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Mediterranean (Zenetos 1997; Morri et al. 1999; Logan et al. 2002), such evidence
could be considered as not conclusive, thus requiring further investigations.

In the light of the high evolutionary rate found in some species of the genus
Crassostrea (Huvet et al. 2000; Launey and Hedgecock 200 l), which is considered as
one the most molecular affine to the genus Pinna (Matsumoto 2003), the genetic
variability of P. nobilis showed in the present research is compatible with a recent
demographic expansion of populations. This inference is suggested by the occurrence of
very low number of point mutations that differentiate haplotypes each other. In such
context, the population expansion dynamics evidenced by star-like network
configuration, mismatch analysis and neutrality tests for the whole area analysed could
be considered as consequence of the environmental stress which affected the species
during last centuries. In fact marine species, after early lowering of diversity and
regression of habitat (Pearson and Rosemberg, 1978; Gray, 1989), overcame
environmental changing with new population demographic expansion generally affected
by evolutionary forces such as founder effect and genetic drift.

Some of these forces seem to bave acted on the current genetic structuring of P.
nobilis in the Mediterranean, which is conceivable as product of repeated bottle-necks
occurred on an ancient panmittic Mediterranean population. Although, this hypothesis is
not corroborated by the low dispersa] capability of the veliger larva] stadium of P.
nobilis evidenced by previous works (Gosling, 2003; Peharda and Vilibié, 2008), the
complex pattem of currents in the Mediterranean may bave also favoured dispersa] and
panmixia in the Mediterranean (Bianchi 2007).

Nowadays, the present genetic pattem of P. nobilis as inferred by analysis of our
data is characterised by a structuring according to the Mediterranean bio-geograpbic

64

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