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a )esser extent, in North Eastem Sardinia and Corsica, was evidenced by Bayesian
model based clustering (Fig. 2a, b) and PCA ordination plots (Fig. 3b, c, d). In North-
Western Sardinia there is no evident geographic barrier to gene flow which may explain
the highly fragmented genetic pattem. On the other hand, its rocky shores are easily
accessible (except those of Argentiera), and P. f erruginea is subject to regular, high
levels of collection for human consumption. These facts lead to hypothesise that past
and present human pressure is the main cause of genetic drift of these populations.
Indeed, several sites are presently characterised by both very low density and reduced
specimen size (Casu, pers. obs.). Although P. ferruginea is known to be able to adjust
the timing ofsex change according to the density oflarge individuals (Rivera-lngraham
et al. 2011 a), the lack of a population of large-sized individuals affects the reproductive
rate (Espinosa et al. 2006).

In Corsica and North-Eastem Sardinia, a less marked spatial genetic structure
than that found in North-Westem Sardinia has been observed (Figs 2a, b, 3c). These
fmdings suggest that gene flow is sufficient enough to prevent high level of genetic
differentiation in this area. However, the pattem of genetic distribution in North-Eastem
Sardinia is quite compiex, due to the overlap of two Bayesian subclusters that do not
match any discrete geographic areas. The high small-scale geographical complexity of
this area, which mostly encompasses an archipelago with complex hydrodynamic
regimes (Pracchi and Terrosu-Asole 1971; Gerigny et al. 2011), may shape the
distribution of genetic variability.

In contrast to the pattems observed for the SCR group, the pattem of ISSR
genetic variability found in the SAS group is rather homogeneous (Figs 2a, b, 3d). The
Mantel correlogram obtained for the SAS group (Fig. 4d) displayed a shape that
resembled IBD or an almost clinal variation. Although we cannot exclude the possibility

26

Gìan Luca Dedola: ANALISI DELLA VARIABILITÀ GENETICA DI PATELLA FéRRUGINEA, PATEL-LA ULl'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