Histological Assessment and Transcriptome Analysis Provide Insights into the Toxic Effects of Perfluorooctanoic Acid to Juvenile Half Smooth Tongue Sole Cynoglossus semilaevis

Perfluorooctanoic acid(PFOA)is a widespread synthetic persistent organic pollutant that may enrich along the food chain and affect the growth,development,reproduction,and lipid metabolism of aquatic organisms,particularly the benthic organisms.How-ever,the toxic effects of PFOA on the half-smooth tongue sole Cynoglossus semilaevis,a commercial benthic fish in China,have rarely been reported.Because juvenile fish are sensitive to environmental pollutants,in the present study,histological assessment and tran-scriptome sequencing were performed to determine the short-term impact of PFOA on juvenile half-smooth tongue soles.Histologi-cal analysis showed that PFOA exposure caused hepatocyte rupture,intestinal villi breakage,increased goblet cell count,and brain ab-normal.Transcriptome results showed that some interesting signaling pathways,such as glycolysis/gluconeogenesis,PPAR signaling pathway and GABAergic synapse signaling pathway,were enriched after PFOA exposure.In addition,some metabolic,immune and neural genes were differentially expressed,which including ependymin,hbb1-like and gad 1,and they were up-regulated after 14 days of exposure.Transcriptome results also indicated that half-smooth tongue sole might improve energy metabolism in response to PFOA toxicity after 7 days of exposure.These findings provide a basis for studying the ecological effects of PFOA on marine benthic fishes.

基于低压静电场技术(LVEF)对半滑舌鳎(Cynoglossus semilaevis)的保鲜效果及微生物菌落影响分析

为研究低压静电场对半滑舌鳎(Cynoglossus semilaevis)在贮藏过程中微生物群落特征,采用高通量测序技术对贮藏至30 d半滑舌鳎的微生物群落碱基信息测序,产生的碱基序列通过OTU聚类、多样性分析、相关性分析等手段,探讨影响半滑舌鳎保鲜效果的微生物群落组成和发育信息。研究不同低压静电场(2000、2500 V/m)对半滑舌鳎贮藏期间微生物群落组成的影响。结果表明:半滑舌鳎有效序列范围为76735~103583,平均长度为428.39~429.17 bp,OTU数目在45~218。贮藏期间样品的微生物多样性和相对丰度均有所下降。在门水平上共鉴定出四种微生物菌门,分别为变形菌门(Proteobacteria)、厚壁菌门(Firmicutes)、拟杆菌门(Bacteroidota)和其他。在属水平上鉴定出11种微生物菌属,分别为不动杆菌属(Acinetobacter)、嗜冷杆菌属(Psychrobacter)、希瓦氏菌属(Shewanella)、环丝菌属(Brochothrix)、漫游菌属(Vagococcus)、发光杆菌属(Photobacterium)、Myroides和肉食杆菌属(Carnobacterium)、假单胞菌属(Pseudomonas)、无色杆菌属(Achromobacter)和其他。贮藏至30 d时,CK组优势腐败菌为环丝菌属,LVEF-1(2000 V/m)和LVEF-2(2500 V/m)组样品中环丝菌属相对丰度显著上升,但是低于CK组,且嗜冷杆菌属和希瓦氏菌属相对丰度仍然占据一定比例。由此推测LVEF能够抑制半滑舌鳎优势腐败菌属相对丰度的增加。研究结果为低压静电场保鲜贮藏半滑舌鳎微生物多样性变化提供了理论基础。

Effects of Starving and Re-feeding Strategies on the Growth Performance and Physiological Characteristics of the Juvenile Tongue Sole(Cynoglossus semilaevis)

