Preliminary Analysis Of Growth And Resistance Differences Among Selected Meretrix Meretrix Populations

The clam Meretrix meretrix is one of the important commercial aquaculture species in China. Although a selective breeding program for genetic quantitative trait improvement of M. meretrix in China was started, the phenotype-artificial selection on M. meretrix still needs to be developed. In this thesis, we intend to analyze mechanisms that make selected populations of M. meretrix have difference in the growth and resistance and identify the gene markers related to the growth and resistance, which would further increase the selection efficiency and make a rapid breeding progress.Firstly, we compared the growth performance (shell length, shell height, shell width and body weight) of four M. meretrix populations and got several important points: Shell color pattern could be inherited steadily after selection; F3SB had growth advantage over F3SP after three generations selection; Growth performance of the selected lines was influenced by the line×site interaction; Encouraging genetic gain could be produce by truncation selection from a hybrid line. The results also indicated shell color patterns, artificial selection, site environments and G×E interactions are some of the reasons make populations of M. meretrix have difference in the growth.Based on the population analyzed above, we made association study of SNPs in some functional important genes with the growth traits of M. meretrix. A FBA gene was identified in M. meretrix and its deduced amino acid residues shared high identity with type I aldolase which involves in energy metabolism. The FBA (MmeFBA) mRNA expression profile was examined and the high expression level in muscle tissue and in pediveliger stage supported that MmeFBA was involved in energy metabolism. In the MmeFBA gene, we identified four intron SNPs and three exon SNPs including a nonsynonymous SNP (mmfbae-2). These SNPs were genotyped in 205 clams from two clam populations with significantly different growth performance. The association analysis indicated three SNPs (mmfbai-1, mmfbai-3 and mmfbae-2) were growth related. This study suggested FBA gene could be a growth related gene and energy metabolism might contribute to the growth of M. meretrix.Three SNPs (MmeSUl-696, MmeSUl-714 and MmeSUl-806) were discovered in a SULT-like gene of M. meretrix. A modified Tm-shift assay was developed to perform polymorphism analysis of the three SNPs in two families and two populations of M. meretrix. The association analysis indicated MmeSUl-806 significantly (P<0.05) correlated with growth traits of M. meretrix. This study suggested SULT gene could be a growth related gene, and the sulfation of xenobiotics and endogenous compounds might be important for the growth of M. meretrix.To analyze mechanisms that make selected populations of M. meretrix have difference in the resistance, we cloned and analyzed a selenium-dependent glutathione peroxidase (MmeGPx) gene. The MmeGPx gene was composed of two introns of 723 bp and 238 bp and an open reading frame (ORF) of 711 bp. The ORF encodes a protein of 237 amino acids with a putative selenocysteine residue encoded by an unusual stop codon. MmeGPx shares a higher level of similarity with human GPx 3 than with other human GPx isozymes. The level of MmeGPx mRNA roughly paralleled GPx enzyme activity in different tissues except in gills, with the highest mRNA expression and enzyme activity occurring in hepatopancreas. MmeGPx mRNA expressions were detected in different larval stages and the results showed that MmeGPx mRNA increased significantly in pediveliger stage, which may be a response to oxidative stress. After challenge of clam with a Vibrio parahaemolyticus-related bacterium (MM21), the expression of MmeGPx was significantly up-regulated at 6 h and 12 h in hepatopancreas, which suggested that MmeGPx may be involved in the immune response to MM21 infection. Then we compared the expression of MmeGPx in hepatopancreas between a selected Vibrio-resistant population and a control population after immersion challenge with MM21. Early up-regulation of MmeGPx was observed in the resistant population. These results suggested that MmeGPx might be involved in maintaining the redox state of immune system, and the early immune response to pathogen infection may help the clam against pathogen infection.