Vibrio Disease, Anti-vibrio Immunity And Vibrio-resistance Selective Breeding Of Clam Meretrix Meretrix

The clam Meretrix meretrix is an important commercial species in the coastal areas of China. Development of M. meretrix aquaculture is beneficial to the exploitation of the beaches and flats. M. meretrix has a broad market which will bring large economic benefits. Same as other cultured mollusks, culture industry of M. meretrix is being negatively affected by the mass disease outbreaks, which have caused heavy economic losses. Disease-resistance selective breeding is an effective method to reduce the impact of disease and keep the sustainable yields of clams. The study about the pathogenicity of pathogen and the immune response mechanism of clam will provide theory and methodology for resistance selective breeding of mollusks.Bacteria, which were isolated from the moribund clams collected from coastal areas of Jiangsu Province of China in the year 2007 when heavy mortality occurred, were identified and characterized in this paper. The results showed that a Vibrio parahaemolyticus strain MM21 and a V. furnissii-related strain MM5 could induce high mortalities of clam M. meretrix. By the means of observation of the histopathological and cytopathological changes in the different tissues of the infected clams, obvious lesions induced by MM21 or MM5 in the tissues of hepatopancreas, gill, and mantle were detected, i.e. the increase of lipid droplets, metabolic disorder, mass secretions, the deposition of granules, and the infestation of the vibrio. The study of pathogenesis and pathogenicity demonstrated that vibrio was the principle pathogen to M. meretrix, among which V. parahaemolyticus could induce the highest mortalities of M. meretrix.The response of immune-related genes of clam M. meretrix to vibrio-infection was analyzed, including an immune-related direct effecter i-type lysozyme and an immune-related indirect effecter heat shock protein 70 (hsp70). The results showed that both in the mRNA level and in the protein level, the expressions of i-type lysozyme in the tissue of hepatopancreas and gill of the vibrio-infected clams were significantly increased compared to those in the control clams. In addition, the recombinant i-type lysozyme displayed antibacterial activity against vibrio, which further demonstrated that i-type lysozyme played an important role in the immune response of clam against vibrio. Similarly, the expressions of hsp70 in the tissue of hepatopancreas and gill of the vibrio-infected clams were also significantly increased, which indicated that hsp70 took part in the anti-vibrio immunity of clams. As a result, i-type lysozyme and hsp70 could reflect the health conditions of clam M. meretrix and be used as parameters to evaluate the effect of resistance breeding.In this paper, vibrio-resistance is chosen as the target and intra-population selection is chosen as the method of the selective breeding. The first filial generation (F1) was obtained from the vibrio-resistant broodstock, which were the ones survived after vibrio-infection. The evaluation based on the mortalities showed that the mortalities of F1 population was significantly lower than that of control population, which indicated that F1 population has gained increased vibrio-resistance by selection. The single nucleotide polymorphism (SNP) in gDNA sequence of i-type lysozyme was analyzed between the obtained resistance population and the control population. The associations study between SNP and resistance showed that ten SNPs were correlated to the vibrio-resistance. Vibrio-resistant F1 population has its own predominant haplotypes compared to the control population. The analysis demonstrated that potential SNP molecular markers could be developed among the SNP of i-type lysozyme and then be used for resistance selective breeding.