Metabolic rate and directional nucleotide substitution in animal mitochondrial DNA

There is marked heterogeneity of nucleotide composition in mitochondrial DNA across divergent animals. Differences in nucleotide composition presumably reflect differences in directional nucleotide substitution for A+T or G+C nucleotides. In mitochondrial DNA, there is A+T directional nucleotide substitution in most (if not all) animals surveyed, and the magnitude of directional A+T nucleotide substitution differs greatly within and among groups. Differences in directional nucleotide substitution among lineages of mammals can be explained by changes in metabolic physiology. This relationship is thought to be mediated by the effect of oxygen radicals because these toxic compounds are by-products of aerobic metabolism and are known mutagens. Association between metabolism and nucleotide composition provides additional evidence in favor of the hypothesis that rates and patterns of nucleotide substitution in mitochondrial DNA can be influenced by factors that impinge on rates of endogenous DNA damage.      

The rate and pattern of nucleotide substitution in Drosophila mitochondrial DNA.

The nucleotide sequences of a segment of mitochondrial DNA (mtDNA) have been determined for nine species or subspecies of the subgenus Drosophila of the genus Drosophila. This segment contains two complete protein-coding genes (i.e., NADH dehydrogenase subunit 1 and cytochrome b) and a transfer RNA gene (tRNA(ser)). The G+C content at third-codon positions for the two protein-coding genes was 1.5 times higher than that in the D. melanogaster species group, which belongs to the subgenus Sophophora. However, there was a substantial difference between the nucleotide frequencies of G and C. The number of nucleotide substitutions per silent site was more than three times higher than that for nuclear DNA, although it was only 60% of that for mammalian mtDNA. Both parametric and nonparametric analyses revealed a strong transition-transversion bias in nucleotide substitution, as was observed in mammalian mtDNA. Moreover, the rate of substitution of A and T for G and C is higher than that for the opposite direction. This bias seems to be responsible for the extremely A+T-rich base composition of Drosophila mtDNA. It is also noted that the rate of transitional change between A and G is higher than that between T and C.     

Site-by-site estimation of the rate of substitution and the correlation of rates in mitochondrial DNA

A maximum likelihood method for independently estimating the relative rate of substitution at different nucleotide sites is presented. With this method, the evolution of DNA sequences can be analyzed without assuming a specific distribution of rates among sites. To investigate the pattern of correlation of rates among sites, the method was applied to a data set consisting of the protein-coding regions of the mitochondrial genome from 10 vertebrate species. Rates appear to be strongly correlated at distances up to 40 codons apart. Furthermore, there appears to be some higher order correlation of sites approximately 75 codons apart. The method of site-by-site estimation of the rate of substitution may also be applied to examine other aspects of rate variation along a DNA sequence and to assess the difference in the support of a tree along the sequence.     

Maximum likelihood estimation of the heterogeneity of substitution rate among nucleotide sites

This paper presents a maximum likelihood approach to estimating the variation of substitution rate among nucleotide sites. We assume that the rate varies among sites according to an invariant+gamma distribution, which has two parameters: the gamma parameter alpha and the proportion of invariable sites theta. Theoretical treatments on three, four, and five sequences have been conducted, and computer program have been developed. It is shown that rho = (1 + theta alpha)/(1 + alpha) is a good measure for the rate heterogeneity among sites. Extensive simulations show that (1) if the proportion of invariable sites is negligible, i.e., theta = 0, the gamma parameter alpha can be satisfactorily estimated, even with three sequences; (2) if the proportion of invariable sites is not negligible, the heterogeneity rho can still be suitably estimated with four or more sequences; and (3) the distances estimated by the proposed method are almost unbiased and are robust against violation of the assumption of the invariant + gamma distribution.      

Intraspecific nucleotide sequence differences in the major noncoding region of human mitochondrial DNA.

