Data from: Immune-challenged fish up-regulate their metabolic scope to support locomotion

Energy-based trade-offs occur when investment in one fitness-related trait diverts energy away from other traits. The extent to which such trade-offs are shaped by limits on the rate of conversion of energy ingested in food (e.g. carbohydrates) into chemical energy (ATP) by oxidative metabolism rather than by the amount of food ingested in the first place is, however, unclear. Here we tested whether the ATP required for mounting an immune response will lead to a trade-off with ATP available for physical activity in mosquitofish (Gambusia holbrooki). To this end, we challenged fish either with lipopolysaccharide (LPS) from E. coli or with Sheep Red Blood Cells (SRBC), and measured oxygen consumption at rest and during swimming at maximum speed 24h, 48h and 7 days post-challenge in order to estimate metabolic rates. Relative to saline-injected controls, only LPS-injected fish showed a significantly greater resting metabolic rate two days post-challenge and significantly higher maximal metabolic rates two and seven days post-challenge. This resulted in a significantly greater metabolic scope two days post-challenge, with LPS-fish transiently overcompensating by increasing maximal ATP production more than would be required for swimming in the absence of an immune challenge. LPS-challenged fish therefore increased their production of ATP to compensate physiologically for the energetic requirements of immune functioning. This response would avoid ATP shortages and allow fish to engage in an aerobically-challenging activity (swimming) even when simultaneously mounting an immune response. Nevertheless, relative to controls, both LPS- and SRBC-fish displayed reduced body mass gain one week post-injection, and LPS-fish actually lost mass. The concomitant increase in metabolic scope and reduced body mass gain of LPS-challenged fish indicates that immune-associated trade-offs are not likely to be shaped by limited oxidative metabolic capacities, but may instead result from limitations in the acquisition, assimilation or efficient use of resources.

当能量被优先分配至某一与适合度相关的性状时，必然会挤占其他性状的能量投入，由此产生基于能量的权衡。但目前学界仍未明确：这类权衡的形成，究竟是受限于食物摄入能量（如碳水化合物）经氧化代谢转化为化学能（三磷酸腺苷（ATP））的速率，还是受制于初始食物摄入量本身。本研究以霍氏食蚊鱼（Gambusia holbrooki）为实验对象，探究引发免疫应答所需的ATP是否会与用于躯体运动的ATP产生权衡。为此，我们分别用大肠杆菌来源的脂多糖（lipopolysaccharide, LPS）以及绵羊红细胞（Sheep Red Blood Cells, SRBC）对食蚊鱼进行免疫刺激，并于免疫刺激后24 h、48 h及7 d时，分别测量其静息状态与最大游速下的耗氧量，以此估算代谢速率。与生理盐水注射对照组相比，仅经LPS处理的食蚊鱼在免疫刺激后2 d的静息代谢率显著升高，且于免疫后2 d及7 d的最大代谢率同样显著提升。这导致LPS处理组食蚊鱼在免疫后2 d的代谢范围显著扩大：它们通过提升最大ATP生成量实现了暂时性过度补偿，其增幅超过了无免疫刺激时游泳所需的ATP需求量。由此可见，经LPS刺激的食蚊鱼会通过提升ATP生成量，从生理层面补偿免疫应答的能量需求。该响应可避免ATP短缺，使得食蚊鱼即便在同时启动免疫应答的情况下，仍能开展需氧挑战性活动（如游泳）。尽管存在上述代偿机制，与对照组相比，经LPS及SRBC处理的食蚊鱼在注射后7 d的体质量增益均出现下降，其中LPS处理组甚至出现了体质量流失。经LPS刺激的食蚊鱼代谢范围升高但体质量增益下降，这一结果表明，免疫相关的权衡不太可能受限于氧化代谢能力，反而可能源于资源获取、同化或高效利用环节的限制。