Towards Improvements in the Estimation of the Coalescent: Implications for the Most Effective Use of Y Chromosome Short Tandem Repeat Mutation Rates

Over the past two decades, many short tandem repeat (STR) microsatellite loci on the human Y chromosome have been identified together with mutation rate estimates for the individual loci. These have been used to estimate the coalescent age, or the time to the most recent common ancestor (TMRCA) expressed in generations, in conjunction with the average square difference measure (ASD), an unbiased point estimator of TMRCA based upon the average within-locus allele variance between haplotypes. The ASD estimator, in turn, depends on accurate mutation rate estimates to be able to produce good approximations of the coalescent age of a sample. Here, a comparison is made between three published sets of per locus mutation rate estimates as they are applied to the calculation of the coalescent age for real and simulated population samples. A novel evaluation method is developed for estimating the degree of conformity of any Y chromosome STR locus of interest to the strict stepwise mutation model and specific recommendations are made regarding the suitability of thirty-two commonly used Y-STR loci for the purpose of estimating the coalescent. The use of the geometric mean for averaging ASD and across loci is shown to improve the consistency of the resulting estimates, with decreased sensitivity to outliers and to the number of STR loci compared or the particular set of mutation rates selected.     

Estimation of Mutation Rates Based on the Analysis of Polypeptide Constituents of Cultured Human Lymphoblastoid Cells

A subclone of a human diploid lymphoblastoid cell line, TK-6, with consistently high cloning efficiency has been used to estimate the rates of somatic mutations on the basis of protein variation detected by two-dimensional polyacrylamide gel electrophoresis. A panel of 267 polypeptide spots per gel was screened, representing the products of approximately 263 unselected loci. The rate of human somatic mutation in vitro was estimated by measuring the proportion of protein variants among cell clones isolated at various times during continuous exponential growth of a TK-6 cell population. Three mutants of spontaneous origin were observed, giving an estimated spontaneous rate of 6 x 10(-8) electrophoretic mutations per allele per cell generation (i.e., 1.2 x 10(-7) per locus per cell generation). Following treatment of cells with N-ethyl-N-nitrosourea, a total of 74 confirmed variants at 54 loci were identified among 1143 clones analyzed (approximately 601,000 allele tests). The induced variants include 65 electromorphs which exhibit altered isoelectric charge and/or apparent molecular weight and nine nullimorphs for each of which a gene product was not detected at its usual location on the gel. The induced frequency for these 65 structural gene mutants is 1.1 x 10(-4) per allele. An excess of structural gene mutations at ten known polymorphic loci and repeat mutations at these and other loci suggest nonrandomness of mutation in human somatic cells. Nullimorphs occurring at three heterozygous loci in TK-6 cells may be caused by genetic processes other than structural gene mutation.     

Mutation analysis of 28 autosomal short tandem repeats in the Chinese Han population

Short tandem repeats (STRs) have been extensively used in forensic genetics. However, according to previous studies, the mutation rates of STRs are relatively high and are affected by many factors. Therefore, it is important to analyze STR mutations and determine the influence of underlying factors on STR mutation rates. Mutation rates of 28 autosomal STRs were determined from 8708 paternity testing cases in the Chinese Han population, and the relationships between STR mutation rates and population, sex, age, allele length and heterozygosity were investigated. A total of 279 mutations were observed at 27 loci in a total of 233,530 meiosis cases, including 273 (97.8%) one-step, 5 (1.8%) two-step and 1 (0.4%) three-step mutations. The overall average mutation rate was 1.19?×?10^–3 (95% CI 1.06?×?10^–3???1.34?×?10^–3) ranging from 0 (TPOX) to 2.79?×?10^–3 (D13S325). Mutation rate comparisons revealed statistically significant differences at several STRs among populations. Paternal mutations occurred more frequently than maternal mutations, at a ratio of 6.04:1, and the mutation rate tended to increase with paternal age. Moreover, our study revealed a bias towards contraction mutations for long alleles and expansion mutations for short alleles. No obvious bias was observed in the overall mutation direction. In addition, STR loci with higher expected heterozygosity (H_exp) tended to have higher mutation rates. This work revealed the relationships between STR mutation rates and several influencing factors, providing useful data and information for further research on STR mutations in forensic genetics.

