Spotted scat (Scatophagus argus) can tolerate a wide range of salinity fluctuations. It is a good model for studying environmental salinity adaptation. Lipid metabolism plays an important role in salinity adaptation in fish. To elucidate the mechanism of lipid metabolism in the osmoregulation, the liver transcriptome was analyzed after 22 d culture with a salinity of 5 ppt (Low-salinity group: LS), 25 ppt (Control group: Ctrl), and 35 ppt (High-salinity group: HS) water by using RNA sequencing (RNA-seq) in spotted scat. RNA-seq analysis showed that 1276 and 2768 differentially expressed genes (DEGs) were identified in the LS vs. Ctrl and HS vs. Ctrl, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the pathways of steroid hormone biosynthesis, steroid biosynthesis, glycerophospholipid metabolism, glycerolipid metabolism, and lipid metabolism were significantly enriched in the LS vs. Ctrl. The genes of steroid biosynthesis (sqle, dhcr7, and cyp51a1), steroid hormone biosynthesis (ugt2a1, ugt2a2, ugt2b20, and ugt2b31), and glycerophospholipid metabolism (cept1, pla2g4a, and ptdss2) were significantly down-regulated in the LS vs. Ctrl. The pathways related to lipid metabolisms, such as fatty acid metabolism, fatty acid biosynthesis, peroxisome proliferator-activated receptor (PPAR) signaling pathway, adipocytokine signaling pathway, fatty acid degradation, and unsaturated fatty acid biosynthesis, were significantly enriched in the HS vs. Ctrl. The genes of unsaturated fatty acid biosynthesis (scd1, hacd3, fads2, pecr, and elovl1) and adipocytokine signaling pathway (g6pc1, socs1, socs3, adipor2, pck1, and pparα) were significantly up-regulated in the HS vs. Ctrl. These results suggest that the difference in liver lipid metabolism is important to adapt to low- and high-salinity stress in spotted scat, which clarifies the molecular regulatory mechanisms of salinity adaptation in euryhaline fish.

Salinity significantly affects physiological and metabolic activities, breeding, development, survival, and growth of marine fish. The greater amberjack (Seriola dumerili) is a fast-growing species that has immensely contributed to global aquaculture diversification. However, the tolerance, adaptation, and molecular responses of greater amberjack to salinity are unclear. This study reared greater amberjack juveniles under different salinity stresses (40, 30, 20, and 10 ppt) for 30 days to assess their tolerance, adaptation, and molecular responses to salinity. RNA sequencing analysis of gill tissue was used to identify genes and biological processes involved in greater amberjack response to salinity stress at 40, 30, and 20 ppt. Eighteen differentially expressed genes (DEGs) (nine upregulated and nine downregulated) were identified in the 40 vs. 30 ppt group. Moreover, 417 DEGs (205 up-regulated and 212 down-regulated) were identified in the 20 vs. 30 ppt group. qPCR and transcriptomic analysis indicated that salinity stress affected the expression of genes involved in steroid biosynthesis (ebp, sqle, lss, dhcr7, dhcr24, and cyp51a1), lipid metabolism (msmo1, nsdhl, ogdh, and edar), ion transporters (slc25a48, slc37a4, slc44a4, and apq4), and immune response (wnt4 and tlr5). Furthermore, KEGG pathway enrichment analysis showed that the DEGs were enriched in steroid biosynthesis, lipids metabolism, cytokine–cytokine receptor interaction, tryptophan metabolism, and insulin signaling pathway. Therefore, this study provides insights into the molecular mechanisms of marine fish adaptation to salinity.

