Axel Janke

The complete mitochondrial (mt) genomes of five marsupial species have been sequenced. The species represent all three South American orders (Didelphimorph

The complete mitochondrial genomes of two reptiles, the common iguana (Iguana iguana) and the caiman (Caiman crocodylus), were sequenced in order to investigate phylogenetic questions of tetrapod evolution. The addition of the two species allows analysis of reptilian relationships using data sets other than those including only fast–evolving species. The crocodilian mitochondrial genomes seem to have evolved generally at a higher rate than those of other vertebrates. Phylogenetic analyses of 2889 amino–acid sites from 35 mitochondrial genomes supported the bird–crocodile relationship, lending no support to the Haematotherma hypothesis (with birds and mammals representing sister groups). The analyses corroborated the view that turtles are at the base of the bird–crocodile branch. This position of the turtles makes Diapsida paraphyletic. The origin of the squamates was estimated at 294 million years (Myr) ago and that of the turtles at 278 Myr ago. Phylogenetic analysis of mammalian relationships using the additional outgroups corroborated the Marsupionta hypothesis, which joins the monotremes and the marsupials to the exclusion of the eutherians.

The complete mitochondrial genome of the alligator, Alligator mississippiensis, was sequenced. The size of the molecule is 16,642 nucleotides. Previously reported rearrangements of tRNAs in crocodile mitochondrial genomes were confirmed and, relative to mammals, no other deviations of gene order were observed. The analysis of protein-coding genes of the alligator showed an evolutionary rate that is roughly the same as in mammals. Thus, the evolutionary rate in the alligator is faster than that in birds as well as that in cold-blooded vertebrates. This contradicts hypotheses of constant body temperatures or high metabolic rate 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 survivors of the mass extinction at the Cretaceous/Tertiary boundary. On the basis of mitochondrial protein-coding genes, the Haemothermia hypothesis, which defines birds and mammals as sister groups and thus challenges the traditional view, could be rejected. Maximum-likelihood branch length data of amino acid sequences suggest that the divergence between the avian and crocodilian lineages took place at approximately equal to 254 MYA.