Nucleotide sequences of the major noncoding region of human mitochondrial DNA (mtDNA) from 95 human placentas have been determined. These sequences include at least a 482-bp-long region encompassing most of the D-loop-forming region. Comparisons of these sequences with those previously determined have revealed remarkable features of nucleotide substitutions and insertion/deletion events. The nucleotide diversity among the sequences is estimated as 1.45%, which is three- to fourfold higher than the corresponding value estimated from restriction-enzyme analysis of whole mtDNA genome. A hypervariable region has also been defined. In this 14-bp region, 17 different sequences were detected. More than 97% of the base changes are transitions. A significantly nonrandom distribution of nucleotide substitutions and sequence length variations were also noted. The phylogenetic analysis indicates that diversity among the negroids is much larger than that among the caucasoids or the mongoloids. In fact, part of the negroids first diverged from other humans in the phylogenetic tree. A striking finding in the phylogenetic analysis is that the mongoloids can be separated into two distinct groups. Divergence of part of the mongoloids follows the earliest divergence of part of the negroids. The remainder of the mongoloids subsequently diverged together with the caucasoids. This observation confirmed our earlier study, which clearly demonstrated, by the restriction-enzyme analysis, existence of two distinct groups in the Japanese.     

A microarray system for genotyping 150 single nucleotide polymorphisms in the coding region of human mitochondrial DNA

We established a genotyping system for a panel of 150 SNPs in the coding regions of mitochondrial DNA based on multiplex tag-array minisequencing. We show the feasibility of this system for simultaneous identification of individuals and prediction of the geographical origin of the mitochondrial DNA population lineage of the sample donors by genotyping the panel of SNPs in 265 samples representing nine different populations from Africa, Europe, and Asia. Nearly 40,000 genotypes were produced in the study, with an overall genotyping success rate of 95% and accuracy close to 100%. The gene diversity value of the panel of 150 SNPs was 0.991, compared to 0.995 for sequencing 500 nucleotides of the hypervariable regions I and II of mtDNA. For 17 individuals with identical sequences in the hypervariable regions of mtDNA, our panel of SNPs increased the power of discrimination. We observed 144 haplotypes that correspond to previously determined mitochondrial "haplogroups," and they allowed prediction of the origin of the maternal population lineage of 97% of the analyzed samples.     

Variability of the human mitochondrial DNA control region sequences in the Lithuanian population

The Lithuanians and Latvians are the only two Baltic cultures that survived until today. Since the Neolithic period the native inhabitants of the present-day Lithuanian territory have not been replaced by any other ethnic group. Therefore the genetic characterization of the present-day Lithuanians may shed some light on the early history of the Balts. We have analysed 120 DNA samples from two Lithuanian ethnolinguistic groups (Aukstaiciai and Zemaiciai) by direct sequencing of the first hypervariable segment (HVI) of the control region of mitochondrial DNA (mtDNA) and restriction enzyme digestion for polymorphic site 00073. On the basis of specific nucleotide substitutions the obtained sequences were classified to mtDNA haplogroups. This revealed the presence of almost all European haplogroups (except X) in the Lithuanian sample, including those that expanded through Europe in the Palaeolithic and those whose expansion occurred during the Neolithic. Molecular diversity indices (gene diversity 0.97, nucleotide diversity 0.012 and mean number of pairwise differences 4.5) were within the range usually reported in European populations. No significant differences between Aukstaiciai and Zemaiciai subgroups were found, but some slight differences need further investigation.     

Substitution rate variation among sites in hypervariable region 1 of human mitochondrial DNA

More than an order of magnitude difference in substitution rate exists among sites within hypervariable region 1 of the control region of human mitochondrial DNA. A two-rate Poisson mixture and a negative binomial distribution are used to describe the distribution of the inferred number of changes per nucleotide site in this region. When three data sets are pooled, however, the two-rate model cannot explain the data. The negative binomial distribution always fits, suggesting that substitution rates are approximately gamma distributed among sites. Simulations presented here provide support for the use of a biased, yet commonly employed, method of examining rate variation. The use of parsimony in the method to infer the number of changes at each site introduces systematic errors into the analysis. These errors preclude an unbiased quantification of variation in substitution rate but make the method conservative overall. The method can be used to distinguish sites with highly elevated rates, and 29 such sites are identified in hypervariable region 1. Variation does not appear to be clustered within this region. Simulations show that biases in rates of substitution among nucleotides and non-uniform base composition can mimic the effects of variation in rate among sites. However, these factors contribute little to the levels of rate variation observed in hypervariable region 1.     

