Patterns of nucleotide misincorporations during enzymatic amplification and direct large-scale sequencing of ancient DNA

Benchmarking Metagenomic Classifiers on Simulated Ancient and Modern Metagenomic Data

Taxonomic profiling of ancient metagenomic samples is challenging due to the accumulation of specific damage patterns on DNA over time. Although a number of methods for metagenome profiling have been developed, most of them have been assessed on modern metagenomes or simulated metagenomes mimicking modern metagenomes. Further, a comparative assessment of metagenome profilers on simulated metagenomes representing a spectrum of degradation depth, from the extremity of ancient (most degraded) to current or modern (not degraded) metagenomes, has not yet been performed. To understand the strengths and weaknesses of different metagenome profilers, we performed their comprehensive evaluation on simulated metagenomes representing human dental calculus microbiome, with the level of DNA damage successively raised to mimic modern to ancient metagenomes. All classes of profilers, namely, DNA-to-DNA, DNA-to-protein, and DNA-to-marker comparison-based profilers were evaluated on metagenomes with varying levels of damage simulating deamination, fragmentation, and contamination. Our results revealed that, compared to deamination and fragmentation, human and environmental contamination of ancient DNA (with modern DNA) has the most pronounced effect on the performance of each profiler. Further, the DNA-to-DNA (e.g., Kraken2, Bracken) and DNA-to-marker (e.g., MetaPhlAn4) based profiling approaches showed complementary strengths, which can be leveraged to elevate the state-of-the-art of ancient metagenome profiling.

Museum DNA reveals a new, potentially extinct species of rinkhals (Serpentes: Elapidae: Hemachatus) from the Eastern Highlands of Zimbabwe

Genetic information plays a pivotal role in species recognition and delimitation, but rare or extinct animals can be difficult to obtain genetic samples from. While natural history wet collections have proven invaluable in the description of novel species, the use of these historical samples in genetic studies has been greatly impeded by DNA degradation, especially because of formalin-fixation prior to preservation. Here, we use recently developed museum genomics approaches to determine the status of an isolated population of the elapid snake genus Hemachatus from Zimbabwe. We used multiple digestion phases followed by single strand sequencing library construction and hybridisation capture to obtain 12S and 16S rDNA sequences from a poorly preserved tissue sample of this population. Phylogenetic and morphological analyses in an integrated taxonomic framework demonstrate that the Zimbabwean rinkhals population represents an old and highly distinct lineage, which we describe as a new species, Hemachatus nyangensis sp. nov. Our phylogenetic dating analysis is compatible with venom spitting having evolved in response to the threat posed by early hominins, although more data are required for a robust test of this hypothesis. This description demonstrates the power of museum genomics in revealing rare or even extinct species: Hemachatus from Zimbabwe are only known from a small area of the Eastern Highlands known for high endemism. No living specimens have been seen since the 1980s, most likely due to dramatic land-use changes in the Eastern Highlands, suggesting that the species could be extinct. In view of its recognition as a highly distinct lineage, urgent action is required to determine whether any populations survive, and to safeguard remaining habitat.

The Biological Hierarchy, Time, and Temporal 'Omics in Evolutionary Biology: A Perspective

Sequencing data-genomics, transcriptomics, epigenomics, proteomics, and metabolomics-have revolutionized biological research, enabling a more detailed study of processes, ranging from subcellular to evolutionary, that drive biological organization. These processes, collectively, are responsible for generating patterns of phenotypic variation and can operate over dramatically different timescales (milliseconds to billions of years). While researchers often study phenotypic variation at specific levels of biological organization to isolate processes operating at that particular scale, the varying types of sequence data, or 'omics, can also provide complementary inferences to link molecular and phenotypic variation to produce an integrated view of evolutionary biology, ranging from molecular pathways to speciation. We briefly describe how 'omics has been used across biological levels and then demonstrate the utility of integrating different types of sequencing data across multiple biological levels within the same study to better understand biological phenomena. However, single-time-point studies cannot evaluate the temporal dynamics of these biological processes. Therefore, we put forward temporal 'omics as a framework that can better enable researchers to study the temporal dynamics of target processes. Temporal 'omics is not infallible, as the temporal sampling regime directly impacts inferential ability. Thus, we also discuss the role the temporal sampling regime plays in deriving inferences about the environmental conditions driving biological processes and provide examples that demonstrate the impact of the sampling regime on biological inference. Finally, we forecast the future of temporal 'omics by highlighting current methodological advancements that will enable temporal 'omics to be extended across species and timescales. We extend this discussion to using temporal multi-omics to integrate across the biological hierarchy to evaluate and link the temporal dynamics of processes that generate phenotypic variation.

