The Complete Mitochondrial Genomes Of Three Crocodiles And Phylogeny Of Inter-genera In Crocodylidae

The nucleotide sequences of the complete mitochondrial DNA (mtDNA) molecules of the salt-water crocodile (Crocodylus porosus), the African slender-snouted crocodile (Crocodylus cataphractus) and the African dwarf crocodile (Osteolaemus tetraspis) were determined. The size of molecules was 16917 bp, 16847 bp and 16873 bp, respectively. The organization of molecules conformed to that of other vertebrate mtDNA, including 22 tRNAs, 2 rRNAs, 13 protein-coding genes, as well as a control region. The arrangement of the genome was the same as that of published crocodilians. However, it differed from that of other vertebrates, i.e. the rearrangement of tRNAPhe, and the tRNASer-tRNAHis-tRNALeu cluster, which showed conservation of the crocodilian gene arrangement.Based on mtDNA genome and the control conserved regions of crocodiles, the phylogenetic analyses supported that crocodiles were grouped into two families, Alligatoridae and Crocodylidae, with the latter encompassing traditional Crocodylidae plus Gavialis/Tomistoma; C. cataphractus was not a member of Crocodylus or Ostcolaemus, and was treated as an unattached genus Mecistops. Within Crocodylidae, the inter-genera relationship was ((Gavialis + Tomistoma) + (Crocodylus + (Mecistops + Ostcolaemus))).The divergence times were estimated using the ML branch lengths. The divergence among extant crocodilians was estimated to have taken place at≈145 MYA. The split between Gavialis/Tomistoma and Crocodylus was placed at≈86 MYA. The divergence between Mecistops/Ostcolaemus and Crocodylus was estimated at≈53 MYA in the Early Tertiary. Those divergence times estimated here were roughly consistent with that based on some molecular data, however, that was slightly earlier than fossils record.