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Hepatitis B virus surface antigen (HBsAg) encoded by the S gene is highly expressed during the replication cycle of hepatitis B virus (HBV). However, the frequent usage of tryptophan in HBsAg, which leads to a high cost of biosynthesis, is inconsistent with the high expression level of this protein. Tryptophan-truncated mutation of HBsAg, that is, a tryptophan to stop codon mutation resulting in truncated HBsAg, might help to maintain its high expression with lower biosynthetic cost. We aimed to investigate the prevalence of tryptophan-truncated S quasispecies in treatment-naïve patients with chronic hepatitis B (CHB) by applying CirSeq as well as a site-by-site algorithm developed by us to identify variants at extremely low frequencies in the carboxyl terminus of HBsAg. A total of 730 mutations were identified in 27 patients with CHB, varying from seven to 56 mutations per sample. The number of synonymous mutations was much higher than that of nonsynonymous mutations in the reverse transcriptase (RT) coding region and vice versa in the S coding region, implying that the evolutionary constraints on the RT and S genes might be different. We showed that 25 (92.6 %) of 27 patients had at least one S-truncated mutation, most of which were derived from tryptophan, indicating a high prevalence of tryptophan-truncated S mutations in treatment-naïve patients with CHB. In terms of the RT gene, 21 (77.8 %) patients had pre-existing drug-resistant mutations, while no truncated mutations were detected. Our findings that tryptophan-truncated S quasispecies and drug-resistant RT mutants were highly prevalent in treatment-naïve patients with CHB provide new insights into the composition of the HBV population, which might help optimize the treatment and management of patients with CHB.

Background Kluyveromyces marxianus, the known fastest-growing eukaryote on the earth, has remarkable thermotolerance and capacity to utilize various agricultural residues to produce low-cost bioethanol, and hence is industrially important to resolve the imminent energy shortage crisis. Currently, the poor ethanol tolerance hinders its operable application in the industry, and it is necessary to improve K. marxianus’ ethanol resistance and unravel the underlying systematical mechanisms. However, this has been seldom reported to date. Results We carried out a wild-type haploid K. marxianus FIM1 in adaptive evolution in 6% (v/v) ethanol. After 100-day evolution, the KM-100d population was obtained; its ethanol tolerance increased up to 10% (v/v). Interestingly, DNA analysis and RNA-seq analysis showed that KM-100d yeasts’ ethanol tolerance improvement was not due to ploidy change or meaningful mutations, but founded on transcriptional reprogramming in a genome-wide range. Even growth in an ethanol-free medium, many genes in KM-100d maintained their up-regulation. Especially, pathways of ethanol consumption, membrane lipid biosynthesis, anti-osmotic pressure, anti-oxidative stress, and protein folding were generally up-regulated in KM-100d to resist ethanol. Notably, enhancement of the secretory pathway may be the new strategy KM-100d developed to anti-osmotic pressure, instead of the traditional glycerol production way in S. cerevisiae. Inferred from the transcriptome data, besides ethanol tolerance, KM-100d may also develop the ability to resist osmotic, oxidative, and thermic stresses, and this was further confirmed by the cell viability test. Furthermore, under such environmental stresses, KM-100d greatly improved ethanol production than the original strain. In addition, we found that K. marxianus may adopt distinct routes to resist different ethanol concentrations. Trehalose biosynthesis was required for low ethanol, while sterol biosynthesis and the whole secretory pathway were activated for high ethanol. Conclusions This study reveals that ethanol-driven laboratory evolution could improve K. marxianus’ ethanol tolerance via significant up-regulation of multiple pathways including anti-osmotic, anti-oxidative, and anti-thermic processes, and indeed consequently raised ethanol yield in industrial high-temperature and high-ethanol circumstance. Our findings give genetic clues for further rational optimization of K. marxianus’ ethanol production, and also partly confirm the positively correlated relationship between yeast’s ethanol tolerance and production. Electronic supplementary material The online version of this article (10.1186/s13068-019-1393-z) contains supplementary material, which is available to authorized users.