Starving and re-feeding trials were conducted to evaluate the effects of starvation duration and recovery times on the growth performance and physiological characteristics of juvenile tongue sole Cynoglossus semilaevis, which included growth, body composition, intestinal morphology and digestive enzymes activities. The fish starved for one(D1), two(D2) and four(D4) days, respectively, were recovered for three(R3, R6, R12), seven(R7, R14, R28) and eleven(R11, R22, R44) folds of starvation days while the control fish were fed constantly. The experiment was conducted for 96 days. The fish individuals in D1R11, D2R14 and D2R22 caught up in weight with the control, indicating that complete compensatory growth existed in these fish individuals. The highest body weight gain and total specific growth rates were observed in D2R22. Food intake increased significantly in most treatments in comparison with that of the control except for D1R7 and D2R22(P < 0.05). Food conversion efficiency and apparent digest coefficient in D2R22 were significantly promoted, being higher than that of the control(P < 0.05). The activities of trypsin and lipase were found to be closely related to the growth performance. The activities of liver and hindgut trypsin, also midgut and hindgut lipase in D2R22 were significantly higher than those of the control(P < 0.05). The fold height(HF) of foregut and midgut increased significantly in D2R22, and HF decreased significantly in D1R3, D2R6 and D4R12(P < 0.05). On our findings, we may conclude that the optimum starving and re-feeding strategy is starving for 2 days and re-feeding for 22 days. And in this starving and re-feeding strategy, the compensatory growth could be mostly attributed to the promotion of food conversion efficiency and digestibility coefficiency.

Evaluation of reference genes for quantitative real-time PCR analysis of gene expression during early development processes of the tongue sole(Cynoglossus semilaevis)

Differential expression of genes is crucial to growth and development of fish. To select the appropriate genes for gene normalization during Cynoglossus semilaevis early developmental process, eight candidate reference genes （ACTB, B2M, EF1A, GADPH, RPL7, TUBA, UBCE and 18S） were tested for their adequacy by using quantitative real-time PCR. The results showed that the expression of all the examined genes exhibited tissue dependent variations in the mature C. semilaevis. EFIA was listed as the most stable reference among the 14 tissues by RefFinder. Furthermore, the recommended comprehensive ranking of the stability determined by RefFinder showed that 18S was the most stable gene during the early developmental stages （from oosphere to 90 days old） in this study. However, when divided the Ct value data of the above mentioned early developmental stages into two separate periods （embryo and post-hatching periods）, TUBA and 18S represented the most stable references of these two developmental periods, respectively. Consequently, the reference gene should be carefully and accurately chosen even for studies of the same species at various developmental processes. The relevant data may help in selecting appropriate reference genes for mRNA expression analysis, and is of great value in the studies of fish growth and development.

Phenotypic and genetic parameter estimation of juvenile growth and bottom color traits in half-smooth tongue sole,Cynoglossus semilaevis

Half-smooth tongue sole（Cynoglossus semilaevis） is a commercially important fish species. There were no reports about accurate genetic parameters of growth traits in C. semilaevis, despite efforts aimed at the culture of this species. Therefore, the main goal of this study was to estimate the parameters for the early growth traits（total length, body depth and body weight） of C. semilaevis based on the data obtained from 80 families. The trait of fish with pure white bottom or not（named as the trait of bottom color） was also analyzed. All the heritabilities estimated for the traits were medium, which varied from 0.210 to 0.362. Genetic correlations among growth traits were highly positive, varying from 0.913 to 0.959, indicating that selection of one of the three traits would result in correlated increase in the others. Also, genetic correlations between bottom color and growth traits were positive and varying from 0.241 to 0.353, suggesting that selection for the bottom color can be utilized to enhance the selection of growth traits. In addition, fish from the top performing 16 full-sib families were selected as broodstock, based on the above analysis and the comparison of family breeding value. This research provides an important basic material to implement selective breeding in C. semilaevis.