Marker‐5, which is linked to the major sex‐determination gene amhy on LG23, has been used for sex genotyping of various strains of Nile tilapia. Marker‐5 specifically amplifies fragments of approximately 1,422 and 982 bp of the X and Y chromosomes, respectively. Herein, we evaluated Marker‐5 polymorphism in Nile tilapia of Hainan strain, China. Agarose gel electrophoresis showed that in 13 out of 85 female fish, Marker‐5 amplified a larger band, distinct from the standard X‐specific band. Sanger sequencing showed that this larger band was 1,688 bp in length and showed high similarity with both 1,422‐bp X‐specific and 982‐bp Y‐specific fragments. The larger band was proved to be a female sex‐linked allele of Marker‐5 and was labeled as X_L. When the polymerase chain reaction (PCR) products of X_LY fish, amplified by Marker‐5, were tested by agarose gel electrophoresis, an unusual band, smaller than X_L but larger than that of Y fragment, was observed. The unusual band could be a heteroduplex of the X_L and Y fragments. In very few XX_L male, amhy was found to be linked with 1,422‐bp fragment amplified by Marker‐5. Understanding the Marker‐5 is useful for its utilization in sex‐controlled breeding in farmed Nile tilapia.

Greater amberjack (Seriola dumerili) is an important commercial fish for its high growth rate and excellent flesh quality. However, its sensitivity to variations of water salinity poses challenges to the cage culture. In this study, the greater amberjack were reared in the optimum salinity (30 ppt, CK) and undesired regimes (10 and 40 ppt) for 72 hours. The molecular adaptive mechanisms to salinity stress were revealed by the comparative transcriptome analysis for the gills and kidneys. In gills, a total of 445 and 423 differentially expressed genes (DEGs) were identified in 10 and 40 ppt salinity stress groups, respectively. Those DEGs were involved in cartilage and skeletal development, ions transport, and immune response. The major ion secretion and osmoregulation transport proteins gene slc12a2/nkcc1 and cftr expression levels were significantly down-regulated at 10 ppt, but slightly activated at 40 ppt, compared with the control group. The expression changes in response to the Na⁺, K⁺ movement, and Cl^- ion secretion reduced under the hypo-osmotic exposure and ion excretion boost upon hyper-salinity stress. Meanwhile, the cartilage and skeletal development were enhanced in the gills by hypo- or hyper-salinity stimuli, which is critical for maintaining gill structures and improving respiration and osmoregulation under salinity stress. In kidneys, 600 and 539 DEGs were identified in 10 and 40 ppt groups, respectively. Those DEGs were enriched in oxygen transport, pronephros development, regulation of growth, blood coagulation, ion transmembrane transport, and immune response. While the known renal Na⁺/Cl^– co-transporter gene slc12a3/ncc expression level was significantly down-regulated at 10 ppt, the organic cation transporter 2 gene slc22a2, ammonium transmembrane transport gene rhd and rhag expression levels were overexpressed under the hyper-salinity condition at 40 ppt, contributing to the salts secretion and ammonium transport regulation, to combat the osmotic influx of salts following the drink of seawater and elevated ammonia production upon high salinity stress. These findings advance our knowledge of adaptative mechanisms to the salinity stress and provide theoretical guidance for the optimal breeding mode for the aquaculture of greater amberjack.

