Population genetic analyses
Population genetic
divergence
The microsatellite marker global FST value was
0.031 across all populations. The pairwise FSTvalues across all fifteen comparisons ranged from -0.0001 to 0.100
(Table 3). The Fisher’s exact test for microsatellite allele frequency
distributions showed a highly significant population effect (p ≤
0.001) in all but one population pairwise comparison
(Utqiagvik/Svalbard; uncorrected p = 0.011), although that
comparison was significant prior to Bonferroni correction (Table 3). The
microsatellite pairwise FST values also showed
highly significant population differentiation in 13/15 population pairs
(FST : 0.009-0.100, p ≤ 0.012) comparisons
(Table 3). The population pairs Alert/Mitivik Island and
Utqiagvik/Svalbard (FST : -0.0001 for both pairs,
uncorrected p -values of 0.564 and 0.464, respectively) did not
show significant population differentiation before or after the
sequential Bonferroni correction (Table 3). Combined results from the
Fisher’s exact test and FST estimation at neutral
markers provide evidence of significant divergence between all
population pairs, with the exception of Utqiagvik/Svalbard and
Alert/Mitivik Island population pairs which exhibited weak isolation.
The SNP marker global FST value was 0.022 across
all populations. The SNP marker pairwise FSTvalues across all fifteen comparisons ranged from 0.004 to 0.053 (Table
4). The Fisher’s exact test for functional SNP marker allele frequency
distribution showed highly significant population differentiation in
9/15 population pairs (p ≤ 0.009), with non-significant
differentiation for: Aleutian Island/Pribilof Island, Alert/Utqiagvik,
Alert/Mitivik Island, Alert/Svalbard, Utqiagvik/Mitivik Island, and
Mitivik Island/Svalbard (uncorrected p -values: 0.56-0.88; Table
4). The SNP marker pairwise FST estimates matched
the Fisher’s exact test results as the same population pairs (listed
above) showed significant divergence (FST :
0.024-0.053, p ≤ 0.003; Table 4) and the remaining pairs showed
non-significant divergence (FST : 0.004-0.013,p ≥ 0.039; Table 4). Broadly, the combined SNP Fisher’s exact
test and FST estimation did not show significant
genetic differentiation between the non-migratory populations (Aleutian
and Pribilof islands), and among a majority (exception:
Utqiagvik/Svalbard comparison) of the migratory populations (Alert,
Utqiagvik, Mitivik Island, and Svalbard). However, all migratory -
non-migratory population comparisons did show significant
differentiation with the SNP marker data. The Utqiagvik/Svalbard and
Alert/Mitivik Island population pairs had significant levels of SNP
marker differentiation, although they were not significantly divergent
based on neutral (microsatellite) marker data. Overall, our analyses
show substantial genetic divergence among our six sampled populations.
Neighbour-joining
cluster diagrams
The neighbour-joining (NJ) cluster diagrams (Figure 2) based on
microsatellite and SNP genotypes show similar overall patterns of
divergence. The microsatellite marker data show three strongly supported
clusters (100% branch support): Utqiagvik & Svalbard, Alert & Mitivik
Island, and Aleutian & Pribilof islands; although Aleutian and Pribilof
islands also show strongly supported divergence (100% branch support)
from each other (Figure 2a). On the other hand, the SNP genotype data do
not show as strong support for population clustering among the six
populations. However, the Aleutian & Pribilof islands, and Utqiagvik &
Alert population pairs show strong patterns of divergence (99% and
100% branch support) between these populations and the others based on
SNP marker data (Figure 2b).
Selection signatures
at SNP loci