Large-Scale Screening of Growth-Related Variants in Chinese Tongue Sole(Cynoglossus semilaevis)

Growth is the most valuable economic trait for improving aquaculture fish species,since fast growth can reduce labor cost and make more economic benefits.However,the knowledge about how many and which genes are related to the growth of Chi-nese tongue sole is limited.High-throughput sequencing screening of variants is a fast,economical and accurate assay to identify genes related to growth in crops,livestock and other aquaculture products.In the present study,genome-wide resequencing of 30 female Chinese tongue sole individuals from large and small groups to screen growth trait-related variations(SNPs,InDels)was carried out.In total,6545735 SNPs and 1016745 InDels were detected,while 31 genes related to growth traits were identified.Their functions were mainly involved in muscle architecture,post-embryonic development,neurosensory development and hormone regulation.Fur-thermore,18 of the 31 genes related to growth trait are located on W chromosome,indicating that W chromosome plays an important role in regulating the body size of female tongue soles.The markers and genes identified in our study can be applied to selective breeding of tongue sole and thus will promote the aquaculture industry and increase economic efficiency.

Location of Vibrio anguillarum resistance-associated trait loci in half-smooth tongue sole Cynoglossus semilaevis at its microsatellite linkage map

A cultured female half-smooth tongue sole(C ynoglossus semilaevis) was crossed with a wild male, yielding the fi rst fi lial generation of pseudo-testcrossing from which 200 fi sh were randomly selected to locate the V ibrio anguillarum resistance trait in half-smooth tongue sole at its microsatellite linkage map. In total, 129 microsatellites were arrayed into 18 linkage groups, ≥4 each. The map reconstructed was 852.85 c M in length with an average spacing of 7.68 c M, covering 72.07% of that expected(1 183.35 cM). The V. anguillarum resistance trait was a composite rather than a unit trait, which was tentatively partitioned into Survival time in Hours After V. anguillarum Infection(SHAVI) and Immunity of V. A nguillarum Infection(IVAI). Above a logarithm of the odds(LOD) threshold of 2.5, 18 loci relative to SHAVI and 3 relative to IVAI were identifi ed. The 3 loci relative to IVAI explained 18.78%, 5.87% and 6.50% of the total phenotypic variation in immunity. The microsatellites bounding the 3 quantitative trait loci(QTLs) of IVAI may in future aid to the selection of V. anguillarum-immune half-smooth tongue sole varieties, and facilitate cloning the gene(s) controlling such immunity.

Identification of global alternative splicing and sex-specific splicing via comparative transcriptome analysis of gonads of Chinese tongue sole(Cynoglossus semilaevis)

Chinese tongue sole(Cynoglossus semilaevis)is an economically important marine fish species with a ZZ/ZW sex determination mechanism,which can be influenced by temperature.Alternative splicing(AS)is an important mechanism regulating the expression of genes related to sex determination and gonadal differentiation,but has rarely been reported in fish.In this study,to explore the molecular regulatory mechanisms of sex determination and gonadal differentiation,we combined isoform and RNA sequencing(Iso-Seq and RNA-Seq)to perform transcriptome profiling of male and female gonads in C.semilaevis.In total,81883 and 32341 full-length transcripts were obtained in males and females,respectively.A total of 8279 AS genes were identified,including 2639 genes showing differential AS(DAS)between males and females.Many intersecting DAS genes and differentially expressed genes(DEGs)were enriched in the meiotic cell cycle pathway,and genes related to gonadal differentiation,such as esrrb and wt1a,were found to have sex-specific isoforms.Thus,this study revealed AS events in the gonadal transcriptomes of male and female C.semilaevis,described the characteristics of active transcription in the testes,and identified candidate genes for studying the regulatory mechanisms of AS during gonadal differentiation.

