The mitochondrial genome of a monotreme--the platypus (Ornithorhynchus anatinus).

The complete nucleotide sequence of the mitochondrial genome of a platypus (Ornithorhynchus anatinus) was determined. Its overall genomic organization is similar to that of placental mammals, Xenopus laevis, and fishes. However, it contains an apparently noncoding sequence of 88 base pairs located between the genes for tRNA(Leu)(UUR) and ND1. The base composition of this sequence and its conservation among monotremes, as well as the existence of a transcript from one of the strands, indicate that it may have a hitherto-unknown function. When the protein-coding sequences are used to reconstruct a phylogeny of mammals, the data suggest that monotremes and marsupials are sister groups and thus that placental mammals represent the most ancient divergence among mammals.