Human pigmentation is a highly diverse and complex trait among populations and has drawn particular attention from both academic and non-academic investigators for thousands of years. Previous studies detected selection signals in several human pigmentation genes, but few studies have integrated contribution from multiple genes to the evolution of human pigmentation. Moreover, none has quantified selective pressures on human pigmentation over epochs and between populations. Here, we dissect dynamics and differences of selective pressures during different periods and between distinct populations with new approaches. We use genotype data of 19 genes associated with human pigmentation from 17 publicly available datasets and obtain data for 2346 individuals of six representative population groups from across the world. Our results quantify the strength of natural selection on light pigmentation not only in modern Europeans (0.0259/generation) but also in proto-Eurasians (0.00650/generation). Our results also suggest that several derived alleles associated with human dark pigmentation may be under positive directional selection in some African populations. Our study provides the first attempt to quantitatively investigate the dynamics of selective pressures during different time periods in the evolution of human pigmentation. This article has an associated First Person interview with the first author of the article., Summary: This study developed a novel approach to estimate selection strength over time and applied it on the evolution of human pigmentation.

Human populations around the world encounter various environmental challenges and, consequently, develop genetic adaptations to different selection forces. Identifying the differences in natural selection between populations is critical for understanding the roles of specific genetic variants in evolutionary adaptation. Although numerous methods have been developed to detect genetic loci under recent directional selection, a probabilistic solution for testing and quantifying selection differences between populations is lacking. Here we report the development of a probabilistic method for testing and estimating selection differences between populations. By use of a probabilistic model of genetic drift and selection, we showed that logarithm odds ratios of allele frequencies provide estimates of the differences in selection coefficients between populations. The estimates approximate a normal distribution, and variance can be estimated using genome-wide variants. This allows us to quantify differences in selection coefficients and to determine the confidence intervals of the estimate. Our work also revealed the link between genetic association testing and hypothesis testing of selection differences. It therefore supplies a solution for hypothesis testing of selection differences. This method was applied to a genome-wide data analysis of Han and Tibetan populations. The results confirmed that both the EPAS1 and EGLN1 genes are under statistically different selection in Han and Tibetan populations. We further estimated differences in the selection coefficients for genetic variants involved in melanin formation and determined their confidence intervals between continental population groups. Application of the method to empirical data demonstrated the outstanding capability of this novel approach for testing and quantifying differences in natural selection.

The Y chromosome is one of the best genetic materials to explore the evolutionary history of human populations. Global analyses of Y chromosomal short tandem repeats (STRs) data can reveal very interesting world population structures and histories. However, previous Y-STR works tended to focus on small geographical ranges or only included limited sample sizes. In this study, we have investigated population structure and demographic history using 17 Y chromosomal STRs data of 979 males from 44 worldwide populations. The largest genetic distances have been observed between pairs of African and non-African populations. American populations with the lowest genetic diversities also showed large genetic distances and coancestry coefficients with other populations, whereas Eurasian populations displayed close genetic affinities. African populations tend to have the oldest time to the most recent common ancestors (TMRCAs), the largest effective population sizes and the earliest expansion times, whereas the American, Siberian, Melanesian, and isolated Atayal populations have the most recent TMRCAs and expansion times, and the smallest effective population sizes. This clear geographic pattern is well consistent with serial founder model for the origin of populations outside Africa. The Y-STR dataset presented here provides the most detailed view of worldwide population structure and human male demographic history, and additionally will be of great benefit to future forensic applications and population genetic studies.

Sherpa population is an ethnic group living in south mountainside of Himalayas for hundreds of years. They are famous as extraordinary mountaineers and guides, considered as a good example for successful adaptation to low oxygen environment in Tibetan highlands. Mitochondrial DNA (mtDNA) variations might be important in the highland adaption given its role in coding core subunits of oxidative phosphorylation in mitochondria. In this study, we sequenced the complete mtDNA genomes of 76 unrelated Sherpa individuals. Generally, Sherpa mtDNA haplogroup constitution was close to Tibetan populations. However, we found three lineage expansions in Sherpas, two of which (C4a3b1 and A4e3a) were Sherpa-specific. Both lineage expansions might begin within the past hundreds of years. Especially, nine individuals carry identical Haplogroup C4a3b1. According to the history of Sherpas and Bayesian skyline plot, we constructed various demographic models and found out that it is unlikely for these lineage expansions to occur in neutral models especially for C4a3b1. Nonsynonymous mutations harbored in C4a3b1 (G3745A) and A4e3a (T4216C) are both ND1 mutants (A147T and Y304H, respectively). Secondary structure predictions showed that G3745A were structurally closing to other pathogenic mutants, whereas T4216C itself was reported as the primary mutation for Leber's hereditary optic neuropathy. Thus, we propose that these mutations had certain effect on Complex I function and might be important in the high altitude adaptation for Sherpa people.

