| The major histocompability complex (MHC) is hetero-dimer gene encoding glycoprotein which persents peptides to T4 cell, and plays a central role in immune systerm.In this study, Ctenopharyngodon idellas were challenged with Flabobacterium columnare and Aeromonas hydrophila respectively, polymorphism and the associations between some genetypes and resistance ability of MH-DAB gene were analyzed by PCR-SSCP and sequenceing between the resistant stock and the susceptible stock. Meanwhile, the mRNA level of MH-DAB was dedected by Real-Time RT-PCR in different times after infection. The results were showed as follows:1.The complete exon 2 of MH-DAB gene of Ctenopharyngodon idella was amplified for the first time, the length of cloned sequences was 276bp, and 8 new alleles of MH-DAB were found, which were named as Ctid-DAB*0101,Ctid-DAB*0102, Ctid-DAB*0201, Ctid-DAB*0202, Ctid-DAB*0301,Ctid-DAB*0302, Ctid-DAB*0401 and Ctid-DAB*0402, respectively.2.Phylogenetic analysis of MH-DAB genes in Ctenopharyngodon idellas were showed that the relationship was nearest with the corresponding sequence of carp, such as Labeobarbus intermedius and Rhodeus ocellatus; relatively near with other teleosteis, such as Epinephelus akaara and Salmo Salar; and relatively distant with selachian and mammalian.3.Standardized disease challenges with the bacterium Flabobacterium columnare and Aeromonas hydrophila were revealed eight unique MH-DAB genotypes and six alleles in the Ctenopharyngodon idellas that challenged with Flabobacterium columnare, of which genotypes AA(Ctid-DAB*0101/*0101),DD (Ctid-DAB*0102/*0102), EE (Ctid-DAB*0401/*0401),DE(Ctid-DAB*0102/*0401)and alleles A (Ctid-DAB*0101),D (Ctid-DAB*0102),E (Ctid-DAB*0401)were shared by the resistant stock and the susceptible stock. Four genotypes BB (Ctid-DAB*0201/*0201), CC (Ctid-DAB*0301/*0301),AB(Ctid-DAB*0101/*0201),DF(Ctid-DAB*0102/*0401) and three alleles B (Ctid-DAB*0201),C(Ctid-DAB*0202),F(Ctid-DAB*0301) were only detected in the resistant stock. Using Chi-square Test,genotypes AA (Ctid-DAB*0101/*0101),BB(Ctid-DAB*0201/*0201),EE(Ctid-DAB*0401/*0401),DE (Ctid-DAB*0102/*0401)and alleles A (Ctid-DAB*0101),B(Ctid-DAB*0201),E (Ctid-DAB*0401) were found having a significant association with high resistance to Flabobacterium columnare (P<0.01).The Ctenopharyngodon idellas that challenged with Aeromonas hydrophila were revealed five unique MH-DAB genotypes and four alleles, of which genotype AH(Ctid-DAB*0101/*0402)could only be detected in the susceptible stock, genotype AA (Ctid-DAB*0101/*0101) and allele Ctid-DAB*0101 were found having a significant association with high susceptiviry to Aeromonas hydrophila (P<0.01).4.MH-DAB gene of common Ctenopharyngodon idellas were expressed in all tissues, but the expression level was different, high expression was detected in brain, muscle, spleen, skin and gill, moderate expression in intestine and head kidney, and low expression in liver and heart. Compared with the control group, after 2 days challenged with Flabobacterium columnare, the expression of MH-DAB gene was decreased in liver, heart, stomach, intestine and muscle in the experimental groups,the expression in liver and intestine were decreased by 0.5 and 0.33 times (P<0.05), respectively, but the expression in spleen, gill and skin was increased a little.After 15 days infection, the expression of MH-DAB gene in all tissues was all higher than that in the control group, except in stomach and intestine, and the level of MH-DAB gene expression in skin increased by 2 times (P<0.05) to control group.5.After challenged with Aeromonas hydrophila, the tissues expression of MH-DAB gene had a significant change in Ctenopharyngodon idellas. Compared with control time(Oh),challenged after 6h, the expression of DAB gene was significantly increased in all tissuse (P<0.05), respectively. Challenged after 12h, the expression of DAB gene was decreased in all tissues except in gill and spleen, which was increased by 4.3 times and it reached the top level in gill all times(P<0.05).Challenged after 24h, the expression of DAB gene was decreased in all tissues except in spleen and intestine, which were maximizd by 12.4 and 5.2 times(P<0.01),respectively. Challenged after 48h, there was an increase in head kidney and liver, but a decrease in spleen, intestine and gill, and the expression in head kidney was 4.2 times compareing the control (P<0.05).From 72h to 96h, the expression of DAB was decreased in all tissues.
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