Phenotypic and transcriptomic responses to salinity stress across genetically and geographically divergent Tigriopus californicus populations

Species inhabiting the North American west coast intertidal must tolerate an extremely variable environment, with large fluctuations in both temperature and salinity. Uncovering the mechanisms for this tolerance is key to understanding species’ persistence. We tested for differences in salinity tolerance between populations of Tigriopus californicus copepods from locations in northern (Bodega Reserve) and southern (San Diego) California known to differ in temperature, precipitation, and humidity. We also tested for differences between populations in their transcriptomic responses to salinity. Although these two populations have ~20% mtDNA sequence divergence and differ strongly in other phenotypic traits, we observed similarities in their phenotypic and transcriptomic responses to low and high salinity stress. Salinity significantly affected respiration rate (increased under low salinity and reduced under high salinity) but we found no significant effect of population on respiration or a population by salinity interaction. Under high salinity, there was no population difference in knockdown response but northern copepods had a smaller knockdown under low salinity stress, corroborating previous results for T. californicus. Northern and southern populations had a similar transcriptomic response to salinity based on a principle components analysis, although differential gene expression under high salinity stress was three times lower in the northern population compared to the southern population. Transcripts differentially regulated under salinity stress were enriched for ‘amino acid transport' and ‘ion transport’ annotation categories, supporting previous work demonstrating that the accumulation of free amino acids is important for osmotic regulation in T. californicus.

栖息于北美西海岸潮间带的物种必须耐受极端多变的环境，其温度与盐度均存在剧烈波动。阐明此类耐受机制，是理解物种存续能力的关键所在。我们针对来自加州北部（博德加保护区，Bodega Reserve）与南部（圣迭戈，San Diego）的加州虎猛水蚤（Tigriopus californicus）桡足类（copepod）种群间的盐度耐受差异开展了检测，上述两处生境的温度、降水与湿度均存在显著差异。我们同时检测了两类种群在盐度胁迫下的转录组响应差异。尽管这两个种群的线粒体DNA（mtDNA）序列差异度约为20%，且在其他表型性状上存在显著分化，但我们观察到二者在低盐与高盐胁迫下的表型及转录组响应具有相似性。盐度会显著影响呼吸速率：低盐条件下呼吸速率升高，高盐条件下则降低——但我们未发现种群对呼吸速率存在显著影响，也未观测到种群与盐度的交互作用效应。高盐胁迫下，两类种群的击倒反应无显著差异；但低盐胁迫下，北部种群的击倒反应程度更弱，这一结果验证了此前针对加州虎猛水蚤的相关研究结论。基于主成分分析（PCA，principal components analysis）的结果显示，南北两类种群对盐度的转录组响应相似，不过高盐胁迫下北部种群的差异基因表达量较南部种群低三倍。盐度胁迫下差异调控的转录本，其功能注释显著富集于“氨基酸转运”与“离子转运”类别，这一结果支持了此前的研究结论——即游离氨基酸的积累对于加州虎猛水蚤的渗透压调节至关重要。