Abstract：Complement 9 is involved in the membrane attack complex (MAC) forming with other complement molecules on the target cell membrane, and takes part in the immunoreaction through the target cell disruption mediated by MAC. Single nucleotide polymorphisms (SNPs) analysis of genes functionally related to the streptococcosis resistance of nile tilapia (Oreochromis niloticus) can provide useful information for tilapia molecular breeding. In this study, the genomic fragments of complement 9 gene in O. niloticus (OnC9) were amplified, which contain 11 exons and 10 introns. Then 22 SNPs were identified by using directly sequencing and blasting. The SNPs S1 (C483T), S3 (G954A), S4 (T1180C), S5 (G1539A), S6 (G1627T), S7 (A2590T), S10 (T2876G), S11 (C2915G), S12 (T2939G), S13 (A2942C), S14 (A3544C), S15 (T3604C), S16 (T3670C), S17 (T3841C), S18 (T4339C), S19 (G4494C), S20 (A4678G), S21 (T5790A), and S22 (T5816G) were in introns of OnC9, while the SNPs S2 (C644T), S8 (C2660T), and S9 (A2751T) were in exons. Half of the SNPs in OnC9 was produced by base transition, and half of them was produced by base transversion. In addition, S8 was synonymous mutation and S2, S9 belong to the non-synonymous mutations. 81 tilapias from the resistance group (RG) and 84 tilapias from the susceptible group (SG) were used to analyze genotype polymorphisms by snapshot method. The polymorphisms and genetic parameter of the OnC9 SNPs in SG and RG were calculated by the software Popgen32. The result showed that the polymorphism information content (PIC) of the OnC9 SNPs was 0.09~0.37, suggesting that all the SNPs locus had moderate or intermediate polymorphism. In order to analyze the correlation between the OnC9 SNPs and the phenotype of streptococcosis resistance, the genotype frequency, allele frequency of the 22 SNPs in OnC9 was analyzed by SPSS 20 and tested by chi-square test. The association analysis showed that Two SNPs (S8 and S21) were significantly associated with streptococcosis resistance (P＜0.05). Moreover, all the OnC9 SNPs could formed 4 haplotype blocks and 11 haplotypes from the prediction of haplotypes and linkage disequilibrium analysis used software Haploview 4.2. And 2 of the haplotypes (GTTAG and ATGA) were significantly associated with Streptococcus agalactiae susceptible (P＜0.05). These results indicate that the OnC9 gene can be a candidate for tilapia Streptococcus agalactiae resistance traits which could be useful for molecular breeding of O.niloticus.