Estimation of the number of nucleotide substitutions in the control region of mitochondrial DNA in humans and chimpanzees

Examining the pattern of nucleotide substitution for the control region of mitochondrial DNA (mtDNA) in humans and chimpanzees, we developed a new mathematical method for estimating the number of transitional and transversional substitutions per site, as well as the total number of nucleotide substitutions. In this method, excess transitions, unequal nucleotide frequencies, and variation of substitution rate among different sites are all taken into account. Application of this method to human and chimpanzee data suggested that the transition/transversion ratio for the entire control region was approximately 15 and nearly the same for the two species. The 95% confidence interval of the age of the common ancestral mtDNA was estimated to be 80,000-480,000 years in humans and 0.57-2.72 Myr in common chimpanzees.      

Analysis of phylogenetically reconstructed mutational spectra in human mitochondrial DNA control region

Analysis of mutations in mitochondrial DNA is an important issue in population and evolutionary genetics. To study spontaneous base substitutions in human mitochondrial DNA we reconstructed the mutational spectra of the hypervariable segments I and II (HVS I and II) using published data on polymorphisms from various human populations. An excess of pyrimidine transitions was found both in HVS I and II regions. By means of classification analysis numerous mutational hotspots were revealed in these spectra. Context analysis of hotspots revealed a complex influence of neighboring bases on mutagenesis in the HVS I region. Further statistical analysis suggested that a transient misalignment dislocation mutagenesis operating in monotonous runs of nucleotides play an important role for generating base substitutions in mitochondrial DNA and define context properties of mtDNA. Our results suggest that dislocation mutagenesis in HVS I and II is a fingerprint of errors produced by DNA polymerase gamma in the course of human mitochondrial DNA replication     

Distribution of nucleotide substitutions in human mitochondrial DNA genes

To analyze the distribution pattern of nucleotide substitutions in human mitochondrial DNA (mtDNA), mutational spectra of the mitochondrial genes were reconstructed. The reconstruction procedure is based on the mutation distribution data for 47 monophyletic mtDNA clusters, to which 794 examined mtDNA sequences encoding for tRNAs, rRNAs, and mitochondrial proteins are attributed. One of specific features of mitochondrial mutational spectra revealed was homoplasy of the mutations (the mean mutation number per variable nucleotide site in the coding region varied from 1.09 to 1.43). It was established that in the mtDNA genes maximum mutational constraint fell onto the guanine bases, albeit the content of these bases in the mtDNA L-chains was minimal. Maximal bias towards parallel G to A transitions was observed for rRNA genes, with the protein-and tRNA-encoding genes ranking next. Despite the fact that the differences in the average G-nucleotides content and variability between the genes of two mtDNA segments located between the OriH and OriL were statistically significant, the results did not provide the conclusion that the G-nucleotide instability observed in the mtDNA L-spectra was determined by the mechanism of asynchronous mtDNA replication, along with the deamination of cytosines in the H-chain regions, which remained single-stranded during replication.Translated from Genetika, Vol. 41, No. 1, 2005, pp. 93–99.Original Russian Text Copyright © 2005 by Malyarchuk.     

Direct estimation of the mitochondrial DNA mutation rate in Drosophila melanogaster

Mitochondrial DNA (mtDNA) variants are widely used in evolutionary genetics as markers for population history and to estimate divergence times among taxa. Inferences of species history are generally based on phylogenetic comparisons, which assume that molecular evolution is clock-like. Between-species comparisons have also been used to estimate the mutation rate, using sites that are thought to evolve neutrally. We directly estimated the mtDNA mutation rate by scanning the mitochondrial genome of Drosophila melanogaster lines that had undergone approximately 200 generations of spontaneous mutation accumulation (MA). We detected a total of 28 point mutations and eight insertion-deletion (indel) mutations, yielding an estimate for the single-nucleotide mutation rate of 6.2 × 10⁻⁸ per site per fly generation. Most mutations were heteroplasmic within a line, and their frequency distribution suggests that the effective number of mitochondrial genomes transmitted per female per generation is about 30. We observed repeated occurrences of some indel mutations, suggesting that indel mutational hotspots are common. Among the point mutations, there is a large excess of G→A mutations on the major strand (the sense strand for the majority of mitochondrial genes). These mutations tend to occur at nonsynonymous sites of protein-coding genes, and they are expected to be deleterious, so do not become fixed between species. The overall mtDNA mutation rate per base pair per fly generation in Drosophila is estimated to be about 10× higher than the nuclear mutation rate, but the mitochondrial major strand G→A mutation rate is about 70× higher than the nuclear rate. Silent sites are substantially more strongly biased towards A and T than nonsynonymous sites, consistent with the extreme mutation bias towards A+T. Strand-asymmetric mutation bias, coupled with selection to maintain specific nonsynonymous bases, therefore provides an explanation for the extreme base composition of the mitochondrial genome of Drosophila.     