Using loop-primer mediated PCR to enhance the detection of poorly preserved DNA

Ancient DNA is vitally important in evolutionary research, and obtaining authentic ancient DNA sequences is critical for a proper analysis. However, it is difficult to acquire amplicons accurately and efficiently from ancient DNA templates using current techniques. Here, we established a loop-primer-mediated amplification method (L-PCR) to obtain target ancient DNA sequences with high accuracy and efficiency. The method was tested using 66 ancient samples (including 27 pig bones or teeth and 39 chicken bones) and serially diluted modern animal DNA templates. Compared to nested PCR, L-PCR was proven to be more efficient and accurate and could obtain more amplicons from both ancient pig samples and chicken bones and detect as low as 10−3 ng/μl modern pig template DNA. The efficiency was at least 100-fold that of the nested PCR. The results suggest that L-PCR is advantageous for obtaining authentic DNA sequences from poorly preserved or recalcitrant ancient specimens.

Genome sequencing of up to 6,000-yr-old Citrullus seeds reveals use of a bitter-fleshed species prior to watermelon domestication

Iconographic evidence from Egypt suggests that watermelon pulp was consumed there as a dessert by 4,360 BP. Earlier archaeobotanical evidence comes from seeds from Neolithic settlements in Libya, but whether these were watermelons with sweet pulp or other forms is unknown. We generated genome sequences from 6,000- and 3,300-year-old seeds from Libya and Sudan, and from worldwide herbarium collections made between 1824 and 2019, and analyzed these data together with resequenced genomes from important germplasm collections for a total of 131 accessions. Phylogenomic and population-genomic analyses reveal that (1) much of the nuclear genome of both ancient seeds is traceable to West African seed-use “egusi-type” watermelon (Citrullus mucosospermus) rather than domesticated pulp-use watermelon (Citrullus lanatus ssp. vulgaris); (2) the 6,000-year-old watermelon likely had bitter pulp and greenish-white flesh as today found in C. mucosospermus, given alleles in the bitterness regulators ClBT and in the red color marker LYCB; and (3) both ancient genomes showed admixture from C. mucosospermus, C. lanatus ssp. cordophanus, C. lanatus ssp. vulgaris, and even South African Citrullus amarus, and evident introgression between the Libyan seed (UMB-6) and populations of C. lanatus. An unexpected new insight is that Citrullus appears to have initially been collected or cultivated for its seeds, not its flesh, consistent with seed damage patterns induced by human teeth in the oldest Libyan material.

Isometric artifacts from polymerase chain reaction‐massively parallel sequencing analysis of short tandem repeat loci: An emerging issue from a new technology?

The recent introduction of polymerase chain reaction (PCR)‐massively parallel sequencing (MPS) technologies in forensics has changed the approach to allelic short tandem repeat (STR) typing because sequencing cloned PCR fragments enables alleles with identical molecular weights to be distinguished based on their nucleotide sequences. Therefore, because PCR fidelity mainly depends on template integrity, new technical issues could arise in the interpretation of the results obtained from the degraded samples. In this work, a set of DNA samples degraded in vitro was used to investigate whether PCR‐MPS could generate “isometric drop‐ins” (IDIs; i.e., molecular products having the same length as the original allele but with a different nucleotide sequence within the repeated units). The Precision ID GlobalFiler NGS STR panel kit was used to analyze 0.5 and 1 ng of mock samples in duplicate tests (for a total of 16 PCR‐MPS analyses). As expected, several well‐known PCR artifacts (such as allelic dropout, stutters above the threshold) were scored; 95 IDIs with an average occurrence of 5.9 IDIs per test (min: 1, max: 11) were scored as well. In total, IDIs represented one of the most frequent artifacts. The coverage of these IDIs reached up to 981 reads (median: 239 reads), and the ratios with the coverage of the original allele ranged from 0.069 to 7.285 (median: 0.221). In addition, approximately 5.2% of the IDIs showed coverage higher than that of the original allele. Molecular analysis of these artifacts showed that they were generated in 96.8% of cases through a single nucleotide change event, with the C > T transition being the most frequent (85.7%). Thus, in a forensic evaluation of evidence, IDIs may represent an actual issue, particularly when DNA mixtures need to be interpreted because they could mislead the operator regarding the number of contributors. Overall, the molecular features of the IDIs described in this work, as well as the performance of duplicate tests, may be useful tools for managing this new class of artifacts otherwise not detected by capillary electrophoresis technology.

