Screening And Analysis Of MHC II B Alleles Associated With Disease Resistance In Turbot

Along with fast development of aquaculture, fish diseases occured frequently as the result of high breeding density and worsen environment. It is necessary to breed anti-pathogen stock for aquaculture development. The problems also need to be resolved for turtot, which is an important breeding fish in China. As MHC II B genes play a crucial role in innate and adaptive immune responses and a lot of evidences proved the association between different MHC II B alleles and resistance or susceptibility to pathogens. We studied the MHC polymorphism and association between alleles and resistance to Edwardsiella tarda here.1. Characterization of turbot pathogenTurbot pathogen- Edwardsiella tarda was characterized by 16S rDNA sequence analysis. The pathogen was islated from abdomen cavity as single clone during disease burst period and preserved in our lab. Part of 16S rDNA was amplified with 16S rDNA conserved primer 8F and 1492R of bacteria and sequenced. Three sequences with 1505bp length were obtained with small difference. High similarities were discovered with Edwardsiella ictaluri and Edwardsiella tarda by blast analysis in Genbank. Higher similarity with Edwardsiella tarda was revealed by phylogenetic tree analysis. And one of these sequenes, 16S rDNA-2, shared 100% similarity with Edwardsiella tarda via alignment analysis with Edwardsiella ictaluri and Edwardsiella tarda. Considering sequencing error and the difference of different copies of 16S rDNA, the pathogen of turbot was characterized as Edwardsiella tarda.2. MHC II B polymorphism and associations with pathogen resistance in turbotFour types of intron 1 were discovered, named as A, B, C and D, which were different at their length resulting from the repeats of a 19bp repeat element. There is a CT/GT rich region at 3` end of intron1, which is the potential site recognized by RecA, indicating that recombination might have happened.59 alleles in total were discovered from 42 susceptible and 43 resistant individuals, including 35 of type A, 6 of type B, 13 of type C and 5 of type D. 25 of them were used to analyze distribution between resistant and susceptible stocks. Some alleles appeared in both susceptible and resistant stock with different frequency, while some only appeared in susceptible or resistant stock with low frequency. The allele Scma-DBB1*04 was discovered to be associated with susceptibility to Edwardsiella tarda with frequency 7% in resistant stock and 42.9% in susceptible (p=0.000). The allele Scma-DBB2*01 was associated with resistance, 44.2% in resistant stock and 19% in susceptible (p=0.013).