DNA Methylation of pacap Gene Is Involved in Growth Superiority of Female Half Smooth Tongue Sole(Cynoglossus semilaevis)in Different Salinities

Pituitary adenylate cyclase activating polypeptide(PACAP)and growth hormone-releasing hormone(GHRH)play important roles in the GH/IGF growth axis in fishes.To determine whether epigenetic change is involved in the regulation of pacap and ghrh responses to low salinity stress in Cynoglossus semilaevis,the correlation between growth traits,DNA methylation status and gene expression level in low salinity(15,S15)and optimal salinity(30,S30)at day 7(D7)and day 60(D60)were analyzed.Results showed that exposure to low salinity for 60 days attenuated C.semilaevis growth rate.Under low salinity,DNA methylation level of pacap promoter increased in females and decreased in males at day 7,but inverted at day 60.Additionally,pacap expression was up-regulated in both males and females.The pacap promoter methylation level was higher and expression level was lower in female than in male.The results suggest that pacap promoter methylation level is negatively correlated to mRNA level and positively correlated to body weight,while gene expression level is negatively related with body weight.With the decrease of salinity,DNA methylation level of ghrh promoter and exon1,as well as its gene expression displayed minor changes.Overall,pacap gene seems to play an important role in fish growth,contributing to female growth superiority,while ghrh gene seems not pertinent under salinity stress.The results indicate that low salinity potentially affects fish growth through regulating DNA methylation in pacap promoter.This study expands the understanding of the molecular mechanism of how salinity modulates fish growth from the epigenetic perspective.

Genetic variability of half-smooth tongue sole Cynoglossus semilaevis populations using microsatellite markers

Ten highly variable microsatellite loci were performed in order to evaluate the genetic variability of wild and hatchery samples of half-smooth tongue sole Cynoglossu＇s semilaevis. A group of 200 genotypes belonging to four wild samples, Laizhou （LZ）, Weihai （WH）, Qingdao （QD）, Rizhao （RZ） and one hatchery sample, Mingbo （MB）, were screened. All of the： ten microsatellite loci screened in this study showed marked polymorphism. A total of 70 different alleles were observed over all loci. The number of alleles per locus ranged from 4.2 to 1006 the average of expected and observed heterozygosity ranged from 0.67 to 0.82, and from 0：77 to0.87 respectively. A total of 10, 16, 10, 11 and 5 unique alleles each were found in LZ, WH~ QD：RZ and MB populations. The effective number of alleles varied from 2.87 for HSTS_7 to 6.83 for HSTS_h. The number of average genotypes ranged from 6.0 for HSTS a to 15.4 for HSTS_h： As compared with the wild populations, the hatchery population, MB, showed significant genetic changes such as fewer alleles per locus （P〈0.05）, a smaller number of low frequency alleles （P〈0.05）, a small number of unique alleles and a small number of genotypes （P〈0.05）, all indicative of a reduction in genetic diversity.

Genome-wide identification and prediction of long noncoding RNAs in half-smooth tongue sole Cynoglossus semilaevis

At present,the function and profile of long noncoding RNAs(lncRNAs)in half-smooth tongue soles Cynoglossus semilaevis remain poorly understood.Therefore,we identified lncRNAs in the species using large-scale deep sequencing approaches.A total of 96726222 clean reads and 2497 lncRNAs were obtained from the compound samples.In total,1694 known lncRNAs and 803 novel lncRNAs were identified.Most of the novel lncRNAs distributed mainly in a range of 200-3000 nt and contained 2-6 exons.Six novel lncRNAs were selected for expression analysis by quantitative reverse transcription-polymerase chain reaction,of which lnc_770 and lnc_150 were expressed mainly in the ovary.This study provides a valuable resource for lncRNA studies of half-smooth tongue sole and improves our understanding of the function of non-coding RNA in fish.