An empirical evaluation of genotype imputation of ancient DNA

With capabilities of sequencing ancient DNA to high coverage often limited by sample quality or cost, imputation of missing genotypes presents a possibility to increase the power of inference as well as cost-effectiveness for the analysis of ancient data. However, the high degree of uncertainty often associated with ancient DNA poses several methodological challenges, and performance of imputation methods in this context has not been fully explored. To gain further insights, we performed a systematic evaluation of imputation of ancient data using Beagle v4.0 and reference data from phase 3 of the 1000 Genomes project, investigating the effects of coverage, phased reference, and study sample size. Making use of five ancient individuals with high-coverage data available, we evaluated imputed data for accuracy, reference bias, and genetic affinities as captured by principal component analysis. We obtained genotype concordance levels of over 99% for data with 1× coverage, and similar levels of accuracy and reference bias at levels as low as 0.75×. Our findings suggest that using imputed data can be a realistic option for various population genetic analyses even for data in coverage ranges below 1×. We also show that a large and varied phased reference panel as well as the inclusion of low- to moderate-coverage ancient individuals in the study sample can increase imputation performance, particularly for rare alleles. In-depth analysis of imputed data with respect to genetic variants and allele frequencies gave further insight into the nature of errors arising during imputation, and can provide practical guidelines for postprocessing and validation prior to downstream analysis.

Development and Optimization of a Silica Column-Based Extraction Protocol for Ancient DNA

Rapid and cost-effective retrieval of endogenous DNA from ancient specimens remains a limiting factor in palaeogenomic research. Many methods have been developed to increase ancient DNA yield, but modifications to existing protocols are often based on personal experience rather than systematic testing. Here, we present a new silica column-based extraction protocol, where optimizations were tested in controlled experiments. Using relatively well-preserved permafrost samples, we tested the efficiency of pretreatment of bone and tooth powder with a bleach wash and a predigestion step. We also tested the recovery efficiency of MinElute and QIAquick columns, as well as Vivaspin columns with two molecular weight cut-off values. Finally, we tested the effect of uracil-treatment with two different USER enzyme concentrations. We find that neither bleach wash combined with a predigestion step, nor predigestion by itself, significantly increased sequencing efficiency. Initial results, however, suggest that MinElute columns are more efficient for ancient DNA extractions than QIAquick columns, whereas different molecular weight cut-off values in centrifugal concentrator columns did not have an effect. Uracil treatments are effective at removing DNA damage even at concentrations of 0.15 U/µL (as compared to 0.3 U/µL) of ancient DNA extracts.

Epigenomic Modifications in Modern and Ancient Genomes

Epigenetic changes have been identified as a major driver of fundamental metabolic pathways. More specifically, the importance of epigenetic regulatory mechanisms for biological processes like speciation and embryogenesis has been well documented and revealed the direct link between epigenetic modifications and various diseases. In this review, we focus on epigenetic changes in animals with special attention on human DNA methylation utilizing ancient and modern genomes. Acknowledging the latest developments in ancient DNA research, we further discuss paleoepigenomic approaches as the only means to infer epigenetic changes in the past. Investigating genome-wide methylation patterns of ancient humans may ultimately yield in a more comprehensive understanding of how our ancestors have adapted to the changing environment, and modified their lifestyles accordingly. We discuss the difficulties of working with ancient DNA in particular utilizing paleoepigenomic approaches, and assess new paleoepigenomic data, which might be helpful in future studies.

Variability in ITS1 and ITS2 sequences of historic herbaria and extant (fresh) Phalaris species (Poaceae)

BACKGROUND

Phalaris species (Poaceae) occupy diverse environments throughout all continents except Antarctica. Phalaris arundinacea is an important forage, ornamental, wetland restoration and biofuel crop grown globally as well as being a wetland invasive. The nuclear ribosomal internal transcribed spacer (ITS) region has been used for Phalaris barcoding as a DNA region with high nucleotide diversity for Phalaris species identification. Recent findings that P. arundinacea populations in Minnesota USA are most likely native and not European prompted this analysis to determine whether Eurasian vs. native North American P. arundinacea differed in ITS regions. Our objectives were to amplify and compare ITS regions (ITS1 and ITS2) of historic herbaria (1882-2001) and extant (fresh) Phalaris specimens; analyze ITS regions for species-specific polymorphisms (diagnostic SNPs) and compare ITS regions of historic Phalaris specimens with known, extant Phalaris species.

RESULTS

We obtained complete ITS1 and ITS2 sequences from 31 Phalaris historic (herbaria samples, 1908 to 2001) and five extant (fresh) specimens. Herbaria Phalaris specimens did not produce new SNPs (single nucleotide polymorphisms) not present in extant specimens. Diagnostic SNPs were identified in 8/12 (66.6%) Phalaris species. This study demonstrates the use of herbaria tissue for barcoding as a means for improved species identification of Phalaris herbaria specimens. No significant correlation between specimen age and genomic DNA concentration was found. Phalaris arundinacea showed high SNP variation within its clade, with the North American being distinctly different than other USA and most Eurasian types, potentially allowing for future identification of specific SNPs to geographic origin.

