The colonisation of freshwater environments by marine fishes has historically been considered a result of adaptation to low osmolality. However, most marine fishes cannot synthesise the physiologically indispensable fatty acid, docosahexaenoic acid (DHA), due to incomplete DHA biosynthetic pathways, which must be adapted to survive in freshwater environments where DHA is poor relative to marine environments. By analysing DHA biosynthetic pathways of one marine and three freshwater-dependent species from the flatfish family Achiridae, we revealed that functions of fatty acid metabolising enzymes have uniquely and independently evolved by multi-functionalisation or neofunctionalisation in each freshwater species, such that every functional combination of the enzymes has converged to generate complete and functional DHA biosynthetic pathways. Our results demonstrate the elaborate patchwork of fatty acid metabolism and the importance of acquiring DHA biosynthetic function in order for fish to cross the nutritional barrier at the mouth of rivers and colonise freshwater environments.

海洋鱼类在淡水环境中的殖民化历来被认为是适应低渗透压的结果。然而，由于 DHA 生物合成途径不完整，大多数海洋鱼类无法合成生理上必不可少的脂肪酸，二十二碳六烯酸 (DHA)，必须适应 DHA 相对于海洋环境较差的淡水环境生存。通过分析比目鱼科比目鱼科的一种海洋物种和三种依赖淡水的物种的 DHA 生物合成途径，我们发现脂肪酸代谢酶的功能通过每个淡水物种的多功能化或新功能化独特而独立地进化，使得每种功能组合的酶已经聚合以产生完整和功能性的 DHA 生物合成途径。