Data from: Redder isn’t always better: cost of carotenoids in Chinook salmon eggs

Carotenoids provide animals with many fitness benefits through increased mating success, immune function, gamete quality and antioxidant capacity. Despite these benefits, carotenoids are not utilized equally by all animals, implying trade-offs associated with the pigments; although, few studies have quantified fitness costs of carotenoid pigmentation. Salmon are known for their conspicuous red coloration; however, amongst Chinook salmon (Oncorhynchus tshawytscha), a natural genetic color polymorphism exists (red and white morphs) which results in carotenoid-based color differences in eggs and other tissues. While the fitness benefit of egg carotenoid content on egg incubation survival has been demonstrated, carotenoid pigmentation also results in highly visible eggs vulnerable to predation. Therefore, although white Chinook salmon eggs experience costs in terms of viability, a potential benefit in terms of reduced predation could help explain the maintenance of the polymorphism. Here, using red and white eggs from wild Chinook salmon, we show that increased carotenoid content of salmon eggs leads to greater predation risk. We found that two populations of wild-type rainbow trout (an ecologically relevant predator) showed a significant bias for red eggs over white eggs under choice experiments, where red eggs were consumed first twice as often and significantly faster than white eggs. Our study suggests that trade-offs between red and white Chinook salmon during the egg stage provide an evolutionary mechanism promoting the maintenance of the unique Chinook salmon color polymorphism in nature, while also, for the first time, demonstrating a direct fitness cost of carotenoids in salmon.

类胡萝卜素（carotenoids）可通过提升交配成功率、免疫功能、配子质量与抗氧化能力，为动物带来诸多适合度收益。尽管存在上述益处，但并非所有动物都能同等利用类胡萝卜素，这暗示该色素存在权衡关系；不过目前鲜有研究量化类胡萝卜素着色带来的适合度成本。鲑鱼以其醒目的红色体色闻名；然而在奇努克鲑鱼（Oncorhynchus tshawytscha）中存在天然的体色遗传多态性（红色与白色两种形态），使得其卵粒与其他组织呈现基于类胡萝卜素的颜色差异。尽管已有研究证实卵粒类胡萝卜素含量对胚胎孵化存活率的适合度益处，但类胡萝卜素着色同时会使卵粒变得极易被识别，从而面临更高的捕食风险。因此，尽管白色奇努克鲑鱼卵粒的存活力存在劣势，但捕食压力降低所带来的潜在益处，或可解释该多态性在自然中的维持机制。本研究以野生奇努克鲑鱼的红色与白色卵粒为实验材料，证实鲑鱼卵粒的类胡萝卜素含量越高，其面临的捕食风险也越高。我们在选择实验中发现，两个种群的野生型虹鳟（具有生态相关性的捕食者）均表现出对红色卵粒的显著偏好：红色卵粒被优先捕食的频率是白色卵粒的两倍，且被摄食的速度也显著快于白色卵粒。本研究表明，奇努克鲑鱼红色与白色卵粒在胚胎阶段的权衡关系，为该独特体色多态性在自然中的维持提供了一种进化机制；同时本研究也首次证实了鲑鱼体内类胡萝卜素带来的直接适合度成本。