CONCLUSIONS

While not as efficient as extant specimens to obtain DNA, Phalaris herbaria specimens can produce high quality ITS sequences to evaluate historic genetic resources and facilitate identification of new species-specific barcodes. No correlation between DNA concentration and age of historic samples (119 year range) occurred. Considerable polymorphism was exhibited in the P. arundinacea clade with several N. American accessions being distinct from Eurasian types. Further development of within species- and genus-specific barcodes could contribute to designing PCR primers for efficient and accurate identification of N. American P. arundinacea. Our finding of misidentified Phalaris species indicates the need to exercise stringent quality control measures on newly generated sequence data and to approach public sequence databases in a critical way.

Phylogeographic analysis delimits three evolutionary significant units of least chipmunks in North America and identifies unique genetic diversity within the imperiled Peñasco population

Although least chipmunks (Neotamias minimus) are a widely distributed North American species of least concern, the southernmost population, N. m. atristriatus (Peñasco least chipmunk), is imperiled and a candidate for federal listing as a subspecies. We conducted a phylogeographic analysis across the N. minimus range to assess genomic differentiation and distinctiveness of the N. m. atristriatus population. Additionally, we leveraged the historical component of sampling to conduct a temporal analysis of N. minimus genetic diversity and also considered climate change effects on range persistence probability by projecting a species distribution model into the IPCC5 RCP 2.6 and 8.5 scenarios. We identified three geographically structured groups (West, North, and South) that were supported by both mitochondrial and nuclear data. N. m. atristriatus grouped within a unique South subclade but were not reciprocally monophyletic from N. m. operarius, and nuclear genome analyses did not separate N. m. atristriatus, N. m. caryi, and N. m. operarius. Thus, while least chipmunks in the Southwest represent an evolutionary significant unit, subspecies distinctions were not supported and listing of the Peñasco population as a Distinct Population Segment of N. m. operarius may be warranted. Our results also support consideration of populations with North and West mitogenomes as two additional evolutionary significant units. We found that N. minimus genetic diversity declined by ~87% over the last century, and our models predicted substantial future habitat contraction, including the loss of the full contemporary ranges of N. m. atristriatus, N. m. arizonensis, and N. m. chuskaensis.

De novo whole-genome assembly and resequencing resources for the roan (Hippotragus equinus), an iconic African antelope

Roan antelope (Hippotragus equinus) is the second-largest member of the Hippotraginae (Bovidae), and is widely distributed across sub-Saharan mesic woodlands. Despite being listed as "Least Concern" across its African range, population numbers are decreasing with many regional Red List statuses varying between Endangered and Locally Extinct. Although the roan antelope has become an economically-important game species in Southern Africa, the vast majority of wild populations are found only in fragmented protected areas, which is of conservation concern. Genomic information is crucial in devising optimal management plans. To this end, we report here the first de novo assembly and annotation of the whole-genome sequence of a male roan antelope from a captive-breeding program. Additionally, we uncover single-nucleotide variants (SNVs) through re-sequencing of five wild individuals representing five of the six described subspecies. We used 10X Genomics Chromium chemistry to produce a draft genome of 2.56 Gb consisting of 16,880 scaffolds with N50 = 8.42 Mb and a BUSCO completeness of 91.2%. The draft roan genome includes 1.1 Gbp (42.2%) repetitive sequences. De novo annotation identified 20,518 protein-coding genes. Genome synteny to the domestic cow showed an average identity of 92.7%. Re-sequencing of five wild individuals to an average sequencing depth of 9.8x resulted in the identification of a filtered set of 3.4x10⁶ bi-allelic SNVs. The proportion of alternative homozygous SNVs for the individuals representing different subspecies, as well as differentiation as measured by PCA, were consistent with expected divergence from the reference genome and among samples. The roan antelope genome is a valuable resource for evolutionary and population genomic questions, as well as management and conservation actions.