     

银染法检测短串联重复序列(STR)位点PCR产物鉴别MRC-5细胞

通过采用银染法鉴别短串联重复序列聚合酶链式反应(STR)位点的PCR产物来鉴别人二倍体细胞MRC-5株主细胞库、工作细胞库及限制代细胞。运用PCR方法对细胞库的9个STR位点(CSF1PO、TPOX、TH01、F13A01、FESFPS、vWA、D16S539、D7S820、D13S317)和性别鉴别位点Am elogen in进行扩增,变性聚丙烯酰胺凝胶电泳分离,银染法显影技术,检测MRC-5主细胞库、工作细胞库及限制代细胞的遗传标记。与ATCC公布的MRC-5的荧光STR图谱的8个STR位点和Am elogen in位点相比,MRC-5主细胞库、工作细胞库、限制代细胞的银染STR图谱的9个STR位点和Am elogen in位点,荧光法和银染法重叠的8个位点(CSF1PO、TPOX、TH01、FESFPS、vWA、D16S539、D7S820、D13S317和Am elogen in)数据完全吻合,说明细胞鉴别试验成立。STR图谱作为细胞鉴别的方法简单、易行、准确。     

How important is DNA replication for mutagenesis?

Rates of mutation and substitution in mammals are generally greater in the germ lines of males. This is usually explained as resulting from the larger number of germ cell divisions during spermatogenesis compared with oogenesis, with the assumption made that mutations occur primarily during DNA replication. However, the rate of cell division is not the only difference between male and female germ lines, and mechanisms are known that can give rise to mutations independently of DNA replication. We investigate the possibility that there are other causes of male-biased mutation. First, we show that patterns of variation at approximately 5,200 short tandem repeat (STR) loci indicate a higher mutation rate in males. We estimate a ratio of male-to-female mutation rates of approximately 1.9. This is significantly greater than 1 and supports a greater rate of mutation in males, affecting the evolution of these loci. Second, we show that there are chromosome-specific patterns of nucleotide and dinucleotide composition in mammals that have been shaped by mutation at CpG dinucleotides. Comparable patterns occur in birds. In mammals, male germ lines are more methylated than female germ lines, and these patterns indicate that differential methylation has played a role in male-biased vertebrate evolution. However, estimates of male mutation bias obtained from both classes of mutation are substantially lower than estimates of cell division bias from anatomical data. This discrepancy, along with published data indicating slipped-strand mispairing arising at STR loci in nonreplicating DNA, suggests that a substantial percentage of mutation may occur in nonreplicating DNA.     

Contrasting genetic structure in Plasmodium vivax populations from Asia and South America

Populations of Plasmodium falciparum show striking differences in linkage disequilibrium, population differentiation and diversity, but only fragmentary data exists on the genetic structure of Plasmodium vivax. We genotyped nine tandem repeat loci bearing 2-8 bp motifs from 345 P. vivax infections collected from three Asian countries and from five locations in Colombia. We observed 9-37 alleles per locus and high diversity (He=0.72-0.79, mean=0.75) in all countries. Numbers of multiple clone infections varied considerably: these were rare in Colombia and India, but > 60% of isolates carried multiple alleles in at least one locus in Thailand and Laos. However, only one or two of the nine loci show >1 allele in many samples, suggesting that mutation within infections may result in overestimation of true multiple carriage rates. Identical nine-locus genotypes were frequently found in Colombian populations, contributing to strong linkage disequilibrium. These identical genotypes were strongly clustered in time, consistent with epidemic transmission of clones and subsequent breakdown of allelic associations, suggesting high rates of inbreeding and low effective recombination rates in this country. In contrast, identical genotypes were rare and loci were randomly associated in all three Asian populations, consistent with higher rates of outcrossing and recombination. We observed low but significant differentiation between different Asian countries (standardized FST = 0.13-0.45). In comparison, we see greater differentiation between collection locations within Colombia (standardized FST = 0.4-0.7), and strong differentiation between continents (standardized FST = 0.48-0.79). The observed heterogeneity in multiple clone carriage rates, linkage disequilibrium and population differentiation are similar in some, but not all, respects to those observed in P. falciparum, and have important implications for the design of association mapping studies, and interpretation of P. vivax epidemiology.     

