Nile tilapia (Oreochromis niloticus) is one of the most important food fishes in global aquaculture. The optimal rearing temperature for Nile tilapia is 27–30 °C; however, in some Asian breeding areas, such as south China, water temperatures in summer frequently exceed 35 °C for several days. Potential effects of long-term exposure to high temperatures on the survival and metabolism of tilapia are unclear. In this study, genetically improved farmed tilapia, age six weeks, were exposed to water temperatures of 28, 32, and 36 °C for 15 weeks. Mean survival rates and tolerance to hypoxia were significantly reduced, and respiratory rates were increased in fish reared at 36 °C, compared to the 28 and 32 °C treatments (p < 0.05). Partial temperature compensation for routine metabolic rates was observed after 15 weeks at 36 °C. Gill morphology changes in the 36 °C treatment included curling of the lamellae and hyperplasia of the filament end, which became more pronounced after acute hypoxia (0.2 mg/L O2). Transcriptomics demonstrated that expression of numerous genes related to aerobic metabolism was altered in the 36 °C treatment, including down-regulation of nine genes of the tricarboxylic acid cycle. In summary, high temperature affected tilapia gill morphology, reduced hypoxia tolerance, and inhibited aerobic metabolism, thus ultimately threatening tilapia viability and survival.

Domestication can be defined as the artificial selection in animals to achieve morphological, physiological, and developmental conformity to human needs, with the aim of improving various limitations in species under a human feeding environment. The future sustainability of aquaculture may rely partly on the availability of numerous domesticated fish species. However, the underlying adaptive mechanisms that result in the domestication of fish are still unclear. Because they are poikilothermic, temperature is a key environmental element that affects the entire life of fish, so studying the association between physiological and behavioral changes in low-temperature domesticated fish can provide a model for understanding the response mechanisms of fish under cold stress. Through 5 generations and 10 years of artificial selection at low temperatures, we used cold-tolerant fugu as a biological model to compare transcriptome changes in brain and liver tissues to study the effects of cold stress on fish. It was found that the expression of genes such as apoptosis, p53, oxidative phosphorylation, and mitochondrial β-oxidation in the brain of cold-tolerant fugu was significantly lower than the wild type due to cold stress, while excessive energy metabolism would lead to the production of reactive oxygen species (ROS) and exacerbate the brain damage, thus causing rollover and coma. Meanwhile, under cold stress, the signaling pathways involved in glycogenolysis and lipid metabolism, such as insulin signaling, adipocytokines, and mTOR signaling pathways, were significantly up-regulated in the liver of cold-tolerant fugu. Although the mitochondrial β-oxidation pathway was increased in cold-tolerant fugu liver tissues, the transcriptome was not enriched in apoptotic. These phenomena predict that in response to low-temperature conditions, cold-tolerant fugu employs a dynamic inter-organ metabolic regulation strategy to cope with cold stress and reduce damage to brain tissues.

Grass carp (Ctenopharyngodon idella) is an important economic and ecological freshwater fish. Grass carp is a highly adaptive and has been introduced around the world, making it a good model to explore genomic divergence in diverse ecosystems. Habitat colonization often requires extensive adaptation to cope with environmental challenges from temperature and light variations; however, the associated genomic mechanisms underlying evolution are unclear. To evaluate the genetic diversity and selective adaptation in this species, we improved the draft genome of female grass carp using a fine genetic map and performed whole-genome resequencing to generalize genetic differentiations and variations among eight populations across its distributional range in Asia. Population structures analysis using principal component, linkage disequilibrium decay, and admixture analyses revealed that the Asian grass carp comprises two independent and genetically distinct populations, which might have resulted from geographical distributions with contrasting environments. The South Asia population exhibited reduced genetic diversity and a distinct population structure compared with those of the China and Vietnam populations. Genome selected sweep analysis revealed many diverged genomic regions containing genes enriched for the nervous system and epigenetic diversity-related genetic variation. Finally, among 27 candidate genes located in selected regions, the oxytocin gene had the highest Fst value, and is involved in parturition, social behavior, growth, and developmental processes, possibly contributing to the local adaptation of South Asia grass carp. Our results suggested that grass carp genomic divergence has been triggered and maintained by geographic isolation, revealing the genomic basis of adaptation in this species.