THE COMPLETE MITOCHONDRIAL GENOME OF THE WALLAROO (MACROPUS-ROBUSTUS)AND THE PHYLOGENETIC RELATIONSHIP AMONG MONOTREMATA, MARSUPIALIA, ANDEUTHERIA

The complete mitochondrial DNA (mtDNA) (16,896 nt) of the wallaroo (Ma cropus robustus) was sequenced. The concatenated amino acid sequences of 12 mitochondrial protein-coding genes of the wallaroo plus those of a number of other mammals were included in a phylogenetic study of ea rly mammalian divergences. The analysis joined monotremes and marsupia ls (the Marsupionta hypothesis) to the exclusion of eutherians. The an alysis rejected significantly the commonly acknowledged Theria hypothe sis, according to which Marsupialia and Eutheria are grouped together to the exclusion of Monotremata. The region harboring the gene for lys ine tRNA (tRNA-Lys) in the mtDNA of other vertebrates is in the wallar oo occupied by a sequence (tRNA-Lys) that lacks both an anticodon loop as well as the anticodon for the amino acid lysine. An alternative tR NA-Lys gene was not identified in any other region of the mtDNA of the wallaroo, suggesting that a tRNA-Lys of nuclear origin is imported in to marsupial mitochondria. Previously described RNA editing of tRNA-As p and rearrangement of some tRNA genes were reconfirmed in the mtDNA o f the wallaroo. The divergence between Monotremata/Marsupialia and Eut heria was timed to approximate to 130 million years before present (MY BP). The same calculations suggested that Monotremata and Marsupialia diverged approximate to 115 MYBP and that Australian and American mars upials separated approximate to 75 MYBP. The findings also show that m any, probably most, extant eutherian orders had their origin in middle to late Cretaceous times, 115-65 MYBP.