用多重PCR检测上海地区汉族人群9个STR基因座的多态性

利用多重PCR和四色荧光（5－FAM,JOE,NED和ROX）自动化检测技术调查上海地区汉族人群D3S1358、vWA、FGA、D8S1179、D21S11、D18S51、D5S818、D13S317、D7S820等9个STR基因座多态性分布并计算 该9个基因座的的基因频率（Pi)、个体鉴别力（DP）、无偏倚期望杂合性（H）、多态性信息含量（PIC）和非父排除概率（PE）。结果显示：9个STR基因座的基因型分布符合Hardy-Weinberg平衡,9个STR基因座中FGA基因座的DP值最高为0.9584,D8S1179的H值最高为0.9403,D18S51的PIC值最高为0.8560,D18S51的PE值最高为0.7391,9个STR基因座累积个体鉴别力（CDP)为0.9999996,累积非父排除能力（CPE）为0.99991。9个STR基因座适合作为中国人群的遗传标志,用于人类学、遗传疾病基因连锁分析、法医学亲子鉴定和个体识别等研究领域。     

Polymorphism profile of nine short tandem repeat Loci in the Han chinese

Nine short tandem repeat (STR) markers (D3S1358, VWA, FGA, THO1, TPOX, CSFIPO, D5S818, D13S317, and D7S820) and a sex-identification marker (Amel-ogenin locus) were amplified with multiplex PCR and were genotyped with a four-color fluorescence method in samples from 174 unrelated Han individuals in North China. The allele frequencies, genotype frequencies, heterozygosity, probability of discrimination powers, probability of paternity exclusion and Hardy-Weinberg equilibrium expectations were determined. The results demonstrated that the genotypes at all these STR loci in Han population conform to Hardy-Weinberg equilibrium expectations. The combined discrimination power (DP) was 1.05×10-10 within nine STR loci analyzed and the probability of paternity exclusion (EPP) was 0.9998. The results indicate that these nine STR loci and the Amelo-genin locus are useful markers for human identification, paternity and maternity testing and sex determination in forensic sciences.     

High mutation rate of a long microsatellite allele in Drosophila melanogaster provides evidence for allele-specific mutation rates

Within recent years, microsatellite have become one of the most powerful genetic markers in biology. For several mammalian species, microsatellite mutation rates have been estimated on the order of 10(- 3)-10(-5). A recent study, however, demonstrated mutation rates in Drosophila melanogaster of at least one order of magnitude lower than those in mammals. To further test this result, we examined mutation rates of different microsatellite loci using a larger sample size. We screened 24 microsatellite loci in 119 D. melanogaster lines maintained for approximately 250 generations and detected 9 microsatellite mutations. The average mutation rate of 6.3 x 10(-6) is identical to the mutation rate from a previous study. Most interestingly, all nine mutations occurred at the same allele of one locus (DROYANETSB). This hypermutable allele has 28 dinucleotide repeats and is among the longest microsatellite reported in D. melanogaster. The allele-specific mutation rate of 3.0 x 10(-4) per generation is within the range of mammalian mutation rates. Future microsatellite analyses will have to account for the dramatic differences in allele-specific mutation rates.      