Establishment and characterization of a new cell line derived from half-smooth tongue sole Cynoglossus semilaevis kidney

In this study,we established a new cell line(Cynoglossus semilaevis kidney cells,CSK)from kidney of the half-smooth tongue sole(Cynoglossus semilaevis).The cells were subcultured over 1000 days and passaged for more than 100 times.Additionally,CSK cells were optimally maintained in Dulbecco's modified Eagle medium nutrient mixture F-12 supplemented with HEPES,antibiotics,fetal bovine serum,2-mercaptoethanol,and basic fibroblast growth factor.The optimum growth temperature for CSK cells was 25℃,and the cells showed a fibroblast-like phenotype.Chromosome analysis revealed that CSK cells had a normal diploid karyotype with 2 n=42.CSK cells were susceptible to Grouper nervous necrosis virus(NNV),and cytopathic ef fects were observed at 3–5 days postinfection.The NNV sensitivity of CSK cells was related to the high abundance of virions in the cytoplasm,as observed by electron microscopy.Additionally,CSK cells could be successfully transfected with a green fluorescent protein reporter plasmid,and fluorescent signals were easily observed.Finally,immunocytochemistry analysis showed that CSK cells were supporting cells.Overall,we established this new cell line,which may have potential applications in the identification of viral pathogens af fecting the half-smooth tongue sole.

Histopathological Observation of Cynoglossus semilaevis Günther with Abdominal Dropsy in Industrial Aquaculture

[ Objective] This study aimed to investigate and analyze the histopathological changes in various tissues and organs of Cynoglossus semilaevis Gunther with abdominal dropsy. [ Method] By HE staining, pathological tissues of main organs of C. semilaevis Giinther with abdominal dropsy were observed. [ Result] According to the results of histopathological observation, C. semilaevis Gilnther with abdominal dropsy exhibited a large amount of ascites in the abdominal cavity; small intestinal mucosal epithelial cells were denatured and necrotic, and shed into the enteric cavity, with submucosal vascular congestion; the liver and pancreas tissues were denatured and necrotic, and the morphology of most liver cells disappeared. However, there was no remarkable pathological change in the heart, spleen and other organs. [ Conclusion] C. semilaevis Gtinther with abdominal dropsy exhibited serious pathological and histochemical lesions in the small intestine, liver and pancreas, causing metabolic disorders.

Cloning, expression profiling and promoter functional analysis of bone morphogenetic protein 2 in the tongue sole(Cynoglossus semilaevis)

BMP2 plays crucial roles in vertebrate developmental process and acts as a bone inducer during osteogenesis. We present here the molecular cloning of bmp2 cDNA from the marine flatfish Cynoglossus semilaevis, and the analysis of bmp2 expression profiling and promoter function. The full length of bmp2 cDNA sequence is 2 048 bp,which encodes a protein of 422 amino acids. Tissue expression distribution of bmp2 was examined in 14 tissues of mature individuals by quantitative real time PCR（qRT-PCR）. The results revealed that bmp2 was expressed ubiquitously, and the highest expression level was detected in the spinal cord. Moreover, bmp2 expression levels were detected at 15 sampling time points of early developmental stages（egg, larva, juvenile and fingerling stages）.The highest expression level of bmp2 was observed at the gastrula stage, which was about ten times higher than those at the other three embryo stages. Whole-mount in situ hybridization showed that the bmp2 signal was strongly detected at the location of the crown-like larval fin, heart and liver, and slightly expressed in the notochord at one day post hatch（dph）; then the expression of bmp2 started to be concentrated in notochord at three dph. Subsequently, we characterized the 5′-flanking region of bmp2 by testing the promoter activity by Luciferase reporter assays. Positive regulatory region was detected at the location of –179 to ＋109. The predicted transcription factor binding sites（E-box binding factors, zinc finger transcription factor, etc.） in this region might participate in the transcriptional regulation of the bmp2 gene.

