Pairwise selection at SNP loci
The overall pairwiseG’ST values ranged from 0.007 to 0.4508 for the
microsatellite markers, and from 0.0076 to 0.0655 for the SNP locus
markers among the fifteen comparisons, depending on the population
compared (Supplementary Material S8).
Controlling for the neutral processes (99% CI using microsatellite
genotype data) for all fifteen pairwise population comparisons of the
101 SNP loci (1286 possible G’ST values), we
mostly observed signatures of stabilizing selection (51%) and genetic
drift (38%), followed by undetermined (7%) and divergent selection
(4%). We observed roughly equivalent patterns of divergence among the
gene function categories (Supplementary Material S9). The distribution
of the types of selection did not differ significantly among different
functional categories (χ2 = 20.33, p =
0.32).
We detected signatures of stabilizing selection across all SNP loci in
all but six population comparisons (Figure 4), where the neutralG’ST ranges included zero (Alert/Utqiagvik,
Alert/Mitivik Island, Alert/Svalbard, Utqiagvik/Mitivik Island,
Utqiagvik/Svalbard, and Mitivik Island/Svalbard). For these six
comparisons, the SNP loci that had G’ST values
less than the neutral expected range (i.e., negativeG’ST values) were identified as “undetermined”.
Therefore, we are likely underestimating overall stabilizing selection
effects. Overall, we observed that 67.5% - 96.3% of the SNP loci are
under stabilizing selection among the nine pairwise population
comparisons where we were able to test for signatures of stabilizing
selection (Figure 4, Supplementary Material S8).
We did not observe any signatures of divergent selection in six
(Aleutian Islands/Alert, Aleutian Islands/Utqiagvik, Aleutian
Islands/Mitivik Island, Aleutian Islands/Pribilof Islands, Aleutian
Islands/Svalbard, and Alert/Utqiagvik) out of fifteen pairwise
population comparisons (Figure 4). For the remaining nine population
pairs, we observed that 1.09%-23.1% of the SNP loci exhibited
divergent selection (Figure 4, Supplementary Material S8). The
Utqiagvik/Svalbard population comparison showed the most divergence
(23.1%) based on our selected functional locus SNPs, followed by
Pribilof Islands/Svalbard and Alert/Mitivik Island population
comparisons which exhibited directional divergence at 7.23%, and 6.60%
of tested SNP loci, respectively (Supplementary Material S8).
To compare selection signatures across all pairwise population
comparisons, SNP marker data would have to be 100% available (i.e., not
classified as “undetermined” in any comparison). Thus, only a minority
(11/101) of SNP marker loci could be broadly compared across all
pairwise comparisons (Figure 5; Table 5). Based on those 11 SNP marker
loci, high levels of stabilizing selection signatures were generally
observed when the non-migratory populations (Aleutian & Pribilof
islands) were compared to each other or with migratory populations
(Alert, Utqiagvik, Svalbard, and Mitivik Island), whereas comparisons
among the migratory populations showed mixed signatures of neutral
processes and divergent selection depending on the SNP locus (Table 5).
Of these 11 SNP loci, 7 showed evidence of divergent selection in at
least one population comparison, while the Utqiagvik/Svalbard population
pair comparison showed 6 of these 11 loci under divergent selection
(Table 5). Broadly, the divergent genes from pairwise comparisons in
this subset were associated with cellular housekeeping, lipid
metabolism, nervous system development, reproduction, and stress (Table
5, Supplementary Material S10).