中国东乡族9个STR基因座遗传多态性研究

选择9个STR基因座,采用四色荧光标记STR基因扫描技术,对中国甘肃省特有民族——东乡族的群体遗传多态性进行研究。同时检测94个无关个体血液样本,共检出72种等位基因,基因频率的分布在0.0053~0.5825之间;检出182种基因型,基因型频率分布在0.0106~0.2660之间;9个STR位点基因型分布均符合Hardy-Weinberg平衡定律（P＞0.05）。9个STR位点多态信息量（polymorphism information content,PIC）均大于0.6378,杂合度(heterozygosity,H) 均大于0.6500,个体识别力(discrimination power,DP)均大于0.8216,非父排除率(probabilities of paternity exclusion,PPE) 均大于 0.4903。种族比较结果显示,甘肃东乡族与白种人及黑种人在绝大多数位点存在显著差异（P＜0.05）,而9个STR位点与汉族群体的遗传差异均不显著（P＞0.05）。研究结果丰富了中华民族基因数据库,在人类群体遗传学及法医学研究领域有重要应用价值。 Abstract:Genetic distribution for nine STR loci was determined in a Chinese Dongxing ethnic group based on STR genescan marked by fluorescence.Seventy-Two alleles and 182 genotypes were observed in 94 unrelated Chinese Dongxiang individuals,with the corresponding gene frequency and genotype frequency being 0.0053~0.5825 and 0.0106~0.2660 respectively.The genotypes of nine STR loci were in accordance with the Hardy-Weinberg equilibrium (P＞0.05).The statistical analysis of nine STR loci showed PIC( polymorphism information content,PIC)≥0.6378,H(heterozygosity,H) ≥0.6500,DP (discrimination power,DP) ≥0.8216,PPE(probabilities of paternity exculation,PPE) ≥0.4903.The result indicated that there was a significant difference between Dongxiang ethnic group and the white and the black.There was no significant difference in Han nationality.These result filled the Dongxiang ethnic group-a specific group of Chinese into the genetic database and played an important role in Chinese population genetic study and forensic medicine application.     

Genetic variation and de novo mutations in the parthenogenetic Caucasian rock lizard Darevskia unisexualis

Unisexual all-female lizards of the genus Darevskia that are well adapted to various habitats are known to reproduce normally by true parthenogenesis. Although they consist of unisexual lineages and lack effective genetic recombination, they are characterized by some level of genetic polymorphism. To reveal the mutational contribution to overall genetic variability, the most straightforward and conclusive way is the direct detection of mutation events in pedigree genotyping. Earlier we selected from genomic library of D. unisexualis two polymorphic microsatellite containing loci Du281 and Du215. In this study, these two loci were analyzed to detect possible de novo mutations in 168 parthenogenetic offspring of 49 D. unisexualis mothers and in 147 offspring of 50 D. armeniaca mothers. No mutant alleles were detected in D. armeniaca offspring at both loci, and in D. unisexualis offspring at the Du215 locus. There were a total of seven mutational events in the germ lines of four of the 49 D. unisexualis mothers at the Du281 locus, yielding the mutation rate of 0.1428 events per germ line tissue. Sequencing of the mutant alleles has shown that most mutations occur via deletion or insertion of single microsatellite repeat being identical in all offspring of the family. This indicates that such mutations emerge at the early stages of embryogenesis. In this study we characterized single highly unstable (GATA)(n) containing locus in parthenogenetic lizard species D. unisexualis. Besides, we characterized various types of mutant alleles of this locus found in the D. unisexualis offspring of the first generation. Our data has shown that microsatellite mutations at highly unstable loci can make a significant contribution to population variability of parthenogenetic lizards.     

Effect of repeat copy number on variable-number tandem repeat mutations in Escherichia coli O157:H7

Variable-number tandem repeat (VNTR) loci have shown a remarkable ability to discriminate among isolates of the recently emerged clonal pathogen Escherichia coli O157:H7, making them a very useful molecular epidemiological tool. However, little is known about the rates at which these sequences mutate, the factors that affect mutation rates, or the mechanisms by which mutations occur at these loci. Here, we measure mutation rates for 28 VNTR loci and investigate the effects of repeat copy number and mismatch repair on mutation rate using in vitro-generated populations for 10 E. coli O157:H7 strains. We find single-locus rates as high as 7.0 x 10(-4) mutations/generation and a combined 28-locus rate of 6.4 x 10(-4) mutations/generation. We observed single- and multirepeat mutations that were consistent with a slipped-strand mispairing mutation model, as well as a smaller number of large repeat copy number mutations that were consistent with recombination-mediated events. Repeat copy number within an array was strongly correlated with mutation rate both at the most mutable locus, O157-10 (r2= 0.565, P = 0.0196), and across all mutating loci. The combined locus model was significant whether locus O157-10 was included (r2= 0.833, P < 0.0001) or excluded (r2= 0.452, P < 0.0001) from the analysis. Deficient mismatch repair did not affect mutation rate at any of the 28 VNTRs with repeat unit sizes of >5 bp, although a poly(G) homomeric tract was destabilized in the mutS strain. Finally, we describe a general model for VNTR mutations that encompasses insertions and deletions, single- and multiple-repeat mutations, and their relative frequencies based upon our empirical mutation rate data.     

