluster A for K = 2 in a eastward direction, from the
Atlantic to eastern Mediterranean may cope with this scenario (Fig. 2c). A similar
pattem but decreasing westwards, from eastem Mediterranean to the Atlantic, was
observed in cluster B. These patterns reflected in each region the relative proportion of
individuals assigned to cluster A or B, according to a membership coefficient :::=: 0.8
(Vaha et al. 2007). Most of individuals from the Atlantic were assigned to cluster A
(66%), whereas in the eastem Mediterranean 81% of individuals were assigned to
cluster B (Fig. 4). The similar numbers of individuals assigned to either cluster A or B
in the western Mediterranean (40 and 39 %), suggest this region may be actually a
contact zone (Fig. 4). Noteworthy, similar clinal patterns have been reported in P.

139

Gìan Luca Dedola: ANALISI DELLA VARIABILITÀ GENETICA DI PATELLA FéRRUGINEA, PATEL.LA UL l'SSIPONENSIS (MOLLUSCA: GASTROPODA) E PINNA
NOBILIS (MOLLUSCA: BIVALVIA): lLCONTRJBUTO DEl DATI MOLECOLARI ALLA CONSERVAZIONE DI SPECIE MINACCIATE · Tesi di douorato in Scienze
della natura e delle sue risorse • indiri7.zo BK>logi.a Ambientale, Università degli Studi di Sassarl