A SMRT approach for targeted amplicon sequencing of museum specimens (Lepidoptera)-patterns of nucleotide misincorporation

Natural history collections are a valuable resource for molecular taxonomic studies and for examining patterns of evolutionary diversification, particularly in the case of rare or extinct species. However, the recovery of sequence information is often complicated by DNA degradation. This article describes use of the Sequel platform (Pacific Biosciences) to recover the 658 bp barcode region of the mitochondrial cytochrome c oxidase I (COI) gene from 380 butterflies with an average age of 50 years. Nested multiplex PCR was employed for library preparation to facilitate sequence recovery from extracts with low concentrations of highly degraded DNA. By employing circular consensus sequencing (CCS) of short amplicons (circa 150 bp), full-length barcodes could be assembled without a reference sequence, an important advance from earlier protocols which required reference sequences to guide contig assembly. The Sequel protocol recovered COI sequences (499 bp on average) from 318 of 380 specimens (84%), much higher than for Sanger sequencing (26%). Because each read derives from a single molecule, it was also possible to quantify the incidence of substitutions arising from DNA damage. In agreement with past work on sequence changes induced by DNA degradation, the transition C/G → T/A was the most prevalent category of change, but its rate of occurrence (4.58E−4) was so low that it did not impede the recovery of reliable sequences. Because the current protocol recovers COI sequence from most museum specimens, and because sequence fidelity is unaffected by nucleotide misincorporations, large-scale sequence characterization of museum specimens is feasible.

Herbarium Specimens: A Treasure for DNA Extraction, an Update

With the expansion of molecular techniques, the historical collections have become widely used. The last boom started with using next- and second-generation sequencing in which massive parallel sequencing replaced targeted sequencing and third-generation technology involves single molecule technology. Studying plant DNA using these modern molecular techniques plays an important role in understanding evolutionary relationships, identification through DNA barcoding, conservation status, and many other aspects of plant biology. Enormous herbarium collections are an important source of material especially for taxonomic long-standing issues, specimens from areas difficult to access or from taxa that are now extinct. The ability to utilize these specimens greatly enhances the research. However, the process of extracting DNA from herbarium specimens is often fraught with difficulty related to such variables as plant chemistry, drying method of the specimen, and chemical treatment of the specimen. The result of these applications is often fragmented DNA. The reason new sequencing approaches have been so successful is that the template DNA needs to be fragmented for proper library building, and herbarium DNA is exactly that. Although many methods have been developed for extraction of DNA from herbarium specimens, the most frequently used are modified CTAB and DNeasy Plant Mini Kit protocols. Nine selected protocols in this chapter have been successfully used for high-quality DNA extraction from different kinds of plant herbarium tissues. These methods differ primarily with respect to their requirements for input material (from algae to vascular plants), type of the plant tissue (leaves with incrustations, sclerenchyma strands, mucilaginous tissues, needles, seeds), and further possible applications (PCR-based methods, microsatellites, AFLP or next-generation sequencing).

Competitive mapping allows for the identification and exclusion of human DNA contamination in ancient faunal genomic datasets

BACKGROUND

After over a decade of developments in field collection, laboratory methods and advances in high-throughput sequencing, contamination remains a key issue in ancient DNA research. Currently, human and microbial contaminant DNA still impose challenges on cost-effective sequencing and accurate interpretation of ancient DNA data.

RESULTS

Here we investigate whether human contaminating DNA can be found in ancient faunal sequencing datasets. We identify variable levels of human contamination, which persists even after the sequence reads have been mapped to the faunal reference genomes. This contamination has the potential to affect a range of downstream analyses.

CONCLUSIONS

We propose a fast and simple method, based on competitive mapping, which allows identifying and removing human contamination from ancient faunal DNA datasets with limited losses of true ancient data. This method could represent an important tool for the ancient DNA field.

Population Dynamics in Italian Canids between the Late Pleistocene and Bronze Age

Dog domestication is still largely unresolved due to time-gaps in the sampling of regions. Ancient Italian canids are particularly understudied, currently represented by only a few specimens. In the present study, we sampled 27 canid remains from Northern Italy dated between the Late Pleistocene and Bronze Age to assess their genetic variability, and thus add context to dog domestication dynamics. They were targeted at four DNA fragments of the hypervariable region 1 of mitochondrial DNA. A total of 11 samples had good DNA preservation and were used for phylogenetic analyses. The dog samples were assigned to dog haplogroups A, C and D, and a Late Pleistocene wolf was set into wolf haplogroup 2. We present our data in the landscape of ancient and modern dog genetic variability, with a particular focus on the ancient Italian samples published thus far. Our results suggest there is high genetic variability within ancient Italian canids, where close relationships were evident between both a ~24,700 years old Italian canid, and Iberian and Bulgarian ancient dogs. These findings emphasize that disentangling dog domestication dynamics benefits from the analysis of specimens from Southern European regions.

