CLONING AND SEQUENCE EVOLUTION ANALYSIS OF SOX GENESIN BIGHEAD CARP (ARISTICHTHYS NOBILIS)

To amplify Sox genes from bighead carp genome, two pairs of degenerate primers (SoxN and Sox9) were designed for PCR amplification that were utilized for sequencing and analysis of sequence homology and phylogenetic relationships. The results showed that 15 distinct Sox genes encoding the HMG domains were identified in bighead carp, which were assigned to group B, C and E. According to their homology to orthologs of zebrafish, 15 Sox genes were designated as Sox1a, Sox1b, Sox2, Sox3, Sox4a, Sox4b, Sox9a, Sox9b, Sox10, Sox11b, Sox12, Sox14a, Sox14b, Sox19 and Sox21a, respectively. Phylogenetic tree of Sox1, Sox4 and Sox9 nucleotide sequences indicated that the duplication of these three Sox genes occurred before the divergence of bighead carp and zebrafish, and this observation supported the fish-specific whole-genome duplication theory. Using Sox1a, Sox1b and Sox4 as molecular clock markers in phylogenetic analysis, the estimation of the divergence time between bighead carp and zebrafish demonstrated that in original Danioninae, a common ancestor appeared approximately 63.7 million years ago for bighead carp and zebrafish, both of which belong to Cyprinidae. This study would provide important information for further studies of Sox gene replication and genome evolution in fish.