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Genetica (2011) 139:1293–1308 1303

Fig. 5 ISSR dataset: spatial a
autocorrelation analyses of P. 0.3
ferruginea based on Mantel
correlograms. Dotted black lines 0.2
bound the 95% confidence
0.1
interval of the distribution of
permuted r-values. Error bars 0
about r indicate 95% confidence
limits of each r-value estimated -0.1
0 25 50 75 125 150 175 200 250 300 600 1000 1500 1700 1800 2000
by bootstrap. Mantel
correlograms include: a all b
samples; b cluster A; c cluster
0.2
B; and d cluster C, as retrieved
by STRUCTURE analyses.
0.1
Distance classes are plotted on
the x axis; genetic correlation is
0
plotted on the y axis
-0.1
-0.2
0 15 20 30 40 140 160 180
c
0.2

0.1
0

-0.1
-0.2
0 15 30 40 60 80 100 140 180 220 260 310
d
0.08

0.04

0
-0.04

-0.08
0 10 40 100 200 300 400 500 1300 1400 1500 1700 1810
All specimens from Argentiera (North-Western Sardi- hydrodynamics occurring in this area make almost
nia) grouped in cluster B, which encloses all North-Eastern impossible for humans reaching it either from the mainland
Sardinian and Corsican populations (Figs. 3a, 4a). The or sea. The lack of human influence may have contributed
genetic makeup of Argentiera sample may indicate that to the conservation of traits of an ancient genetic vari-
occasional long distance dispersal events, in a background ability. Different authors have demonstrated that the level
of reduced gene flow, may occur along the coasts of North of accessibility of the coast still plays a major role in
of Sardinia and South of Corsica. Although the circulation determining the occurrence of P. ferruginea (Paracuellos
pattern in this area is very variable due to the complex et al. 2003; Guerra-Garcı ´a et al. 2004).
coastal geometry, nonetheless the two prevailing currents A second, interesting result is the observed high degree
flow eastward and westward, respectively (Gerigny et al. of genetic structuring within SCR group. Clear genetic
2011). Our data do not rule out the possibility that sample structuring in North-Western Sardinia and, at a lesser
from Argentiera is representative of a relic population and extent, in North Eastern Sardinia and Corsica, was evi-
that cluster B had a wider geographical distribution in the denced by Bayesian model based clustering (Fig. 3a, b)
past. and PCA ordination plots (Fig. 4b–d). The rocky shores of
Notably, the Argentiera site is the only one among those North-Western Sardinia are easily accessible (except those
of SCR group characterised by hazardous cliff and the high of Argentiera), and P. ferruginea is subject to regular, high

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