Molecular cloning,characterization and expression analysis of Sox9a and Foxl2 genes in half-smooth tongue sole(Cynoglossussemilaevis)

Two transcription factors, Sox9a and Foxl2 were cloned from half-smooth tongue sole （Cynoglossus semilaevis）. Sox9a is a new duplication of C.semilaevis Sox9 gene. The complete cDNA of Sox9a gene was 1 842 bp long coding for 487 amino acids and Foxl2 gene was 1 817 bp coding for 308 amino acids. Sox9a was expressed higher in male brain, pituitary and gonad and Foxl2 were higher in female brain, pituitary and gonad. The expression of Sox9a gene in gonads of neo-males was higher than that of normal females. Sox9a and Foxl2 were expressed higher in gastrula stage than in other stages. In the period of sex differentiation, the expression of Sox9a was first going up and then going down and Foxl2 was higher expressed at 37 dph. The highest expressions of Sox9a and Foxl2 genes occurred in nine-month and 12-month old gonad tissues, respectively. Sox9a gene was considered to have inevitable links with sex reversal, sex differentiation and cell differentiation of embryos and formation of spermatogenic ceils. Foxl2 was considered to play a role in sex differentiation, cell differentiation of embryos but not to be necessary for sex determination and sex reversal.

Identification of differential genes in the ovary relative to the testis and their expression patterns in half-smooth tongue sole (Cynoglossus semilaevis)

Half-smooth tongue sole （Cynoglossus semilaevis） is a rare marine flatfish whose mature ovary and testis greatly differ in volume and weight. The length and weight of mature females are over twice greater than those of mature males. To obtain sufficient information on gonad differentiation and the relationship between gonad development and growth in the fish, we compared gene expression between the ovary and testis using suppression subtractive hybridization （SSH）. Testis cDNAs are subtracted from ovary cDNAs and are used to es- tablish an ovary testis-subtracted cDNA library. A total of 41 nonredundant clones are identified, including 20 known cDNAs, 9 unchar-acterized cDNAs （EST clones）, and 12 novel sequences. For selected genes such as ZPC, RacGAP, survivin, aquaporin, CPEB, O5, O15, and O18, gene expression patterns are analyzed by reverse transcriptase-polymerase chain reaction （RT-PCR）. The results confirm that these genes are only expressed in the ovary and not in the testis, or at higher levels in the ovary than in the testis. At the same time, expressions of certain genes such as ZPC, survivin, aquaporin, CPEB, and O15 are demonstrated to possess sexual dimorphism in the kidney or muscle, and spleen. The results suggest that these genes could play key roles not only in the ovary but in other female tissues as well.

Progress in studies of sex determination mechanisms and sex control techniques in Cynoglossus semilaevis(half-smooth tongue sole)

The Cynoglossus semilaevis(half-smooth tongue sole) is a marine flatfish of great commercial value for fisheries and aquaculture in China.It has a female heterogametic sex determination system(ZW/ZZ) and environmental factors can induce sex-reversal of females to phenotypic males,suggesting that it is a promising model for the study of sex determination mechanisms.Additionally,females grow much faster than males and it is feasible to improve the aquaculture production through sex control techniques.This paper reviews the progress in research on sex determination mechanisms research in our laboratory.We have completed whole-genome sequencing and revealed the genome organization and sex chromosome evolution of C.semilaevis.A putative male determining gene dmrt1 was identified and DNA methylation was verified as having a crucial role in the sex reversal process.Genetic maps and sex-specific biomarkers have been used in a marker-assisted selection breeding program and for differentiation of the fish sex.Development and improvement of sex control technologies,including artificial gynogenesis and production of breeding fry with high proportion of females,is also reviewed.These research advances have provided insight into the regulation of sex determination and enabled efficient sex management in artificial culturing of C.semilaevis.

Transcriptome analysis and candidate gene identification reveals insights into the molecular mechanisms of hypermelanosis in Chinese tongue sole(Cynoglossus semilaevis)