Spotted scat, a marine aquaculture fish, has variable body color development stages during their ontogenesis. However, the regulatory mechanism of body color patterns formation was poorly understood. Thyroid hormones (TH) function as an important endocrine factor in regulating metamorphosis. In this study, exogenous thyroid hormones 3,5,3′-L-triiodothyronine (T3) and its inhibitor thiourea (TU) were used to treat spotted scat juveniles during the metamorphosis stage (from 60 to 90 dpf). The function and molecular mechanism of thyroid hormone signaling in regulating body color patterns formation was revealed, using the micro-observation of pigments cells distribution, colorimetric evaluation and carotenoids concentration measurement by spectrophotometry, and comparative transcriptome analysis. Spotted scat body color patterns consisted of whole body black color, black bar, black and red spots, and its final pattern was formed through the metamorphosis. When spotted scat were treated with the inhibitor TU to disrupt thyroid hormone signaling, the levels of T3 and T4 were significantly decreased, the melanophores numbers were significantly increased, as well as the expression of genes involved in melanin synthesis and melanophore differentiation (tyr, tyrp1, dct, mitf, pmel, oca2, slc24a5, and erbb3) was significantly increased. Besides, the expression of genes associated with carotenoids and pteridine metabolism (apod, pnpla2, rdh12, stard10, xdh, abca1, retsat, scarb1, rgs2, and gch1) and carotenoids accumulation were stimulated, when thyroid hormone signaling was disrupted by TU. On the contrary, the levels of T3 and T4 were significantly elevated in spotted scat treated with T3, which could weaken the skin redness and reduce the number of black spots and melanophores, as well as the number and diameter of larval erythrophores. Notably, unlike melanophores and erythrophores, the differentiation of iridophore was promoted by thyroid hormones, gene related to iridophore differentiation (fhl2-l, fhl2, ltk, id2a, alx4) and guanine metabolism (gmps, hprt1, ppat, impdh1b) were up-regulated after T3 treatment, but they were down-regulated after TU treatment. Above results showed that thyroid hormone signaling might play critical roles in regulation pigments synthesis and deposition, thereby affecting pigment cells (melanophores, iridophores and erythrophores) formation and body color patterns. The mechanisms of hyperthyroid and hypothyroid on different pigment cells development were different. Excess thyroid hormone might impact the rearrangement of melanophore by regulating cell cycle, resulting in the abnormalities of black spots in spotted scat. Meanwhile, the excessed thyroid hormone could reduce the number and diameter of larval erythrophores, as well as weaken the skin redness of juvenile erythrophores, but they were enhanced by the disruption of thyroid hormone. However, the formation of iridophore differentiation and guanine synthesis genes expression were stimulated by thyroid hormones. These findings provide new insights for exploring the formation of body color patterns in fish, and help to elucidate the molecular mechanism of thyroid hormone in regulating pigment cell development and body coloration, and may also contribute to selective breeding of ornamental fish.

Red coloration is considered an economically important trait in some fish species, including spotted scat, a marine aquaculture fish. Erythrophores are gradually covered by melanophores from the embryonic stage. Despite studies of black spot formation and melanophore coloration in the species, little is known about erythrophore development, which is responsible for red coloration. 1-phenyl 2-thiourea (PTU) is a tyrosinase inhibitor commonly used to inhibit melanogenesis and contribute to the visualization of embryonic development. In this study, spotted scat embryos were treated with 0.003% PTU from 0 to 72 h post fertilization (hpf) to inhibit melanin. Erythrophores were clearly observed during the embryonic stage from 14 to 72 hpf, showing an initial increase (14 to 36 hpf), followed by a gradual decrease (36 to 72 hpf). The number and size of erythrophores at 36 hpf were larger than those at 24 and 72 hpf. At 36 hpf, LC–MS and absorbance spectrophotometry revealed that the carotenoid content was eight times higher than the pteridine content, and β-carotene and lutein were the main pigments related to red coloration in spotted scat larvae. Compared with their expression in the normal hatching group, rlbp1b, rbp1.1, and rpe65a related to retinol metabolism and soat2 and apoa1 related to steroid hormone biosynthesis and steroid biosynthesis were significantly up-regulated in the PTU group, and rh2 associated with phototransduction was significantly down-regulated. By qRT-PCR, the expression levels of genes involved in carotenoid metabolism (scarb1, plin6, plin2, apoda, bco1, and rep65a), pteridine synthesis (gch2), and chromatophore differentiation (slc2a15b and csf1ra) were significantly higher at 36 hpf than at 24 hpf and 72 hpf, except for bco1. These gene expression profiles were consistent with the developmental changes of erythrophores. These findings provide insights into pigment cell differentiation and gene function in the regulation of red coloration and contribute to selective breeding programs for ornamental aquatic animals.