THE MTDNA SEQUENCE OF THE OSTRICH AND THE DIVERGENCE BETWEEN PALEOGNATHOUS AND NEOGNATHOUS BIRDS

The complete mitochondrial DNA (mtDNA) molecule of the ostrich, Struth io camelus, was sequenced. The size of the molecule is 16,591 nucleoti des. Since the ostrich represents the paleognathous birds, comparison with the mtDNA of the neognathous chicken, the only avian species repo rted so far in databases, made it possible to identify common and, pro bably, general avian mtDNA characteristics. Relative to other vertebra tes, the avian NADH6 and tRNA-Glu genes are positioned upstream of the control region rather than the cytochrome b gene. The NADH3 gene of t he ostrich is terminated by a stop codon at position 207. Thus, the ge ne is about 130 nucleotides shorter than in other vertebrates. The seq uence for L-strand origin of replication is missing in both birds, and four transfer RNA genes of the two avian mtDNAs deviate from common c haracteristics of tRNAs of vertebrate mtDNAs by having an adenine (and not a thymidine) at position 8. Due to the absence of suitable fossil s, most paleontological datings of avian divergences are conjectural. Molecular dating of the divergence between the ostrich and the chicken indicates that these two avian lineages separated 80-90 MYA. Phylogen etic analysis of complete cytochrome b genes of six avian orders showe d that Passeriformes represent the earliest divergence among recent bi rds, contradicting the commonly accepted notion of a basal position of the Palaeognathae among recent birds.