The mutation rates of di-, tri- and tetranucleotide repeats in Drosophila melanogaster

In a recent study, we reported that the combined average mutation rate of 10 di-, 6 tri-, and 8 tetranucleotide repeats in Drosophila melanogaster was 6.3 x 10(-6) mutations per locus per generation, a rate substantially below that of microsatellite repeat units in mammals studied to date (range = 10(-2)-10(-5) per locus per generation). To obtain a more precise estimate of mutation rate for dinucleotide repeat motifs alone, we assayed 39 new dinucleotide repeat microsatellite loci in the mutation accumulation lines from our earlier study. Our estimate of mutation rate for a total of 49 dinucleotide repeats is 9.3 x 10(-6) per locus per generation, only slightly higher than the estimate from our earlier study. We also estimated the relative difference in microsatellite mutation rate among di-, tri-, and tetranucleotide repeats in the genome of D. melanogaster using a method based on population variation, and we found that tri- and tetranucleotide repeats mutate at rates 6.4 and 8.4 times slower than that of dinucleotide repeats, respectively. The slower mutation rates of tri- and tetranucleotide repeats appear to be associated with a relatively short repeat unit length of these repeat motifs in the genome of D. melanogaster. A positive correlation between repeat unit length and allelic variation suggests that mutation rate increases as the repeat unit lengths of microsatellites increase.      

Short tandem repeat based analysis of genetic variability in Kanarese buffalo of South India

The goal of the present study was assessing genetic diversity within Kanarese buffalo, the dual purpose breed of South India. A total of 48 unrelated animals were genotyped at 23 short tandem repeat (STR markers)loci. The total number of observed alleles was 180 with a mean of 7.83 per locus, which varied from 3 to 12 across different loci. The mean observed and expected heterozygosity in South Kanara buffaloes was estimated to be 0.518 and 0.712 respectively. Within population inbreeding estimate (F _IS) was significantly positive in most of the investigated loci which resulted in significant deviation from Hardy-Weinberg equilibrium at 19 of 23 loci analyzed. Evaluation of South Kanara buffalo population for mutation drift equilibrium revealed no significant heterozygosity excess under three different models of evolution viz. infinite alleles model (IAM), step-wise mutation model (SMM) and two phase model (TPM), thus indicating the absence of any recent genetic bottleneck. The results of the present study will help in formulating rational breeding strategies as well as conservation of this important germplasm.     

Single paternity of clutches and sperm storage in the promiscuous green turtle (Chelonia mydas)

Paternity of 22 green turtle ( Chelonia mydas ) clutches from 13 females of the southern Great Barrier Reef breeding population was determined through microsatellite analyses at five loci, including the analysis of successive clutches for nine of the females. A large number of alleles per locus (10–40) provided probabilities of detecting multiple paternity that were quite high, particularly at all loci combined (99.9%). Although green turtles are promiscuous breeders and there was an expectation of finding extensive multiple paternity, only two clutches were multiply sired and, in these, very few eggs had been fertilized by a secondary male. The rarity of multiple paternity may reflect either a low proportion of multiple matings by females in this population, or sperm competition, possibly resulting from a first-male sperm preference. Additionally, the analysis of > 900 offspring provided data on mutations, which included 20 mutation events that were observed in 27 offspring and involved both maternal and paternal lineages. Most mutations ( n = 16) occurred at a single highly variable locus and their presence emphasizes the need to use multiple loci in paternity studies.     

Mutation Rates for Enzyme and Morphological Loci in Barley ( HORDEUM VULGARE L.)