Temporal and structural genetic variation in reindeer (Rangifer tarandus) associated with the pastoral transition in Northwestern Siberia

Just as the domestication of livestock is often cited as a key element in the Neolithic transition to settled, the emergence of large-scaled reindeer husbandry was a fundamental social transformation for the indigenous peoples of Arctic Eurasia. To better understand the history of reindeer domestication, and the genetic processes associated with the pastoral transition in the Eurasian Arctic, we analyzed archaeological and contemporary reindeer samples from Northwestern Siberia. The material represents Rangifer genealogies spanning from 15,000 years ago to the 18th century, as well as modern samples from the wild Taĭmyr population and from domestic herds managed by Nenetses. The wild and the domestic population are the largest populations of their kind in Northern Eurasia, and some Nenetses hold their domestic reindeer beside their wild cousins. Our analyses of 197 modern and 223 ancient mitochondrial DNA sequences revealed two genetic clusters, which are interpreted as representing the gene pools of contemporary domestic and past wild reindeer. Among a total of 137 different mitochondrial haplotypes identified in both the modern and archaeological samples, only 21 were detected in the modern domestic gene pool, while 11 of these were absent from the wild gene pool. The significant temporal genetic shift that we associate with the pastoral transition suggests that the emergence and spread of reindeer pastoralism in Northwestern Siberia originated with the translocation and subsequent selective breeding of a special type of animal from outside the region. The distinct and persistent domestic characteristics of the haplotype structure since the 18th century suggests little genetic exchange since then. The absence of the typical domestic clade in modern nearby wild populations suggests that the contemporary Nenets domestic breed feature an ancestry from outside its present main distribution, possibly from further South.

A mobile laboratory for ancient DNA analysis

Mobile devices for on-field DNA analysis have been used for medical diagnostics at the point-of-care, forensic investigations and environmental surveys, but still have to be validated for ancient DNA studies. We report here on a mobile laboratory that we setup using commercially available devices, including a compact real-time PCR machine, and describe procedures to perform DNA extraction and analysis from a variety of archeological samples within 4 hours. The process is carried out on 50 mg samples that are identified at the species level using custom TaqMan real-time PCR assays for mitochondrial DNA fragments. We evaluated the potential of this approach in museums lacking facilities for DNA studies by analyzing samples from the Enlène (MIS 2 layer) and the Portel-Ouest cave (MIS 3 deposits), and also performed experiments during an excavation campaign at the Roc-en-Pail (MIS 5) open-air site. Enlène Bovinae bone samples only yielded DNA for the extinct steppe bison (Bison priscus), whereas Portel-Ouest cave coprolites contained cave hyena (Crocuta crocuta spelaea) DNA together, for some of them, with DNA for the European bison sister species/subspecies (Bison schoetensacki/Bb1-X), thus highlighting the cave hyena diet. Roc-en-Pail Bovinae bone and tooth samples also contained DNA for the Bison schoetensacki/Bb1-X clade, and Cervidae bone samples only yielded reindeer (Rangifer tarandus) DNA. Subsequent DNA sequencing analyses confirmed that correct species identification had been achieved using our TaqMan assays, hence validating these assays for future studies. We conclude that our approach enables the rapid genetic characterization of tens of millennia-old archeological samples and is expected to be useful for the on-site screening of museums and freshly excavated samples for DNA content. Because our mobile laboratory is made up of commercially available instruments, this approach is easily accessible to other investigators.

Inter-laboratory study on standardized MPS libraries: evaluation of performance, concordance, and sensitivity using mixtures and degraded DNA

We present results from an inter-laboratory massively parallel sequencing (MPS) study in the framework of the SeqForSTRs project to evaluate forensically relevant parameters, such as performance, concordance, and sensitivity, using a standardized sequencing library including reference material, mixtures, and ancient DNA samples. The standardized library was prepared using the ForenSeq DNA Signature Prep Kit (primer mix A). The library was shared between eight European laboratories located in Austria, France, Germany, The Netherlands, and Sweden to perform MPS on their particular MiSeq FGx sequencers. Despite variation in performance between sequencing runs, all laboratories obtained quality metrics that fell within the manufacturer’s recommended ranges. Furthermore, differences in locus coverage did not inevitably adversely affect heterozygous balance. Inter-laboratory concordance showed 100% concordant genotypes for the included autosomal and Y-STRs, and still, X-STR concordance exceeded 83%. The exclusive reasons for X-STR discordances were drop-outs at DXS10103. Sensitivity experiments demonstrated that correct allele calling varied between sequencing instruments in particular for lower DNA amounts (≤ 125 pg). The analysis of compromised DNA samples showed the drop-out of one sample (FA10013B01A) while for the remaining three degraded DNA samples MPS was able to successfully type ≥ 87% of all aSTRs, ≥ 78% of all Y-STRs, ≥ 68% of all X-STRs, and ≥ 92% of all iSNPs demonstrating that MPS is a promising tool for human identity testing, which in return, has to undergo rigorous in-house validation before it can be implemented into forensic routine casework.

