The complete cytochrome b gene sequences of the large yellow croaker and the small yellow croaker were obtained and analyzed for the nucleotide compositions and the nucleotide pair frequencies. The contents of T、C、A、G、A+T were 28.4%/26.7%、33.0%/34.1%、23.2%/23.8%、15.4%/15.4% 、51.6%/50.5% in the cytb gene sequences of the large/small yellow croakers. Both gene sequences of these two species show similar traits in the nucleotide compositions and the nucleotide pair frequencies. The first codon positions showed no bias to any of the four nucleotides as a whole, while the second and third positions were rich in T and C respectively. 126 nucleotide sites were substituted and sequences similarity was merely 88.95% between these two sequences. The ratio of transitional to transversional substitution was 3.1 with the transitional rate between pyrimidines(C and T) being higher than that between purines(A and G). Most of the base substitutions occurred at the third loci of the codons.
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