Figure 2. Principal component analysis of 276 Clethrionomys
glareolus individuals sampled from 12 populations across Europe using
2,476 SNPs based on genotype likelihood. With percentage of variation
explained for each component displayed on the axes, together the four
components explain 20.1% of variation. Each circle represents an
individual, colours correspond to Regions and sites Abbreviations - C:
Central, E: Eastern, N: North, S: South, W: Western, .fi: Finland, .se:
Sweden, .pl: Poland, .cz: Czechia, .de: Germany, it: Italy, .fr: France,
.ro: Romania
PC1 explained 9% of the variation and sorted the populations by
geography, correlating to both longitude (R2= 0.83)
and latitude (R2=0.49). PC2 (5%) separated West and
East within Regions; in the Northern populations the population west of
the baltic sea (N.se) from the the three Northern populations east of
the Baltic sea (NE1-3.fi), and within Central Europe the three
geographically close populations (C1.d3, C2-3.cz) from the two Eastern
European populations (CE.pl and SE.ro). The third and fourth components
together explained 5.5% of genetic variation, and separated two
populations from the other ten: the northern (N.se) population beyond
the Baltic Sea, and the southern population beyond the Alps (S.it).
In the admixture analysis (Figure 1), the lowest variance of likelihood
was found for 2 or 3 ancestral populations (Figure S2). AssumingK =2, the populations in the southern range of the distribution
are separated from the other populations. Increasing to K =3
additionally separated the three populations east of the Baltic
(NE1-3.fi), increasing to K=4 separated S.it beyond the Alps, and to
K=5 the Swedish population beyond the baltic (N.se). AssumingK =10 (having the next lowest variance of likelihood), clusters
mirrored sampling locations, except for the two pairs of populations
with the least geographic distance, the southern Finnish populations
forming a single cluster, and the Central populations (C2 and C3.cz)
with only some degree of admixture suggested.
A similar pattern emerges from the pairwise population
FST estimates (PPF), which revealed moderate to high
levels of differentiation between populations (Table S4). PPF ranged
between 0.035 (C1.cz vs C2.cz) and 0.555 (N.se and SW1.fr) with an
average fixation index of 0.269 (SD = 0.12). PPF corresponded well with
geographic proximity of populations (Mantel tests genetic and geographic
distance correlation: r = 0.47, p = 0.002), suggesting a strong spatial
pattern of isolation by distance. In accordance with previous results,
the population from Sweden (east of Baltic sea) was more similar to the
central European populations than to the North-Eastern populations.