Molecular Evolution Of Partial Acipenseriformes Species Inferred From Mitochondrial COⅠ Gene

Acipenseriformes is the only existing large-sized Chondrostei species of the Osteichthyes. Their unusual mixture of characters and their ancient status which make them to be the historical ideas for classification and evolution of fishes. We measured 15 different mitochondrial cytochrome c oxidase subunitⅠgene of this group (8 species retrived from GenBank,7 species directly sequenced), analyzed the molecular evolutionary characteristics and reconstructed the phylogenetic trees. The main results are as follows:1. Comparison of 681bp section of the mitochondrial DNA cytochrome c oxidase subunitⅠgene revealed 174 variable sites with 120 parsimoniously informative nucleotide positions. No single insertion or deletion was observed. Outgroup sequence length was 681bp. Base composition bias was obvious in those sequences, average Guanine nucleotide (G) level was significantly lower than the other three nucleotides.2. At the amino acids level, there also observed codon usage bias. The Ts/Tv ratio in the first codon positions was 23.1, in the third codon positions was 6.0, while no variation happened in the second codon positions. The genetic distance (K) between species was ranging from 0.0074 to 0.1576, and the average distance between Acipenseridae and Polyodontidae was 0.1408 based on Kimura's two-parameter model. The number of both transitions and transversions showed an approximately linear relationship with distance, indicated that those sequences haven't saturated.3. Nonsynonymous changes were much less frequent than synonymous changes. Pairwise ratio of nonsynonymous to synonymous (dN/dS) were all below one. The overall mean ratio of dN/dS fixations between species was 0.0221(dN=0.0095±0.0025; dS=0.4306±0.0379), indicated that purifying selection was the predominant force during its evolution. The neutral test demonstrated the evolution of certain gene was deviate from neutral mode, but it didn't reach statistical significant level (P>0.1).4. According to the result of relative rate test, species remained consistent evolution rate except Acipenser stellatus. Based on average genetic distance combined with divergence time between Acipenseridae and Polyodontidae, we applied the molecular clock formula V=K/2T to estimate the mtCOⅠgene evolution rate. The result was 0.048-0.062 subs/site year-110-9.5. Whether the genus Acipenser should be subdivided needs further investigation on the basis of phylogenetic relationships and the genetic differences between species.