Prickly postglacial pioneers: adaptation of threespine stickleback to
freshwater influences fatty acid desaturase (FADS2) copy number in
Southern Greenland
Abstract
The adaptation of marine fish to freshwater environments is a prodigious
example of rapid evolution, and yet the underlying ecological causes of
genomic and phenotypic differentiation are poorly understood for traits
associated with lipid content and composition. Threespine stickleback
(Gasterosteus aculeatus) have repeatedly colonized, and adapted to,
freshwater habitats across the northern hemisphere. These freshwater
populations often show elevated copy number of the fatty acid desaturase
2 gene (FADS2), which increases the biosynthetic capacity of ω-3
long-chain polyunsaturated fatty acids (LC-PUFA). The starkly lower
content of LC-PUFA in freshwater compared to marine prey, especially
docosahexaenoic acid (DHA), likely imposes strong positive selection on
freshwater fish for either increased biosynthesis or greater dietary
acquisition of LC-PUFA. The threespine stickleback populations in
postglacial coastal lakes of Southern Greenland offer an exceptional
opportunity to study how variation in the copy number of FADS2 is
related to abiotic and biotic conditions of lakes and their morphometry.
As expected, we found strong sexual dimorphism in FADS2 copy number in
all populations (19 freshwater, 1 marine, and 1 brackish), and an
increased dimorphism in some freshwater populations. We also found that
FADS2 copy number was negatively correlated, for both males and females,
with the abundance of copepods, which are a DHA-rich food source in the
zooplankton community. Overall, our results suggest that the prey
community context of lakes might influence the process of metabolic
adaptation of marine fish colonizing freshwater ecosystems.