Spontaneous mutation rates were estimated by assaying 84,126 seedlings of a highly homozygous barley line (isogenic line 2025) for five enzyme loci. No mutants were observed in 841,260 allele replications. This result excludes, at probability level 0.95, a spontaneous mutation rate larger than 3.56 x 10^-6/locus/gamete/generation for these enzyme loci. Isogenic line 2025 also was scored for mutants at four loci governing morphological variants. No mutants were observed in 3,386,850 allele replications which indicates that the upper bound for the mutation rate for these loci is 8.85 x 10^-7. It was concluded that, even though spontaneous mutation has been important in creating variability in the barley species at the loci scored, the rate is too low to have much affect on the short-term dynamics of barley populations.     

Somatic stability of microsatellite loci in Eastern white pine,Pinus strobus L

Variation at nuclear- and chloroplast-encoded microsatellite loci was studied among and within clonally propagated individuals of Eastern white pine. Total DNA was extracted and assayed from gamete-bearing tissue (megagametophytes) located on six different branch positions on each of 12 individual genets. No within-individual variation was observed among 12 loci studied. Estimates of numbers of mitotic cell divisions required to produce the tissue used as the source of genomic DNA were obtained by combining tree growth and anatomical data. This allowed for the calculation of upper bound estimates of numbers of mutations per locus per somatic cell division. The estimated somatic mutation rate was found to be substantially lower than those published for genomic microsatellite mutation rates in other plant species.     

Rate of recombinational deletion among human endogenous retroviruses

The fate of most human endogenous retroviruses (HERVs) has been to undergo recombinational deletion. This process involves homologous recombination between the flanking long terminal repeats (LTRs) of a full-length element, leaving a relic structure in the genome termed a solo LTR. We examined loci in one family, HERV-K(HML2), and found that the deletion rate decreased markedly with age: the rate among recently integrated loci was almost 200-fold higher than that among loci whose insertion predated the divergence of humans and chimpanzees (8 x 10(-5) and 4 x 10(-7) recombinational deletion events per locus per generation, respectively). One hypothesis for this finding is that increasing mutational divergence between the flanking LTRs reduces the probability of homologous recombination and thus the rate of solo LTR formation. Consistent with this idea, we were able to replicate the observed rates by a simulation in which the probability of recombinational deletion was reduced 10-fold by a single mutation and 100-fold by any additional mutations. We also discuss the evidence for other factors that may influence the relationship between locus age and the rate of deletion, for example, host recombination rates and selection, and highlight the consequences of recombinational deletion for dating recent HERV integrations.     

中国汉族人群(西安)STR基因扫描与遗传结构

选择9种STR基因位点和Amelogenin基因位点,以测序为基础,研究我国汉族人群STR遗传结构.采用基因自动测序仪建立了10个位点基因分析方法,通过对汉族群体的基因扫描、基因分型和遗传结构分析,获得了STR基因传递特征的大量基因遗传数据,在汉族人群DP为1.05×10-0,EPP为0.9998,为建立我国不同民族STR基因数据库、基因资源研究与保护奠定了基础,为生物考古、基因诊断、性别鉴定、个人识别,司法审判、侦察破案提供有力的科学依据.     

Characteristics of loci and individuals are associated with germline microsatellite mutation rates in lesser kestrels (Falco naumanni)

Although microsatellites are one of the most popular tools in genetic studies, their mutational dynamics and evolution remain unclear. Here, we apply extensive pedigree genotyping to identify and analyze the patterns and factors associated with de novo germline mutations across nine microsatellite loci in a wild population of lesser kestrels (Falco naumanni). A total of 10 germline mutations events were unambiguously identified in four loci, yielding an average mutation rate of 2.96x10(-3). Across loci, mutation rate was positively correlated with locus variability and average allele size. Mutations were primarily compatible with a stepwise mutation model, although not exclusively involved single-step changes. Unexpectedly, we found an excess of maternally transmitted mutations (male-to-female ratio of 0.1). One of the analyzed loci (Fn2.14) resulted hypermutable (mutation rate=0.87%). This locus showed a size-dependent mutation bias, with longer alleles displaying deletions or additions of a small number of repeat than shorter alleles. Mutation probability at Fn2.14 was higher for females and increased with parental (maternal) age but was not associated with individual physical condition, multilocus heterozygosity, allele length or allele span. Overall, our results do not support the male-biased mutation rate described in other organisms and suggest that mutation dynamics at microsatellite loci are a complex process which requires further research.