Blind-side hypermelanosis has emerged as a major concern in flatfish aquaculture worldwide,including tongue sole(Cynoglossus semilaevis)in China.The causative gene and the molecular basis are still unclear.In this study,comparative transcriptome analyses were performed using different skin tissues of tongue sole:ocular-side normal(pigmented)skin,blind-side normal(non-pigmented)skin and blind-side hypermelanotic(pigmented)skin.Finally,60 key hypermelanosis-related genes were mined,providing potential candidate gene resources involved in blind-side hypermelanosis.These genes were selected based on the log2(FoldChange)and false discovery rate(FDR)values(with corresponding P-Values<0.05),and they were verified in other species to assess if they were directly or indirectly related to melanogenesis.The protein-protein interaction network of these 60 genes and the relationship between tyr and other key hypermelanosis-related genes were illustrated.The qRT-PCR validation of 16 differentially expressed genes(DEGs)showed that the data of qRT-PCR were consistent with those of RNA-seq.Further analyses revealed that the selected DEGs were significantly overrepresented in several pigment metabolic processes and in the melanogenesis pathway.Our results may imply that blind-side hypermelanosis is a pattern of environmental regulation of gene expression and adaptation in flatfish.Membrane transport proteins(such as OCA2 and SLC45A2)may serve as a“switch”for melanogenesis in tongue sole.Overall,this study provided novel insights into the molecular mechanism of hypermelanosis in flatfish species and will facilitate future selective breeding of tongue sole for this market-favoured trait in aquaculture.

Effects of low and high levels of nano-selenium on intestinal microbiota of Chinese tongue sole(Cynoglossus semilaevis)

Nano-selenium(Se)has attracted growing attention due to its high bioavailability,low toxicity and its possible applications in the prevention of oxidative damage in animal tissues.Recent research works have conveyed that the use of nano-seleniumas food additive could have positive effects on the nutrition of aquatic animals.In this study,the intestinal bacterial communities associated with low and high concentrations of nano-selenium in the diet of Chinese tongue sole were characterized,through 16S rRNA V3–V4 sequencing.Samples were derived from fish fed with diet containing different nano-selenium level,0 mg/kg for the control group,1.6 mg/kg for low dose group,and 2.4 mg/kg for high dose group The intestinal microbial community structure showed a low overall diversity,among which the vibro genus occupied the most important position with more than 90%relative abundance in all three group.Significant differences in specie diversity occurred between the nanoselenium treated(both low dose[IL]and high dose[LH])groups and the control group(IC).Acinetobacter,Arthrobacter,and Streptophytes were the top three enriched genera after the treatment with nano-selenium compared to IC.Meantime,the weight gain rate of the LH group was 9%higher than control.The results revealed the effect of nano-selenium on microbial community structure of gut and growth performance of C.semilaevis,Future research exploring the interaction mechanism between micronutrients and the microbial communities would increase our understanding of the fish nutrition.

Estimation of heritabilities of disease resistance to Edwardsiella tarda and genetic correlations between resistance and growth traits in Chinese tongue sole (Cynoglossus semilaevis)

Chinese tongue sole(Cynoglossus semilaevis)is an important marine flatfish which is widely cultured in coastal areas in China.Infectious bacterial diseases such as Edwardsiella tarda imposed serious threats to this species.A potential strategy to prevent this disease is to select resistance strains.The aim of the present work was to estimate the genetic variation of disease resistance to E.tarda and to evaluate the genetic correlations between resistance traits and growth traits.Three types of models were fitted by using different trait definitions(binary,continuous and categorical).After a 9-day challenge test,the overall survival was 75.4%(ranging from 6.4%to 100%in families),and at test day 6,the overall survival was 50.7%.We set test day 6 and 9 as cut-off point times respectively.The heritabilities of survival traits were ranging from 0.10 to 0.36.Considerably higher heritability values were obtained at day 6 than at day 9,regardless of which model or trait definition used(except Trait 2 in LIN).The genetic correlations between disease resistance traits and growth traits(i.e.body weight and total length)were low and not significant from zero(−0.12–0.24).There is a substantial re-ranking of families when defined resistance as categorical and continuous traits compared to binary trait.These results confirm the existence of genetic variation for resistance against E.tarda and weak genetic correlations indicate that joint genetic improvement of E.tarda resistance and growth is scarcely available.