Summary The power of statistical tests to measure effect sizes in the presence of population stratification is an important issue for the design and analysis of population-based association studies. Comparisons of statistical tests have shown that the power of different statistical approaches varies in different genetic scenarios. However, the impact of stratified population parameters on statistical power is not yet understood in a general statistical framework, particularly the impact of correlated population parameters. To investigate such impact in detail, we implemented a genetic model for population-based association studies with stratified samples and evaluated the impact on power with different genetic scenarios. The investigation shows that correlation between disease prevalence and risk allele frequency among subpopulations impacts statistical power. In a model with five subpopulations and moderate population divergence (Fst= 0.01), the correlation accounts for more than 85% of power difference. Our results also show that the estimation of genetic effect for candidate loci is biased by population divergence. Beneficial alleles could be wrongly characterized as risk alleles when prevalence differences and divergences of risk loci are large among subpopulations.

Substantial increases of linkage disequilibrium (LD) both in magnitude and in range have been observed in recently admixed populations such as African-American (AfA). On the other hand, it has also been shown that LD in AfAs was very similar to that of African. In this study, we attempted to resolve these contradicting observations by conducting a systematic examination of the LD structure in AfAs by genotyping a sample of AfA individuals at 24,341 single nucleotide polymorphisms (SNPs) spanning almost the entire chromosome 21, with an average density of 1.5 kb/SNP. The overall LD in AfAs is similar to that in African populations and much less than that in European populations. Even when the ancestry-informative markers (AIMs) were used, extended LD in AfA was found to be limited to certain magnitude range (0.2 < or = r(2) < or = 0.8) and certain distance range, that is, between-marker distance more than 200 kb. Furthermore, the inclusion of AfA individuals with predominant African ancestry was found to reduce the overall magnitude of LD. Elevation of LD in the AfA population, compared with its parental populations, can only be observed at the markers with large allele frequency differences between 2 parental populations at limited scenario. AfA individuals of wholly African ancestry contribute little to the extended LD in the AfA population, and further genotyping or association analysis conducted using only admixed individuals may lead to higher statistical power and possibly reduced cost.

Reverse transcriptase (RT) mutations contribute to hepatitis B virus resistance during antiviral therapy with nucleos(t)ide analogs. However, the composition of the RT quasispecies and their interactions during antiviral treatment have not yet been thoroughly defined. In this report, 10 patients from each of 3 different virological response groups, i.e., complete virological response, partial virological response and virological breakthrough, were selected from a multicenter trial of Telbivudine treatment. Variations in the drug resistance-related critical RT regions in 107 serial serum samples from the 30 patients were examined by ultra-deep sequencing. A total of 496,577 sequence reads were obtained, with an average sequencing coverage of 4,641X per sample. The phylogenies of the quasispecies revealed the independent origins of two critical quasispecies, i.e., the rtA181T and rtM204I mutants. Data analyses and theoretical modeling showed a cooperative-competitive interplay among the quasispecies. In particular, rtM204I mutants compete against other quasispecies, which eventually leads to virological breakthrough. However, in the absence of rtM204I mutants, synergistic growth of the drug-resistant rtA181T mutants with the wild-type quasispecies could drive the composition of the viral population into a state of partial virological response. Furthermore, we demonstrated that the frequency of drug-resistant mutations in the early phase of treatment is important for predicting the virological response to antiviral therapy.

