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rustica, wbere three lineages (one from Atlantic and western Mediterranean, the others
from eastern Mediterranean) formed a contact zone along southern shores of Italian
peninsula (Sa Pinto et al. 201 0). Interestingly, the presence of clines and a contact zone
may fit the results observed at K = 3 and 4 in which clusters C and D resulted from the
re-allocation of individuals as well as populations belonging to clusters B and A rather
than to a further split of one group. In fact, bayesian clustering of data simulating a
secondary contact zone between two divergent gene pools may result in a further
cluster, grouping admixed individuals/populations of the secondary contact zone
(Francois et al. 20 l 0).
The different frequency with which individuals belonging to clusters A and B are
distributed in the western and eastern Mediterranean may explain the slight, albeit
significant genetic differentiation found between and within these basins and (Table 4).
The genetic homogeneity found between Atlantic and Mediterranean, but not at regional
and sub-regional spatial scales (within and between the Mediterranean basins) may
reflect complex patterns ofconnectivity at smaller geographical scale, where mesoscale
and coastal circulation as well as coastal geometry may affect dispersa) (Ribeiro et al.
2010). Nonetheless, the genetic differentiation among groups found within western
Mediterranean was much less than observed in P. ferruginea wbere at least one fifth of
total variation was due to differences among groups (Casu et al. accepted). Given the
scarce knowledge on the biology ofthese species we could only hypothesise the cause
for such differences. P. ulyssiponensis is found above all over the mid-littoral zone,
whereas P. ferruginea occupies the supralittoral and the high-midlittoral rocky shores,
wbere the two species' distribution overlaps. The partially different vertical distribution
of the species may affect the degree of genetic structure, as it may imply, for instance,
different selective regimes as shown in many species of coastal invertebrates (Kelly and

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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