(c) Nuclear diversity
In contrast to the mitochondrial results, there was no evidence for strong latitudinal or longitudinal diversity gradients in the nuclear dataset. Nuclear genetic diversity declined only weakly towards the poles and did not follow strong longitudinal patterns (Fig. 2C & 3C, Fig. S4). The null model performed better than any models that incorporated latitude or longitude predictors, and neither latitude nor longitude was a significant term in any of the models (Table 3). However, diversity was consistently lower for loci amplified with primers originally developed in another species (Fig. S10) and consistently but weakly declined towards the range edge (Table 3, Fig. S7).
Nuclear diversity was also significantly associated with chlorophyll; all chlorophyll models performed better than the null, and the model including mean chlorophyll performed best overall (Table 2, Table S3). Similarly to the mitochondrial patterns, nuclear genetic diversity peaked at upper-range chlorophyll values (5-10 mg/m3) (Fig. 4F, Fig. S9). Among the SST models, only the one including SST range performed marginally better than the null model, and no SST predictors were significantly related with nuclear genetic diversity (Table 2, Table S2).