The spread of culture and language in human populations is explained by two alternative models: the demic diffusion model, which involves mass movement of people; and the cultural diffusion model, which refers to cultural impact between populations and involves limited genetic exchange between them. The mechanism of the peopling of Europe has long been debated, a key issue being whether the diffusion of agriculture and language from the Near East was concomitant with a large movement of farmers. Here we show, by systematically analysing Y-chromosome and mitochondrial DNA variation in Han populations, that the pattern of the southward expansion of Han culture is consistent with the demic diffusion model, and that males played a larger role than females in this expansion. The Han people, who all share the same culture and language, exceed 1.16 billion (2000 census), and are by far the largest ethnic group in the world. The expansion process of Han culture is thus of great interest to researchers in many fields.

Demographic change of human populations is one of the central questions for delving into the past of human beings. To identify major population expansions related to male lineages, we sequenced 78 East Asian Y chromosomes at 3.9 Mbp of the non-recombining region, discovered >4,000 new SNPs, and identified many new clades. The relative divergence dates can be estimated much more precisely using a molecular clock. We found that all the Paleolithic divergences were binary; however, three strong star-like Neolithic expansions at ∼6 kya (thousand years ago) (assuming a constant substitution rate of 1×10(-9)/bp/year) indicates that ∼40% of modern Chinese are patrilineal descendants of only three super-grandfathers at that time. This observation suggests that the main patrilineal expansion in China occurred in the Neolithic Era and might be related to the development of agriculture.

The genetic origins of Japanese populations have been controversial. Upper Paleolithic Japanese, i.e. Jomon, developed independently in Japanese islands for more than 10,000 years until the isolation was ended with the influxes of continental immigrants about 2,000 years ago. However, the knowledge of origin of Jomon and its contribution to the genetic pool of contemporary Japanese is still limited, albeit the extensive studies using mtDNA and Y chromosomes. In this report, we aimed to infer the origin of Jomon and to estimate its contribution to Japanese by fitting an admixture model with missing data from Jomon to a genome-wide data from 94 worldwide populations. Our results showed that the genetic contributions of Jomon, the Paleolithic contingent in Japanese, are 54.3∼62.3% in Ryukyuans and 23.1∼39.5% in mainland Japanese, respectively. Utilizing inferred allele frequencies of the Jomon population, we further showed the Paleolithic contingent in Japanese had a Northeast Asia origin.

The genome-wide association study (GWAS) has become a routine approach for mapping disease risk loci with the advent of large-scale genotyping technologies. Multi-allelic haplotype markers can provide superior power compared with single-SNP markers in mapping disease loci. However, the application of haplotype-based analysis to GWAS is usually bottlenecked by prohibitive time cost for haplotype inference, also known as phasing. In this study, we developed an efficient approach to haplotype-based analysis in GWAS. By using a reference panel, our method accelerated the phasing process and reduced the potential bias generated by unrealistic assumptions in phasing process. The haplotype-based approach delivers great power and no type I error inflation for association studies. With only a medium-size reference panel, phasing error in our method is comparable to the genotyping error afforded by commercial genotyping solutions.

To date, most genome-wide association studies (GWAS) and studies of fine-scale population structure have been conducted primarily on Europeans. Han Chinese, the largest ethnic group in the world, composing 20% of the entire global human population, is largely underrepresented in such studies. A well-recognized challenge is the fact that population structure can cause spurious associations in GWAS. In this study, we examined population substructures in a diverse set of over 1700 Han Chinese samples collected from 26 regions across China, each genotyped at approximately 160K single-nucleotide polymorphisms (SNPs). Our results showed that the Han Chinese population is intricately substructured, with the main observed clusters corresponding roughly to northern Han, central Han, and southern Han. However, simulated case-control studies showed that genetic differentiation among these clusters, although very small (F(ST) = 0.0002 approximately 0.0009), is sufficient to lead to an inflated rate of false-positive results even when the sample size is moderate. The top two SNPs with the greatest frequency differences between the northern Han and southern Han clusters (F(ST)0.06) were found in the FADS2 gene, which associates with the fatty acid composition in phospholipids, and in the HLA complex P5 gene (HCP5), which associates with HIV infection, psoriasis, and psoriatic arthritis. Ingenuity Pathway Analysis (IPA) showed that most differentiated genes among clusters are involved in cardiac arteriopathy (p10(-101)). These signals indicating significant differences among Han Chinese subpopulations should be carefully explained in case they are also detected in association studies, especially when sample sources are diverse.