Fidelity of nucleotide incorporation by human mitochondrial DNA polymerase

We have examined the fidelity of polymerization catalyzed by the human mitochondrial DNA polymerase using wild-type and exonuclease-deficient (E200A mutation) forms of recombinant, reconstituted holoenzyme. Each of the four nucleotides bind and incorporate with similar kinetics; the average dissociation constant for ground state binding is 0.8 microm, and the average rate of polymerization is 37 x s(-1), defining a specificity constant kcat/Km = 4.6 x 10(7) x m(-1) x s(-1). Mismatched nucleotides show weaker ground-state nucleotide binding affinities ranging from 57 to 364 microm and slower rates of polymerization ranging from 0.013 to 1.16 x s(-1). The kinetic parameters yield fidelity estimates of 1 error out of 260,000 nucleotides for a T:T mismatch, 3563 for G:T, and 570,000 for C:T. The accessory subunit increases fidelity 14-fold by facilitating both ground-state binding and the incorporation rate of the correct A:T base pair compared with a T:T mismatch. Correctly base-paired DNA dissociates from the polymerase at a rate of 0.02 x s(-1) promoting processive polymerization. Thus, the mitochondrial DNA polymerase catalyzed incorporation with an average processivity of 1850, defined by the ratio of polymerization rate to the dissociation rate (37/0.02) and with an average fidelity of one error in 280,000 base pairs.     

Genetic variation of Japanese pink salmon populations inferred from nucleotide sequence analysis of the mitochondrial DNA control region

To estimate genetic variation and structure of pink salmon (Oncorhynchus gorbuscha) populations in Hokkaido, Japan, we analyzed the nucleotide sequence of about 500 bp in a variable portion of the 5′ end of the mitochondrial DNA control region for even- and odd-year broodlines. Sixty-seven haplotypes were detected in the examined individuals. Among these, 25 haplotypes were unique to the even-year broodline, while another 30 haplotypes were unique to the odd-year broodline. Five and three length-heteroplasmic haplotypes were detected in the even-year broodline and odd-year broodline, respectively. The distribution pattern of the 67 haplotypes was different among populations between both broodlines, while not different among populations within the same broodline. The haplotype and nucleotide diversity were higher for even-year broodline populations than for odd-year broodline populations, suggesting greater genetic variation within populations of the even-year broodline. Analysis of molecular variance and pairwise fixation index estimates also demonstrated strong genetic differentiation between even- and odd-year broodlines, although there was no genetic differentiation among populations within the same year broodline. The neutrality tests and mismatch distribution analysis indicate that the demographic history of pink salmon in Japan differs between even- and odd-year populations. Together, these results suggest strong reproductive isolation between the even- and odd-year broodlines of pink salmon, and high gene flow with broodlines due to straying.     

A gradient of silent substitution rate in the human pseudoautosomal region

It has been demonstrated that recombination in the human p-arm pseudoautosomal region (p-PAR) is at least twenty times more frequent than the genomic average of approximately 1 cM/Mb, which may affect substitution patterns and rates in this region. Here I report the analysis of substitution patterns and rates in 10 human, chimpanzee, gorilla, and orangutan genes across the p-PAR. Between species silent divergence in the p-PAR forms a gradient, increasing toward the telomere. The correlation of silent divergence with distance from the p-PAR boundary is highly significant (rho = 0.911, P < 0.001). After exclusion of the CpG dinucleotides this correlation is still significant (rho = 0.89, P < 0.01), thus the substitution rate gradient cannot be explained solely by the differences in the extent of methylation across the p-PAR. Frequent recombination in the PAR may result in a relatively strong effect of biased gene conversion (BGC), which, because of the increased probability of fixation of the G or C nucleotides at (A or T)/(G or C) segregating sites, may affect substitution rates. BGC, however, does not seem to be the factor creating the substitution rate gradient in the p-PAR, because the only gradient is still detactable if only A<-->T and G<-->C substitutions are taken into account (rho = 0.82, P < 0.01). I hypothesize that the substitution rate gradient in the p-PAR is due to the mutagenic effect of recombination, which is very frequent in the distal human p-PAR and might be lower near the p-PAR boundary.     