More than skin and bones: Comparing extraction methods and alternative sources of DNA from avian museum specimens

Next-generation sequencing has greatly expanded the utility and value of museum collections by revealing specimens as genomic resources. As the field of museum genomics grows, so does the need for extraction methods that maximize DNA yields. For avian museum specimens, the established method of extracting DNA from toe pads works well for most specimens. However, for some specimens, especially those of birds that are very small or very large, toe pads can be a poor source of DNA. In this study, we apply two DNA extraction methods (phenol-chloroform and silica column) to three different sources of DNA (toe pad, skin punch and bone) from 10 historical avian museum specimens. We show that a modified phenol-chloroform protocol yielded significantly more DNA than a silica column protocol (e.g., Qiagen DNeasy Blood & Tissue Kit) across all tissue types. However, extractions using the silica column protocol contained longer fragments on average than those using the phenol-chloroform protocol, probably as a result of loss of small fragments through the silica column. While toe pads yielded more DNA than skin punches and bone fragments, skin punches proved to be a reliable alternative source of DNA and might be especially appealing when toe pad extractions are impractical. Overall, we found that historical bird museum specimens contain substantial amounts of DNA for genomic studies under most extraction scenarios, but that a phenol-chloroform protocol consistently provides the high quantities of DNA required for most current genomic protocols.

Temporal genomic contrasts reveal rapid evolutionary responses in an alpine mammal during recent climate change

Many species have experienced dramatic changes in their abundance and distribution during recent climate change, but it is often unclear whether such ecological responses are accompanied by evolutionary change. We used targeted exon sequencing of 294 museum specimens (160 historic, 134 modern) to generate independent temporal genomic contrasts spanning a century of climate change (1911-2012) for two co-distributed chipmunk species: an endemic alpine specialist (Tamias alpinus) undergoing severe range contraction and a stable mid-elevation species (T. speciosus). Using a novel analytical approach, we reconstructed the demographic histories of these populations and tested for evidence of recent positive directional selection. Only the retracting species showed substantial population genetic fragmentation through time and this was coupled with positive selection and substantial shifts in allele frequencies at a gene, Alox15, involved in regulation of inflammation and response to hypoxia. However, these rapid population and gene-level responses were not detected in an analogous temporal contrast from another area where T. alpinus has also undergone severe range contraction. Collectively, these results highlight that evolutionary responses may be variable and context dependent across populations, even when they show seemingly synchronous ecological shifts. Our results demonstrate that temporal genomic contrasts can be used to detect very recent evolutionary responses within and among contemporary populations, even in the face of complex demographic changes. Given the wealth of specimens archived in natural history museums, comparative analyses of temporal population genomic data have the potential to improve our understanding of recent and ongoing evolutionary responses to rapidly changing environments.

Old wild wolves: ancient DNA survey unveils population dynamics in Late Pleistocene and Holocene Italian remains

Background

The contemporary Italian wolf (Canis lupus italicus) represents a case of morphological and genetic uniqueness. Today, Italian wolves are also the only documented population to fall exclusively within the mitochondrial haplogroup 2, which was the most diffused across Eurasian and North American wolves during the Late Pleistocene. However, the dynamics leading to such distinctiveness are still debated.

Methods

In order to shed light on the ancient genetic variability of this wolf population and on the origin of its current diversity, we collected 19 Late Pleistocene-Holocene samples from northern Italy, which we analyzed at a short portion of the hypervariable region 1 of the mitochondrial DNA, highly informative for wolf and dog phylogenetic analyses.

Results

Four out of the six detected haplotypes matched the ones found in ancient wolves from northern Europe and Beringia, or in modern European and Chinese wolves, and appeared closely related to the two haplotypes currently found in Italian wolves. The haplotype of two Late Pleistocene samples matched with primitive and contemporary dog sequences from the canine mitochondrial clade A. All these haplotypes belonged to haplogroup 2. The only exception was a Holocene sample dated 3,250 years ago, affiliated to haplogroup 1.

Discussion

In this study we describe the genetic variability of the most ancient wolf specimens from Italy analyzed so far, providing a preliminary overview of the genetic make-up of the population that inhabited this area from the last glacial maximum to the Middle Age period. Our results endorsed that the genetic diversity carried by the Pleistocene wolves here analyzed showed a strong continuity with other northern Eurasian wolf specimens from the same chronological period. Contrarily, the Holocene samples showed a greater similarity only with modern sequences from Europe and Asia, and the occurrence of an haplogroup 1 haplotype allowed to date back previous finding about its presence in this area. Moreover, the unexpected discovery of a 24,700-year-old sample carrying a haplotype that, from the fragment here obtained, falls within the canine clade A, could represent the oldest evidence in Europe of such dog-rich clade. All these findings suggest complex population dynamics that deserve to be further investigated based on mitochondrial or whole genome sequencing.

