Guppy (Poecilia reticulata) can adapt to a wide range of salinity changes. To investigate the gene expression changes in the guppy exposed to seawater, we characterized its gill transcriptome using RNA sequencing. Experimental fish were exposed to salinity increase from 0 to 30‰ within 4 days, while control fish were cultured in freshwater (0‰ salinity). Seven days after salinity exposure, the gills were sampled and the mortality within 2 weeks was recorded. No significant difference in the cumulative mortality at the second week was found between the two groups. Transcriptomic analysis identified 3477 differentially expressed genes (DEGs), including 1067 upregulated and 2410 downregulated genes. These DEGs were enriched in several biological processes, including ion transport, ion homeostasis, ATP biosynthetic process, metabolic process, and immune system process. Oxidative phosphorylation was the most activated pathway. DEGs involved in the pathway “endoplasmic reticulum (ER)-mediated phagocytosis,” “starch and sucrose metabolism,” and “steroid biosynthesis” were mainly downregulated; chemokines and interleukins involved in “cytokine-cytokine receptor interaction” were differentially expressed. The present results suggested that oxidative phosphorylation had essential roles in osmoregulation in the gills of seawater acclimated guppy, during which the decline in the expression of genes encoding V-ATPases and calreticulin had a negative effect on the phagocytosis and immune response. Besides, several metabolic processes including “starch and sucrose metabolism” and “steroid biosynthesis” were affected. This study elucidates transcriptomic changes in osmotic regulation, metabolism, and immunity in seawater acclimated guppy.

孔雀鱼（Poecilia reticulata) 能适应大范围的盐度变化。为了研究暴露于海水的孔雀鱼的基因表达变化，我们使用 RNA 测序对其鳃转录组进行了表征。实验鱼在 4 天内暴露于盐度从 0 增加到 30‰，而对照鱼在淡水（0‰ 盐度）中养殖。盐分暴露后 7 天，对鳃取样并记录 2 周内的死亡率。两组之间在第二周的累积死亡率没有显着差异。转录组学分析确定了 3477 个差异表达基因 (DEG)，包括 1067 个上调基因和 2410 个下调基因。这些DEGs在多种生物过程中富集，包括离子转运、离子稳态、ATP生物合成过程、代谢过程和免疫系统过程。氧化磷酸化是最活跃的途径。参与“内质网（ER）介导的吞噬作用”、“淀粉和蔗糖代谢”和“类固醇生物合成”途径的DEGs主要被下调；参与“细胞因子-细胞因子受体相互作用”的趋化因子和白介素差异表达。目前的结果表明，氧化磷酸化在海水驯化孔雀鱼鳃的渗透调节中具有重要作用，在此期间，编码 V-ATPases 和钙网蛋白的基因表达的下降对吞噬作用和免疫反应产生负面影响。此外，包括“淀粉和蔗糖代谢”和“类固醇生物合成”在内的几个代谢过程受到影响。这项研究阐明了渗透调节、代谢、