Sequence comparisons were made from 2214 bp of mitochondrial DNA cloned from six Pacific salmonid species. These sequences include the genes for ATPase subunit 6, cytochrome oxidase subunit 3, NADH dehydrogenase subunit 3, NADH dehydrogenase subunit 4L, tRNA(GLY), and tRNA(ARG). Variation is found at 338 silent and 12 nonsilent positions of protein coding genes and 10 positions in the two tRNA sequences. A single 3-bp length difference was also detected. In all pairwise comparisons the sequence divergence observed in the fragment was higher than that previously predicted by restriction enzyme analysis of the entire molecule. The inferred evolutionary relationship of these species is consistent between methods. The distribution of silent variation shows a complex pattern with greatly reduced variation at the junctions of genes. The variation in the tRNA sequences is concentrated in the DHU loop. The close relationship of these species and extensive sequence analyzed allows for an analysis of the spectrum of substitutions that includes the frequencies of all 12 possible substitutions. The observed spectrum of substitutions is related to potential pathways of spontaneous substitution. The salmonid sequences show an extremely high ratio of silent to replacement substitutions. In addition the amino acid sequences of the four proteins coded in this fragment show a consistently high level of identity with the Xenopus sequences. Taken together these data are consistent with a slower rate of amino acid substitution among the cold-blooded vertebrates when compared to mammals.
Amplification by the polymerase chain reaction and sequencing of a highly variable segment (199 base pairs) of mitochondrial DNA (mtDNA) in 491 fish from 22 streams and 9 hatcheries revealed 9 mtDNA types in steelhead trout (Oncorhynchus mykiss), 5 types in coho salmon (O. kisutch), and 6 types in chinook salmon (O. tshawytscha). Likelihood chi-square analysis of frequencies of mtDNA types in eight paired comparisons of hatchery and geographically proximate wild populations of Oncorhynchus showed significant differences in mtDNA genotypes. More mtDNA types were found, on average, in contemporary hatchery populations for each species than in geographically proximate wild stocks. Factors potentially leading to genetic differences in hatchery and wild stocks sampled in California were historic introductions of geographically divergent populations into hatchery brood stocks and lack of introgression of geographically divergent genotypes from the hatchery into wild populations.
The nucleotide sequences of the mitochondrial origin of light-strand replication and the five tRNA genes surrounding it were determined for three marsupials. The region was found to be rearranged, leaving only the tRNA(Tyr) gene at the same position as in placental mammals and Xenopus. Distribution of the same rearranged genotype among two marsupial families indicates that the events causing the rearrangements took place in an early marsupial ancestor. The putative mitochondrial light-strand origin of replication in marsupials contains a hairpin structure similar to other vertebrate origins and; in addition, extensive flanking sequences that are not found in other vertebrates. Sequence comparisons among the marsupials as well as placentals indicate that the tRNA(Tyr) gene has been evolving under more constraints than the other tRNA genes.
1986. Mitochondrial DNA analysis of Pacific salmonid evolution. Can. J. Zool. 64: 1058-1064. The salmonids of Pacific North America are a group of closely related species with complex life histories and interesting distribution. Previous studies of their evolution and population structure have involved classical morphological and genetic techniques. We have analysed both intra-and inter-specific variation in mitochondria1 DNA sequences of the five North American species of the genus Oncorhynchus and the rainbow trout species, Salmo gairdneri. Cleavage sites for 13 different restriction enzymes were sampled, comparing an average of 48 sites per individual, or approximately 1.7% of the genome. No obvious size variation in the 16 500 + 500 base pair length was observed. Levels of intraspecific variation detected in the chum salmon and rainbow trout were 0.24 + 0.23 and 0.45 +-0.26%, respectively. This variation was population specific; no variation was detected within any of the populations sampled, suggesting the existence of population substructuring. Estimates of divergence between species range from 2.46 + 0.72% in the coho-chinook salmon comparison to 6.88 + 1.27% between coho and chum salmon. The phylogenetic relationship among these species, based on the levels of sequence divergence, organizes the species into three distinct groups. One group includes the pink and chum salmon while a second group contains the coho and chinook salmon, as well as the rainbow trout. The sockeye salmon are distinct from both groups. Although most of these results are in accordance with classical analyses, the relationship of the rainbow trout to the coho and chinook salmon suggests different interpretations of the evolution of life histories and morphological traits in these closely related species. THOMAS, W. K., R. E. WITHLER et A. T. BECKENBACH. 1986. Mitochondrial DNA analysis of Pacific salmonid evolution.Can. J. Zool. 64: 1058-1064. Les Salmonidae de la c6te nord-amCricaine du Pacifique constituent un groupe d'especes trks apparentkes qui ont des cycles biologiques complexes et dont la rCpartition est intkressante. Les Ctudes antkrieures sur leur Cvolution et sur la structure des populations ont utilisC des techniques morphologiques et gCnCtiques classiques. On trouvera ici une analyse de la variation intraspkcifique et de la variation interspkcifique des sCquences d'ADN des mitochondries chez cinq espkces nord-amkricaines d'oncorhynchus et chez la truite arc-en-ciel, Salmo gairdneri. Les points de sCparation de 13 enzymes de restriction ont Ct C CtudiCs, ce qui a permis de comparer en moyenne 48 points par individu, ou environ 1,7% du gCnome. I1 n'y pas de variation Cvidente de longueur entre les 16 500 + 500 paires de bases. L'importance de la variation intraspkcifique a Ct C CvaluCe a 0,24 + 0,23 chez le saumon keta, et a0,45 + 0,26% chez la truite arc-en-ciel. Cette variation est spicifique a la population; il n'y a pas de variation interne chez aucune des populations CtudiCes, ce qui semble indiquer qu'il existe des s...
The entire nucleotide sequence of the mitochondrial genome of the American opossum, Didelphis virginiana, was determined. Two major features distinguish this genome from those of other mammals. First, five tRNA genes around the origin of light strand replication are rearranged. Second, the anticodon of tRNA(Asp) is posttranscriptionally changed by an RNA editing process such that its coding capacity is altered. When the complete protein-coding region of the mitochondrial genome is used as an outgroup for placental mammals it can be shown that rodents represent an earlier branch among placental mammals than primates and artiodactyls and that artiodactyls share a common ancestor with carnivores. The overall rates of evolution of most of the mitochondrial genome of placentals are clock-like. Furthermore, the data indicate that the lineages leading to the mouse and rat may have diverged from each other as much as 35 million years ago.
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