THE COMPLETE MITOCHONDRIAL GENOME OF ALLIGATOR-MISSISSIPPIENSIS AND THE SEPARATION BETWEEN RECENT ARCHOSAURIA (BIRDS AND CROCODILES)

The complete mitochondrial genome of the alligator, Alligator mississi ppiensis, was sequenced. The size of the molecule is 16,642 nucleotide s. Previously reported rearrangements of tRNAs in crocodile mitochondr ial genomes were confirmed and, relative to mammals, no other deviatio ns of gene order were observed. The analysis of protein-coding genes o f the alligator showed an evolutionary rate that is roughly the same a s in mammals. Thus, the evolutionary rate in the alligator is faster t han that in birds as well as that in cold-blooded vertebrates. This co ntradicts hypotheses of constant body temperatures or high metabolic r ate being correlated with elevated molecular evolutionary rates. It is commonly acknowledged that birds are the closest living relatives to crocodiles. Birds and crocodiles represent the only archosaurian suviv ors of the mass extinction at the Cretaceous/Tertiary boundary. On the basis of mitchondrial protein-coding genes, the Haemothermia hypothes is, which defines birds and mammals as sister groups and thus challeng es the traditional view, could be rejected. Maximum-likelihood branch length data of amino acid sequences suggest that the divergence betwee n the avian and crocodilian lineages took place at approximate to 254 MYA.