Estimation Of Genetic Parameters Of Disease-resistant Traits And Selective Breeding For Disease Resistance Strains In Chinese Tongue Sole (Cynoglossus Semiliaevis)

Chinese tongue sole(Cynoglossus semilaevis)is an important marine flatfish which is widely cultured in coastal areas in China.With the development of production scale and intension of aquaculture industry of tongue sole,high-stress rearing environmental conditions such as high stocking density,inadequate nutrition and problems related to water quality should be responsible for frequent bacterial disease outbreaks with significant production losses.Given this,in this paper survival data obtained from challenge tests,natural outbreaks and juvenile survival was used for estimating genetic parameters.Summary results were as follows:1.By using V.harveyi challenge test data of(involving 50 full-sib families and 8547 individuals),heritabilities of resistance traits and genetic correlations between resistance traits and growth traits(body weight and total length)were estimated.Longitudinal linear models and cross-sectional threshold models were fitted by using different trait definitions(binary and categorical).After a 14-day test,the overall challenge test survival was 57.86%,family survival rates ranging from 9.3% to 94.3%.The heritabilities of survival were ranging from 0.05 to 0.39,depending on the models used and trait definitions,estimates obtained by sire-dam models were higher than animal models.The genetic correlations between resistance and two growth traits were moderately positive(0.27 – 0.51).Very high Pearson correlations(0.860 – 0.998)between full-sib family EBVs by using different models fitted with same or similar trait definitions,however,the Pearson correlations(0.412 – 0.494)between full-sib family EBVs by using different models fitted with different trait definitions were moderate high,which indicating the similar predictive ability between models with same or similar trait definitions.The favourable heritabilities and medium positive genetic correlations indicate that joint genetic improvement of vibriosis resistance and growth performance would be feasible.2.The genetic variation of disease resistance to E.tarda and the genetic correlations between resistance traits and growth traits were estimate by using challenge test survival data(involving 39 families and 3727 individuals).After a 9-day challenge test,the overall mortality 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.Three types of models were fitted by using different trait definitions(BTS,DD and BTDS).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 DD 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 continuous traits(DD)and repetitive 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.3.To verify whether the resistance of vibriosis(caused by co-infections of different Vibrio spp.in natural outbreak)can be included in breeding programs,genetic analysis of resistance to vibriosis based on natural outbreak survival data were carried out by using four statistical models(three cross-sectional models and one longitudinal model).The magnitude of the genetic variation in the resistance of vibriosis was estimated through a 56-day natural outbreak test of 15912 individuals from 78 full-sib families.Variance components and heritabilities were estimated at two cut-off points respectively,i.e.day 35 with 49.2% cumulative mortality and day 56 with 71.6% finial cumulative mortality.Heritabilities of resistance to vibriosis were low to moderate,where values at day 56(0.04 – 0.21)were larger than at day 35(0.03 – 0.10)in each corresponding model.The Spearman rank correlations between family EBVs for different models were very high(>.98),indicating a nearly identical ranking of families.Compare to three simpler cross-section models,the longitudinal model taken time until death into account demonstrated the highest model accuracy of family selection.These results confirmed the existence of genetic variation for resistance to vibriosis and provided novel insights into the selective breeding for disease resistance to vibriosis by using natural outbreak survival data in tongue sole.4.As we all known,it is of great importance to increase the natural disease resistance of farmed fish.Genetic variances and heritabilities of natural disease resistance were estimated by using juvenile natural survival data(involving three year-classes,two generations with 221 full-sib families,195589 individuals).Survival was defined as binary trait(dead / alive)fitted in two cross-sectional models(i.e.cross-sectional linear sire-dam model(CLM)and cross-sectional threshold(logit)sire-dam model(CTM)).Heritabilities of survival were estimated with each year-class dataset and with complete dataset.Heritability estimates varied among year-classes regardless of model used.On the observed(CLM)and underlying(CTM)scale with complete dataset,the heritabilities were 0.09 ± 0.04 and 0.13 ± 0.06 respectively.The Pearson correlations between full-sib family EBVs in two models were very high(>.98).Both models performed nearly identical predictive ability with very high selection accuracy(>.99),the model accuracy obtained from CLM(0.993)was slightly higher than CTM(0.991).Very high Pearson and Spearman(ranking)correlations(>.98)between full-sib family EBVs,indicating similar ranking of families in the both models.The selection response was 0.06,the expected genetic gain was 11.8%,which means that the juvenile natural survival is expected to achieve an improvement of 11.8% units per generation.These results confirmed the existence of genetic variation for juvenile natural survival and highlighted the enormous potential for improving natural survival by selective breeding in tongue sole.