Fitness of aquatic animals can be limited by the scarcity of nutrients such as long-chain polyunsaturated fatty acids, especially docosahexaenoic acid (DHA). DHA availability from diet varies among aquatic habitats, imposing different selective pressures on resident animals to optimize DHA acquisition and synthesis. For example, DHA is generally poor in freshwater ecosystems compared to marine ecosystems. Our previous work revealed that, relative to marine fishes, several freshwater fishes evolved higher copy numbers of the fatty acid desaturase2 (Fads2) gene, which encodes essential enzymes for DHA biosynthesis, likely compensating for the limited availability of DHA in freshwater. Here, we demonstrate that Fads2 copy number also varies between freshwater sticklebacks inhabiting lakes and streams with stream fish having higher Fads2 copy number. Additionally, populations with benthic-like morphology possessed higher Fads2 copy number than those with planktivore-like morphology. This may be because benthic-like fish mainly feed on DHA-deficient prey such as macroinvertebrates whereas planktivore-like fish forage more regularly on DHA-rich prey, like copepods. Our results suggest that Fads2 copy number variation arises from ecological divergence not only between organisms exploiting marine and freshwater habitats but also between freshwater organisms exploiting divergent resources.

长链多不饱和脂肪酸，尤其是二十二碳六烯酸 (DHA) 等营养物质的缺乏可能会限制水生动物的健康。饮食中的 DHA 可用性因水生栖息地而异，对常驻动物施加不同的选择压力，以优化 DHA 的获取和合成。例如，与海洋生态系统相比，淡水生态系统中的 DHA 通常较差。我们之前的研究表明，相对于海洋鱼类，一些淡水鱼类进化出了更高拷贝数的脂肪酸去饱和酶2 ( Fads2 ) 基因，该基因编码了 DHA 生物合成的必需酶，这可能弥补了淡水中 DHA 有限的可用性。在这里，我们证明了Fads2栖息在湖泊和溪流中的淡水棘鱼的拷贝数也有所不同，其中流鱼具有更高的Fads2拷贝数。此外，具有底栖动物形态的种群比具有浮游动物形态的种群具有更高的Fads2拷贝数。这可能是因为底栖鱼类主要以缺乏 DHA 的猎物（如大型无脊椎动物）为食，而类似浮游动物的鱼更经常以富含 DHA 的猎物（如桡足类）为食。我们的研究结果表明，Fads2拷贝数变异不仅源于利用海洋​​和淡水栖息地的生物之间的生态分歧，还源于利用不同资源的淡水生物之间的生态分歧。