Phylogenetic perspective on the relationships and evolutionary history of the Acipenseriformes

The Acipenseriformes, as one of the earliest extant vertebrates, plays an important role in the evolution of fishes and even the whole vertebrates. Here we collected and analyzed all complete mitochondrial genomes of Acipenseriformes species. Phylogenetic analyses demonstrated that the polytomous branch included Acipenseridae and Polyodontidae formed five clades. The Polyodontidae clade and the Scaphirhynchus clade both were monophyletic group, whereas the Acipenser species and the Huso species both were polyphyletic group. The Bayesian divergence times showed that the origin time for Acipenseriformes was at 318.0 Mya, which was similar to the some previous results of 312.1 Mya, 346.9 Mya and 389.7 Mya. The result was in good consistent with the paleontological data available and the split time of the Pacific and Atlantic Oceans from the Jurassic to the Cretaceous (Laurasia splits in North America and Eurasia). The dN/dS ratios showed the evolutionary rates gradually slow down in five major Acipenseriformes clades from the Clade A (the Pacific sturgeons species) to Clade C (the genus Scaphirhynchus), which was related to the process of geographical formation.