A new perspective on avian phylogeny - a study based on mitochondrial genomes

Sammanfattning: The Class Aves (birds) is generally divided into two main groups - the Palaeognathae and the Neognathae. Paleognathous birds are the flightless ratites and the tinamous, while Neognathae include all other extant birds (more than 9,000 species). This opinion has prevailed for more than a century and is primarily based on morphological studies of the avian bony palate. In this study, the relationships between Palaeognathae and Neognathae along with other avian divergences have been investigated in phylogenetic analyses of complete mitochondrial (mt) genomes as well as in analyses of the mt cytochrome b gene. The complete mt genomes of the ostrich (Struthioniformes; Struthio camelus), the greater rhea (Struthioniformes; Rhea americana) and the rook (Passeriformes; Corvus frugilegus) were sequenced. At the time when this study was initiated and conducted, only one complete avian mt genome had been presented, that of the chicken (Galliformes; Gallus gallus), but this year four other complete avian mt genomes (representing the orders Anseriformes, Falconiformes and Passeriformes) were released. The phylogenetic analyses were carried out on the concatenated nucleotide and amino acid sequences (9,597 and 3,199 characters respectively) of twelve mt protein-coding genes from avian, mammalian, reptilian and piscine taxa. The results of the phylogenetic analyses did not support the most commonly accepted view of a basal divergence between paleognaths and neognaths. Instead, the earliest divergence among the taxa included was between the Passeriformes and all other birds including the ratites which were reconstructed at a more apical position in the tree as a sister group of galliforms and anseriforms. The origin of the Passeriformes was estimated to the early Cretaceous, about 120 MYBP (million years before present). A basal position of the ratites has in fact been questioned before, suggesting that ratites are not "primitive" birds but that ratite morphology includes a number of juvenile or neotenous traits (for example the downy plumage and the paleognathous palate) which have been maintained in adulthood. Because of the lack of pre-Tertiary passerine fossils (> 65MYBP) the order Passeriformes is generally regarded as one of the most recent avian lineages. It has been proposed, however, that the order Passeriformes originated already in the Cretaceous and that a recent origin of passerines is due to a misinterpretation of the fossil record. It is probable that the discrepancies between this study and the previous ones are attributable to the considerable length of the present dataset. Most importantly, however, the inclusion of a non-avian outgroup permits a different rooting strategy than used in most studies of avian relationships to date.