| Turbot(Scophthalmus maximus L.)is a cold-water,carnivorous and bottom fish,which distributs in the Atlantic coast of Europe.With fast growth,strong tolerance to cold water,wide salinity suitability,less diseases,kill well and delicious flavor,turbot is the most widely cultured commercial flatfish in the world.From its introduction into China by academician Lei Jilin of Yellow sea fisheries research institute in 1992,turbot aquaculture has developed into an important mariculture industry with an annual production of 60,000 t.From 2006,selection breeding of turbot has been carried out for ten years and some progress have made.At the same time,some problems are exposed,such as lack of effective pedigree and small genetic gain.In this study,genetic gain was first assessed using recorded weight data and pedigree in past ten years.Then,two new methods,that combining molecular marker and BLUP,were developed to estimate genetic parameter and breeding values,which are expected to speed up selection progress.1.Response to selection for body weight in turbotGenetic parameters and response to selection were estimated for body weight in turbot.The data consisted of 10952 individuals of 508 full-sib families from three generations(G0,G1,and G2).The heritability estimates for G0,G1,and G2 were 0.11±0.08,0.18±0.09,and 0.17±0.07,respectively.Over three generations,the heritability estimate was 0.19±0.04.Maternal and common environmental effects were 0.10±0.04,0.14±0.04,and 0.13±0.03 within each generation and 0.13±0.01 across generations.The selection differential in growth was 18.24 g in G0 and 21.19 g in G1 corresponding to an average of 21.41 g per generation.The genetic gains were also calculated,they were 22.06 g in G1 and 11.93 g in G2,corresponding to 6.36%and 3.52%body weight.The total genetic gain after two generations was 10.10%body weight,which indicated that the selective breeding program for the body weight trait in turbot was successful.2.Estimating genetic parameters for growth traits with molecular relatedness in turbotSelective turbot breeding is restricted because there is no reliable pedigree data.The objective of this studywas to evaluate the performance of a molecular relatedness method to estimate genetic parameters for growth traits in turbot.The experimental population consisted of 843 15-month-old turbot from 79 full-sib families produced via fertilization involving 50 sires and 34 dams.Twenty unlinked microsatellite loci in this population were genotyped to calculate their molecular relatedness.Both molecular relatedness and pedigree were used to construct an additive genetic matrix to apply to the same animal model and estimate genetic parameters and breeding value.Thereafter,we compared the accuracy of two estimators by cross validation.Heritability for body weight and length were 0.33± 0.15 and 0.24 ±0.14,respectively,based on pedigree and both were 0.23± 0.04 based on molecular relatedness.Genetic correlation and phenotypic correlationwere 0.96±0.02 and 0.87±0.01,respectively,based on molecular relatedness and 0.99 ±0.02 and 0.89±0.01,respectively,based on pedigree.Cross validation revealed that the accuracy of estimated breeding values was 0.85 with pedigree and 0.92 with molecular relatedness.These results suggest that molecular relatedness is a feasible approach to genetic parameter estimation when pedigree information is either inaccurate or absent.3.Identification of SNP releted to body weight in five month age turbot and its application in marker assist BLUPIn order to further identify single nucleotide poly morphism(SNP)associated with the early growth traits of turbot.TaqMan Probe Real Time PCR was used to genotyping four SNPs loci of seven turbot families,180 individuals in total.The candidate four SNPs were analyzed based on the high-density genetic linkage map.At first,normal distribution tests and homageneity of variance tests were used to verify whether data sets were Suitable for general linear model analysis.Then,general linear models containing family effect,genetype effect and their interaction effect were used to analyze the association between SNP genetypes and body weight.Our results showed that only one locus,SNP105,was associated with body weight significantly(P<0.01),which indicates traits associated markers based on mapping population were tended to not associate with trait in a more complex population due to differences in genetic background.Verification of SNP 105 provided the possibility of molecular marker assist breeding.Four candidate SNPs loci were further used to genotyping mixed group of 927 juvenile fish from 40 families.Data sets were analyzed by four marker assisted BLUP model,which contained fix effets of genotype.The same results as general linear models were obtained and only SNP 105 was associated with body weight significantly(P<0.01).Fixed effect of genotype AA is larger than those of GA and GG,which indicates individuals with genotype AA in SNP 105 locus have advantages of growth.There are some differences between estimated breeding values based on marker assisted BLUP and common BLUP,and pearson correlation coefficient is 0.75.The ranks of family average breeding values basen on two method were also different.There are seven in the first 20 families based on marker assisted BLUP were not in the first 20 based on the other method,which indicated marker assisted BLUP changed the selection results greatly.4.Expression of Cygb-2 mRNA in various tissues and its association analysis with body weightA Cygb-2 gene was found to be closed to the SNP 105 with physical distance of about 1600bp by sequence alignment,which showed significant association with the body weight trait.To further to confirm whether the association was caused by Cygb-2 gene,we analyzed the relative expression levels of this gene in different tissues with the method of real time fluorescent quantitative PCR and did regression analysis using the highest tissue relative expression level and body weight data.The results showed that the Cygb-2 gene mRNA were highly expressed in brain.Nevertheless regression analysis results did not support the Cygb-2 gene relative expression level in brain had a significant effect on body weight in five months juvenile turbot. |
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