The discovery of the block-like structure of linkage disequilibrium (LD) in human populations holds the promise of delineating the etiology of common diseases. However, understanding the magnitude, mechanism, and utility of between-population LD sharing is critical for future genome-wide association studies. In this study, substantial LD sharing between six non-African populations was observed, although much less between African-American and non-African, based on 20,000 SNPs of chromosome 21. We also demonstrated the respective roles of recombination and demographic events in shaping LD sharing. Furthermore, we showed that the haplotype-tagged SNPs chosen from one population are portable to the others in East Asia. Therefore, we concluded that the magnitude of LD sharing between human populations justifies the use of representative populations for selecting haplotype-tagged SNPs in genome-wide association studies of complex diseases.

The M129V mutation in the human prion protein gene (PRNP) is the primary site linked to susceptibility to prion diseases in humans. The heterozygous state of this allele has been proven to be more resistant to prion-related diseases such as Creutzfeldt–Jakob disease. Based on a study of the common genetic variations, it has been proposed that balancing selection has played a critical role in shaping the distribution of 129V in worldwide populations. Kreitman and Di-Rienzo [Kreitman and Di-Rienzo (2004)Trend Genet 20:300–304] challenged this hypothesis by pointing out that the exclusion of polymorphisms with low frequency may introduce an ascertainment bias and, in turn, lead to a wrong conclusion. By studying sequence variations in samples from the Human Genome Diversity Project, Soldevila et al. [Soldevila et al. (2005) Trends Genet 21:389–391] showed that this ascertainment bias does exist. We argue that the evidence presented by Soldevila et al. may be compromised by the samples tested since they were selected from a population that may be substructured. In this study, we re-evaluated the hypothesis of balancing selection in a natural Chinese population using a much longer segment encompassing the entire genomic region of the PRNP gene (15 kb). We showed that the pattern of genetic variation in PRNP is not consistent with the presence of balancing selection in this gene.

Testing for random mating of a population is important in population genetics, because deviations from randomness of mating may indicate inbreeding, population stratification, natural selection, or sampling bias. However, current methods use only observed numbers of genotypes and alleles, and do not take advantage of the fact that the advent of sequencing technology provides an opportunity to investigate this topic in unprecedented detail. To address this opportunity, a novel statistical test for random mating is required in population genomics studies for which large sequencing datasets are generally available. Here, we propose a Monte-Carlo-based-permutation test (MCP) as an approach to detect random mating. Computer simulations used to evaluate the performance of the permutation test indicate that its type I error is well controlled and that its statistical power is greater than that of the commonly used chi-square test (CHI). Our simulation study shows the power of our test is greater for datasets characterized by lower levels of migration between subpopulations. In addition, test power increases with increasing recombination rate, sample size, and divergence time of subpopulations. For populations exhibiting limited migration and having average levels of population divergence, the statistical power approaches 1 for sequences longer than 1 Mbp and for samples of 400 individuals or more. Taken together, our results suggest that our permutation test is a valuable tool to detect random mating of populations, especially in population genomics studies.

An accurate estimate of the divergence time between Native Americans is important for understanding the initial entry and early dispersion of human beings in the New World. Current methods for estimating the genetic divergence time of populations could seriously depart from a linear relationship with the true divergence for multiple populations of a different population size and significant population expansion. Here, to address this problem, we propose a novel measure to estimate the genetic divergence time of populations. Computer simulation revealed that the new measure maintained an excellent linear correlation with the population divergence time in complicated multi-population scenarios with population expansion. Utilizing the new measure and microsatellite data of 21 Native American populations, we investigated the genetic divergences of the Native American populations. The results indicated that genetic divergences between North American populations are greater than that between Central and South American populations. None of the divergences, however, were large enough to constitute convincing evidence supporting the two-wave or multi-wave migration model for the initial entry of human beings into America. The genetic affinity of the Native American populations was further explored using Neighbor-Net and the genetic divergences suggested that these populations could be categorized into four genetic groups living in four different ecologic zones. The divergence of the population groups suggests that the early dispersion of human beings in America was a multi-step procedure. Further, the divergences suggest the rapid dispersion of Native Americans in Central and South Americas after a long standstill period in North America.