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Gills are the location of gas exchange and also the rst target organ of sh response for environmental stress. As a multifunctional organ, its energy supply, when faced with insu cient dissolved oxygen in the water, remain unclear. In this study, largemouth bass were subjected to hypoxia stress (1.2 mg/L) for 24 h and 12 h reoxygenation (R12) to evaluate energy supply strategy of gills. Under hypoxia, gills respond rstly and rapidly by increasing respiratory rate and changing gene expression. Transcriptome data showed that 2026 differential expressed genes (DGEs) were detected in the early stage of hypoxia exposure, including genes related to oxygen sensor and transport. KEGG functional analysis of DEGs revealed that the glycolysis/gluconeogenesis pathway was enriched across all the sampling points (0, 4, 8, 12, 24h, R12). The gene expression and enzyme activity of three rate-limiting enzymes (hexokinase, phosphofructokinase-6, pyruvate kinase) in glycolysis pathway were signi cantly increased. The increase of glycolysis products pyruvate and lactic acid, as well as the number of mitochondria, suggest the enhancement of aerobic and anaerobic metabolism of glucose. These results suggest that gill of largemouth bass enhanced energy supply during acute exposure to hypoxia stress.

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In this study, we explored the effects of 4 weeks of intermittent hypoxic exposure (IHE) on liver angiogenesis and related regulatory mechanisms in largemouth bass (Micropterus salmoides). The results indicated that the O 2 tension for loss of equilibrium (LOE) decreased from 1.17 to 0.66 mg/L after 4 weeks of IHE. Meanwhile, the red blood cell (RBC) and hemoglobin concentrations significantly increased during IHE. Our investigation also found that the observed increase in angiogenesis was correlated with a high expression of related regulators, such as Jagged, phosphoinositide-3-kinase (PI3K), and mitogen-activated protein kinase (MAPK). After 4 weeks of IHE, the overexpression of factors related to angiogenesis processes mediated by HIF-independent pathways (such as nuclear factor kappa-B (NF-κB), NADPH oxidase 1 (NOX1), and interleukin 8 (IL8)) was correlated with the accumulation of lactic acid (LA) in the liver. The addition of cabozantinib, a specific inhibitor of VEGFR2, blocked the phosphorylation of VEGFR2 and downregulated the expression of downstream angiogenesis regulators in largemouth bass hepatocytes exposed to hypoxia for 4 h. These results suggested that IHE promoted liver vascular remodeling by the regulation of angiogenesis factors, presenting a potential mechanism for the improvement of hypoxia tolerance in largemouth bass.

The largemouth bass (Micropterus salmoides) has become a cosmopolitan species due to its widespread introduction as game or domesticated fish. Here a high-quality chromosome-level reference genome of M. salmoides was produced by combining Illumina paired-end sequencing, PacBio single molecule sequencing technique (SMRT) and High-through chromosome conformation capture (Hi-C) technologies. Ultimately, the genome was assembled into 844.88 Mb with a contig N50 of 15.68 Mb and scaffold N50 length of 35.77 Mb. About 99.9% assembly genome sequences (844.00 Mb) could be anchored to 23 chromosomes, and 98.03% assembly genome sequences could be ordered and directed. The genome contained 38.19% repeat sequences and 2693 noncoding RNAs. A total of 26,370 protein-coding genes from 3415 gene families were predicted, of which 97.69% were functionally annotated. The high-quality genome assembly will be a fundamental resource to study and understand how M. salmoides adapt to novel and changing environments around the world, and also be expected to contribute to the genetic breeding and other research.