Distribution of nucleotide substitutions in human mitochondrial DNA genes

To analyze the distribution pattern of nucleotide substitutions in human mitochondrial DNA (mtDNA), mutational spectra of the mitochondrial genes were reconstructed. The reconstruction procedure is based on the mutation distribution data for 47 monophyletic mtDNA clusters, to which 794 examined mtDNA sequences encoding for tRNAs, rRNAs, and mitochondrial proteins are attributed. One of specific features of mitochondrial mutational spectra revealed was homoplasy of the mutations (the mean mutation number per variable nucleotide site in the coding region varied from 1.09 to 1.43). It was established that in the mtDNA genes maximum mutational constraint fell onto the guanine bases, albeit the content of these bases in the mtDNA L-chains was minimal. Maximal bias towards parallel G to A transitions was observed for rRNA genes, with the protein- and tRNA-encoding genes ranking next. Despite the fact that the differences in the average G-nucleotides content and variability between the genes of two mtDNA segments located between the OriH and OriL were statistically significant, the results did not provide the conclusion that the G-nucleotide instability observed in the mtDNA L-spectra was determined by the mechanism of asynchronous mtDNA replication, along with the deamination of cytosines in the H-chain regions, which remained single-stranded during replication.     

Orthodox and unorthodox phylogenetic relationships among tunas revealed by the nucleotide sequence analysis of the mitochondrial DNA control region

A phylogeny of all eight recognized taxa of the genus Thunnus was constructed from approximately 400 base pairs of sequence of the mitochondrial DNA (mtDNA) control region. The PCR-amplified control region I segment studied contained a total of 186 variable sites and 159 phylogenetically informative sites. Diagnostic sequences for every taxon were identified. Neighbour-joining phylogenies supported monophyletic origins of the temperate subgenus Thunnus and of the tropical subgenus Neothunnus . Similar results were obtained by maximum parsimony analyses except that there was no support for a monophyletic origin of the subgenus Thunnus . Bigeye tuna, which have been difficult to place in either subgenus using conventional morphological data, was identified as the sister species of Neothunnus . Within the subgenus Thunnus , the Atlantic bluefin and Southern bluefin tunas were shown to be sister taxa of the highly divergent monophyletic clade formed by the Pacific northern bluefin and the Albacore tunas. The conspecific Atlantic ( T. thynnus thynnus ) and Pacific ( T. t. orientalis ) northern bluefin tunas were more divergent (Tamura-Nei distance 0·145 ± 0·019) from each other than the average distance separating most species-pairs within the genus. Thus, a re-examination of their status as subspecies of T. thunnus is warranted.     

Individual human hair mitochondrial DNA control region heteroplasmy proportions in mothers and children

Due to maternal inheritance, lack of recombination and a high polymorphic density, the mtDNA control region hypervariable (HV) regions are well suited for forensic identification using a maternal relative as the known sample. This analysis can be performed in hair, however, heteroplasmy in this tissue is not rare and can result in an apparent sequence mismatch that complicates this application. There is little data comparing mother and child mtDNA-CR heteroplasmic proportions in hair. In this study, we assayed four hairs per individual in 26 mother-child pairs by TTGE for heteroplasmy across HV1. Single nucleotide heteroplasmy was detected in seven families, and in four families at least two hairs were heteroplasmic. In each of the latter families, sequencing and PCR-RFLP confirmed single nucleotide heteroplasmy in proportions of the variant ranging from < or =10 to > or =90% in the mothers, with far less variability in their children. Sequencing alone would have revealed apparent homoplasmic differences at one nucleotide in these families, possibly resulting in an 'inconclusive' verdict for relatedness of child and mother. However, mother-child heteroplasmic variability did not exceed intra-individual variability in the mothers alone.