RESULTS
The four genetic distance/similarity matrices are presented in
Supplementary Material 2. They are the basis for constructing the
population dendrograms. Six of the ten different dendrograms exhibit the
same topology of two main branches, where each is divided into two
clusters. Cluster 1, henceforth the Eastern Atlantic cluster (EA),
consists of Biarritz, the Canaries and Madeira; cluster 2, the Western
Mediterranean cluster (WM), consists of Bizerte, Málaga and Melilla.
Clusters 1 and 2 both belong to the same main group. Cluster 3, the
Mid-Mediterranean cluster (MM), consists of Bastia, Birżebbuġa and
Pantelleria; Cluster 4, the Eastern Mediterranean cluster (EM), consists
of Bodrum, Dubrovnik, Fažana, Larnaca and Rethymno. Clusters 3 and 4
both belong to the same main group. Two representatives of this
configuration are given in Figs. 2 and 3. We checked the robustness of
the topology presented in this group of dendrograms by repeating the
Ward’s analysis of Fig. 2, 14 times – each time omitting a different
location (thus remaining with 13 locations). Except for the omitted
location, the remaining structure of the four above-mentioned clusters
did not change. Further demonstration of this division into four
clusters is displayed by the PCoA analysis in Fig. 4. The two axes
account for 86.5% of the variance.
Two of the ten different dendrograms exhibit a slightly different
topology of two main branches. One consists of the Eastern Atlantic
cluster only, whereas the other is divided into two sub-branches. One of
these sub-branches consists of the Eastern Mediterranean cluster and the
other is further divided into two clusters – the Western Mediterranean
and the Mid-Mediterranean clusters. Thus, the setup of the four
above-mentioned clusters also exists in these two dendrograms. A
representative of this configuration is given in Fig. 5.
The remaining two dendrograms place Pantelleria in the Western
Mediterranean cluster, next to the geographically nearby Bizerte. A
representative of this configuration is given in Fig. 6.
The mean number of different alleles per position, the mean expected
heterozygosity measures and the mean percentage of polymorphic positions
in each population are presented in Table 2.
The distribution of the number of alleles per position is not
statistically different from a normal distribution (Kolmogorov-Smirnov
test, p = 0.364). In accordance with the results of the
multi-variate analysis, as presented by the genetic dendrograms, we
divide the 14 populations into four groups: EA (the Eastern Atlantic
cluster), WM (the Western Mediterranean cluster), MM (the
Mid-Mediterranean cluster) and EM (the Eastern Mediterranean cluster).
The means (± se) of each group are: 1.361 ± 0.030, 1.782 ± 0.042, 1.857
± 0.039 and 1.942 ± 0.020, for EA, WM, MM and EM, respectively (see Fig.
7a). Using a one-way ANOVA for testing the differences in the mean
number of alleles per position between these four groups, we getF 3,10 = 68.849, p < 0.001.
Post-hoc: EA vs. WM p < 0.001; EA vs. MM p< 0.001; EA vs. EM p < 0.001; WM vs. MMp = 0.841; WM vs. EM p = 0.020; MM vs. EM p =
0.408.
The distribution of the expected heterozygosity is not statistically
different from a normal distribution (Kolmogorov-Smirnov test, p= 0.464). The means (± se) of each group are: 0.069 ± 0.008, 0.154 ±
0.007, 0.205 ± 0.005 and 0.230 ± 0.003, for EA, WM, MM and EM,
respectively (see Fig. 7b). Using a one-way ANOVA for testing the
differences in expected heterozygosity between these four sub-clusters,
we get F 3,10 = 169.899, p <
0.001. Post-hoc: EA vs. WM p < 0.001; EA vs. MMp < 0.001; EA vs. EM p < 0.001; WM vs.
MM p = 0.001; WM vs. EM p < 0.001; MM vs. EMp = 0.041
The distribution of the percentage of polymorphic positions is not
statistically different from a normal distribution (Kolmogorov-Smirnov
test, p = 0.295). The means (± se) of each group are: 33.9% ±
2.7%, 67.2% ± 3.0%, 72.6% ± 3.4% and 77.5% ± 1.62%, for EA, WM,
MM and EM, respectively (see Fig. 7c). Using a one-way ANOVA for testing
the differences in the percentage of polymorphic positions between these
four groups, we get F 3,10 = 57.722, p< 0.001. Post-hoc: EA vs. WM p < 0.001; EA
vs. MM p < 0.001; EA vs. EM p < 0.001;
WM vs. MM p >0.999; WM vs. EM p = 0.084; MM
vs. EM p >0.999.
Focusing more on the differentiation between the populations of the
Eastern and the Western Mediterranean basins, we compared the number of
exclusive alleles in each. Since the number of exclusive alleles is
influenced by the number of locations in a basin, for a balanced
comparison we considered only three of the five locations in the East
(i.e., Bodrum, Larnaca and Rethymno), and compared them to the three
locations in the West (i.e., Bizerte, Málaga and Melilla). In the East,
we counted a total of 15 exclusive alleles (that is, 10 alleles of EF1
and 5 alleles of NaKA that are present in the East, but not in the
West), compared to only 3 alleles (3 of EF1 and none of NaKA) that are
exclusive to the West. Using the exact binomial test, with equal number
of exclusive alleles in each basin as the null hypothesis, we obtained ap -value of 0.008.