THE STRUCTURE, ORGANIZATION AND EVOLUTION OF AVIAN MITOCHONDRIAL DNA

A detailed physical map of the mitochondrial DNA of chickens has been prepared. This map defines the relative positions of 45 restriction endonuclease sites, the relative positions of the D-loop (heavy strand replicative origin) and rRNAs and the approximate map positions of an additional eight to ten mtDNA transcripts.;Mitochondrial RNAs (mtRNAs) were mapped by Northern hybridizations of radiolabelled restriction fragments to filter immobilized mtRNA. These hybridizations have revealed 10-12 mtDNA transcripts; two of these transcripts are present in vast molar excess over all other mtRNAs and presumably represent the rRNAs. The genes for these two rRNAs were further mapped on the chicken mitochondrial genome relative to other restriction sites and the D-loop by additional Northern hybridizations. The results of these studies indicate that the two rRNA genes are adjacent to the D-loop in the relative order: large RNA - small rRNA - D-loop. The organization of the rRNA region of chicken mtDNA thus appears identical to the same region in the mtDNA of all higher animals studied to date. These findings suggest that the organization of animal mitochondrial genomes has been conserved in evolution.;Despite the apparent organizational conservation of animal mtDNAs, recent reports indicate that the nucleotide sequence of animal mtDNAs is not evolutionally conserved. In order to study the intraspecific and interspecific sequence divergence of mtDNA in birds, detailed comparisons were made of restriction endonuclease maps of mtDNA of five species of birds in the avian order Galliforme, including the chicken, Guinea fowl, ring neck pheasant, turkey and Japanese quail. The results of these studies indicate that the Galliforme mtDNAs display from 9% to 18% sequence divergence, depending on the comparison being made. These estimates of sequence divergence, while indicating that the sequence of mtDNAs can undergo extensive divergence, are much lower than the sequence divergence of other animal mtDNAs of similar taxonomic recognition. The significance of these findings is discussed in terms of the evolutionary history and phylogeny of the Galliforme birds. The levels of intraspecific sequence divergence of avian mtDNA also appear to be lower than in other animal mtDNAs. A survey of 30 different chicken mtDNAs revealed only one variant restriction pattern. However, the mtDNA of the jungle fowl, presumed modern ancestor of the chicken, has a restriction map identical to the most common form of chicken mtDNA. It is hypothesized that the mtDNA of chickens has undergone a relatively recent evolutionary bottleneck during the period of domestication.;The D-loop of chicken mtDNA has been localized on the restriction map of chicken mtDNA by (1) electron microscopic visualization of the D-loop in restriction fragments of chicken mtDNA, (2) identification of restriction fragments containing DNA Polymerase I extended D-loops by autoradiography and (3) Southern hybridizations of purified, radiolabelled 7S DNA to restriction fragments of chicken mtDNA. The results of these studies have indicated that the D-loop, with an average size of 700 base pairs (bp), lies within 900 bp of the unique Xba I site in chicken mtDNA.