| Global climate change has been posing great threats to the world aquaculture production.Fish is an important category in aquaculture,understanding their adaptive potential to thermal stress caused by climate change is of great significance to the sustainable and healthy development of marine fish aquaculture industry.Thermal tolerance of fish is affected by both heredity and environment,and it shows certain plasticity to changes in temperature.Thermal plasticity of some fish has been predicted to help fish adapt to climate change,especially for those living in subtropical areas.Large yellow croaker distributes mainly in subtropical and tropical waters,and its culture production ranks first in marine aquaculture of China.In recent years,global warming,extreme heat in summer and cold waves in winter occur frequently,posing great threats to China’s large yellow croaker breeding industry.In the situation that artificial temperature control in mariculture environments is hard to achieve,how will cultured large yellow croaker respond to the increasing thermal pressure caused by climate change?Whether the thermal plasticity of large yellow croaker can help them cope with the threat of climate change?What is the underlying molecular and genetic regulation mechanisms?And how to help them to cope with the potential thermal stress by modern breeding technology?To answer these questions,the ecophysiological and genetic methods were used in this study to investigate the characteristic,genetic and epigenetic mechanisms of the thermal tolerance of cultured large yellow croaker in Sandu’ao basin.The main results are as follows:1.Thermal tolerance and thermal plasticity of large yellow croakerThe experimental large yellow croaker were acclimated at 11℃(present winter average water temperature minus 3℃),18℃,25℃ and 30℃(present summer average water temperature plus 2℃)for three weeks.As a result,the critical thermal limits of large yellow croaker acclimated at 11-30℃ ranged from 5.27℃ to 36.64℃,and the total thermal tolerance zone was 444.74℃2.Thermal limits increased as acclimation temperature rise,and the estimated acclimation response ratios ranged from 0.32 to 0.63,showing obvious thermal plasticity.The estimated post-acclimation thermal sensitivity values(0.80-1.28)for large yellow croaker acclimated at 11-30℃ were close to 1,and the heat-(5.78℃)and cold-thermal safety margins(6.33 ℃)were both high,showing sufficient thermal tolerance.However,performance results showed that longterm exposure to 30℃ would induce energy redistribution and tradeoffs between health/growth and heat tolerance,leading to significantly decreased body weight and increased superoxide dismutase activity in the gill.Due to the shallow culture depth,high density and limited moving spaces in floating sea-cages,as well as the fact that our recorded highest in situ sea surface temperature and warming rate have reached up to 30.1℃ and 1.2℃/d,we speculated that the production of floating sea-cage farmed large yellow croaker is more vulenarble to climate change and the farms currently operating in the main producing areas of China is already at the risk of summer heatwaves.2.Genome-wide transcriptional regulation mechanisms of the thermal plasticity in large yellow croakerThe livers of large yellow croaker acclimated at 11℃(cold group)and 30℃(heat group)were selected for transcriptome sequencing.Comparison between before and after acclimation identified more differentially expressed genes(DEGs)in heat group(642)while higher fold changes of DEGs in cold group(293),indicating the relatively higher transcriptional plasticity for cold stress.The results of GO,KEGG enrichment and protein-protein interaction(PPI)network analysis showed that the plastic transcriptional expression of genes involved in glycometabolism and glucose homeostasis is critical for the thermal plasticity of large yellow croaker.In addition,varied expression of genes related to cell stress response,steroid metabolism and thermogenesis also played important roles in heat and cold plasticity,respectively.Overall,the transcriptional expression pattern in heat-stressed large yellow croaker seems to be in a stress response state.These results suggest that transcriptional plasticity plays an important role in regulating the phenotypic plasticity of thermal tolerance in large yellow croaker.3.Genome-wide methylation analysis revealed the epigenetic regulation mechanism of thermal plasticity in large yellow croakerWhole genome bisulfite sequencing(WGBS)was also conducted with the livers of large yellow croaker acclimated at 11℃(cold group)and 30℃(heat group).A total of 15,813 and 16,508 differentially methylated regions(DMGs)were identified in heatand cold-stressed fish,indicating that thermal acclimation significantly changed the whole genome methylation patterns of large yellow croaker.Further combined analysis with transcription data identified 176 and 81 differentially expressed DMGs in heat and cold groups,respectively.Functional enrichments and PPI analysis of these genes showed that the changed mRNA expression of gck,bpgm,mind and other genes related to glycometabolism and insulin response may be regulated by their changed methylation status under thermal stress,which further mediated the thermal plasticity of large yellow croaker.Moreover,changes in methylation status of genes related to steroid synthesis and lipometabolism,and thermogenesis mediated by thyroid hormone may also play important roles in heat and cold plasticity of large yellow croaker,respectively.In summary,DNA methylation in liver may participate in the regulation of thermal plasticity in large yellow croaker by affecting the transcriptional expression of metabolism related genes.4.GWAS identified candidate loci and genes associated with the acute thermal tolerance of large yellow croakerUsing large yellow croaker from the same cultured population,acute heat(34℃)and cold(8℃)stress experiments were carried out,and dorsal fins from fish showing loss of equilibrium(LOE)during stress were sampled for DNA extraction.Three hundred heat-stressed fish were genotyped using "Ning Xin I"(a 600 K large yellow croaker genotyping array),followed by association analysis using the obtained 304,149 high quality SNPs and "sensitive/tolerant" binary heat-tolerance trait.As a result,5 significant SNPs on 4 chromosomes,as well as a clear peak on Chr19 were identified.The 30 candidate genes predicted from these SNPs function mainly in processes relate to vascular regulation,heat shock response and endoplasmic reticulum stress response.The genotyping of 403 cold-stressed large yellow croaker was conducted by ddRAD sequencing,and 52,392 high quality SNPs were obtained.GWAS analysis based on these SNPs and the LOE times under cold stress identified 4 significant SNPs on 3 chromosomes,and a clear peak on Chr24 was also observed.A total of 46 candidate genes involved in lipid transport and metabolism,cell stress response,glucose metabolism,temperature homeostasis and immunity were identified.Finally,the accuracy of some candidate genes was further validated with a previous transcriptome study and our qPCR experiment.In summary,the thermal tolerance of large yellow croaker is plastic,which can provide them a buffer to cope with future climate change.However,the tradeoff accompanied by heat plasticity can lead to decreased body size and health statue in large yellow croaker exposure to logn-term 30℃ environment or above,leading to declined production in large yellow croaker aquaculture.Changes in mRNA expression of genes related to glycometabolism and insulin response may be regulated by their changed methylation status under thermal stress,which further mediated the thermal plasticity of large yellow croaker.The important candidate loci related to acute heat and cold tolerance of large yellow croaker are probably located on Chr19 and Chr24,respectively. |
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