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the overall leve) of genetic differentiation among populations; then we applied a three
level AMOVA analysis to assess differences among groups of populations. Severa}
grouping schemes that may reflect geographical features such as biogeographical
boundaries, different basins, marine circulation patterns, and/or the presence of islands
were tested (Table 3). The significance ofthe fixation indices associated with the
different groupings was assessed by a non parametric permutation test with 10'000
replicates (Excoffier et al. 1992).

Results

We scored a total of 66 bands over the five primers that give the more
reproducible and reliable banding patterns (Table 2). The percentage of polymorphic
loci averaged over ali samples was 50.6% (± 2.60%), which corresponded to an average
of35.47 bands per population. Ofthese bands only 7.15, on average, were Iocally
common in 50% or less of populations, and 2.25 in 25%. Remarkably, only one private
band was found over 40 locations (Isole delle Formiche, IFO).
Bayesian clustering results evidenced an increasing lnP(D) value from K = l to 12, after
which the curve reached a plateau (Fig. 2a). The presence of a more or less plateau is
due to the way in which the lnP(D) is estimated by approx imation in an ad hoc way
(Pritchard et al. 2000), and occurs for values ofK that are equal or superior to the actual
number ofpopulations in the sample (Garnjer et al. 2004). Furthermore, the increase of
the lnP(D) began to flatten for K > 4, after which the gain of information exhibited
gradually decreasing values (Fig. 2a). This result means that the information brought by
the fifth and the following clusters is much less impmtant than the infmmation brought
by the former ones. Accordingly, the most consistent results across replicate runs were

135

Gian Lucn Dcdola: ANALISI DELLA VARIABILITA GENETICA DI PATI;"LLA FERRU(i!NéA. PATELLA ULYSSIPONéNSIS (MOLLUSCA: GASTROPODA) E PINNA
NOBIUS (MOLLUSCA: BfVALVIA): IL CONTRJBUTO DEl DATI MOLECOLARI ALLA CONSERVAZIONE DI SPECIE MINACCIATE- Tesidi douorato in Scienze
della natura e delle sue risorse • indirizzo BK>Iogìa Ambiemale, Università degli Studi di Sassari