Continental accumulation of fads2 copy numbers allows sticklebacks to thrive across a diversity of nutritional landscapes

Nutrients, including vital organic compounds, vary in availability across ecosystems, with the potential to act as a source of selection for traits that increase nutrient acquisition and biosynthesis. Compared to freshwaters, marine ecosystems are richer in the omega‐3 long‐chain polyunsaturated fatty acid (n‐3 LC‐PUFA) docosahexaenoic acid (DHA). Dietary and metabolic changes during the freshwater establishment process have likely helped ancestrally‐marine fish survive the challenges of freshwater nutritional environments. We explored both genotypic and phenotypic variation among threespine stickleback Gasterosteus aculeatus populations that vary in their history of freshwater establishment and in the nutritional availability of contemporary resources. Specifically, we examined how fads2 copy number, a gene associated with fatty acid synthesis, as well as the fatty acid content of fish and their prey, varies among populations. We found evidence that multiple freshwater stickleback populations have fads2 copy number duplications. Stickleback prey varied in their fatty acid composition across major taxonomic groups and in their relative availability across sites. Diet variation among stickleback populations reflected the relative availability of local prey. However, stickleback DHA phenotypes were remarkably similar across populations, despite wide variation in both fads2 copy numbers and diet. While contents of eicosapentaenoic acid (EPA), another n‐3 LC‐PUFA, were also conserved in muscle tissue of sticklebacks across populations, the contents of other fatty acids varied widely across populations, suggesting that other fatty acids are under less strict metabolic control and reflect more local variation across the landscape. Overall, our results suggest that in spite of genetic variation in synthesis capacity, sticklebacks are able to acquire n‐3 LC‐PUFA across freshwaters. They do appear to do so while consuming a diversity of nutritionally‐variable prey. Future studies should aim to help reveal the rates at which traits related to foraging versus those related to metabolism evolve in response to the nutritional landscape.