Howling from the past: historical phylogeography and diversity losses in European grey wolves

Genetic bottlenecks resulting from human-induced population declines make alarming symbols for the irreversible loss of our natural legacy worldwide. The grey wolf (Canis lupus) is an iconic example of extreme declines driven by anthropogenic factors. Here, we assessed the genetic signatures of 150 years of wolf persecution throughout the Western Palaearctic by high-throughput mitochondrial DNA sequencing of historical specimens in an unprecedented spatio-temporal framework. Despite Late Pleistocene bottlenecks, we show that historical genetic variation had remained high throughout Europe until the last several hundred years. In Western Europe, where wolves nearly got fully exterminated, diversity dramatically collapsed at the turn of the twentieth century and recolonization from few homogeneous relict populations induced drastic shifts of genetic composition. By contrast, little genetic displacement and steady levels of diversity were maintained in Eastern European regions, where human persecution had lesser effects on wolf demography. By comparing prehistoric, historic and modern patterns of genetic diversity, our study hence traces the timeframe and the active human role in the decline of the grey wolf, an emblematic yet controversial animal which symbolizes the complex relationship between human societies and nature conservation.

Methodology for Y Chromosome Capture: A complete genome sequence of Y chromosome using flow cytometry, laser microdissection and magnetic streptavidin-beads

This study is a comparison of the efficiency of three technologies used for Y chromosome capture and the next-generation sequencing (NGS) technologies applied for determining its whole sequence. Our main findings disclose that streptavidin-biotin magnetic particle-based capture methodology offers better and a deeper sequence coverage for Y chromosome capture, compared to chromosome sorting and microdissection procedures. Moreover, this methodology is less time consuming and the most selective for capturing only Y chromosomal material, in contrast with other methodologies that result in considerable background material from other, non-targeted chromosomes. NGS results compared between two platforms, NextSeq 500 and SOLID 5500xl, produce the same coverage results. This is the first study to explore a methodological comparison of Y chromosome capture and genetic analysis. Our results indicate an improved strategy for Y chromosome research with applications in several scientific fields where this chromosome plays an important role, such as forensics, medical sciences, molecular anthropology and cancer sciences.

Enhancing the accuracy of next-generation sequencing for detecting rare and subclonal mutations

Mutations, the fuel of evolution, are first manifested as rare DNA changes within a population of cells. Although next-generation sequencing (NGS) technologies have revolutionized the study of genomic variation between species and individual organisms, most have limited ability to accurately detect and quantify rare variants among the different genome copies in heterogeneous mixtures of cells or molecules. We describe the technical challenges in characterizing subclonal variants using conventional NGS protocols and the recent development of error correction strategies, both computational and experimental, including consensus sequencing of single DNA molecules. We also highlight major applications for low-frequency mutation detection in science and medicine, describe emerging methodologies and provide our vision for the future of DNA sequencing.

Awareness of mutational artefacts in suboptimal DNA samples: possible risk for therapeutic choices

BACKGROUND

Technical biases due to PCR artefacts could represent an insidious obstacle for mutational analysis and precision medicine.

METHODS

The authors report a retrospective analysis by fast COLD-PCR and sequencing of 31 suboptimal tumor DNA samples obtained from FFPE tissues and liquid biopsies.

RESULTS

In FFPE tumor tissues and plasma liquid biopsies of patients with lung and colorectal adenocarcinoma, we observed a significant rate of artefactual KRAS mutations, unveiled by repeated analysis following UDG pretreatment as well as by simple repetition without UDG pretreatment step, thus suggesting a DNA damage different from cytosine deamination. UDG pretreatment was not only unnecessary to contrast artefacts occurrence, but also hampered the efficiency of mutational screening, reducing the analytical sensitivity. Taken individually or considered together, the reduced DNA input per reaction and UDG pretreatment limited the detection of 'real' mutated alleles, decreasing PCR sensitivity enough to hamper distinction between artefactual and true subclonal mutations of KRAS.

CONCLUSIONS

Careful validation of analytical sensitivities should always be carried out through standard controls, and strategies other than UDG pretreatment need to be identified to avoid both amplification of artefactual mutations and failure to identify real subclonal mutations.

Biological Sexing of a 4000-Year-Old Egyptian Mummy Head to Assess the Potential of Nuclear DNA Recovery from the Most Damaged and Limited Forensic Specimens

High throughput sequencing (HTS) has been used for a number of years in the field of paleogenomics to facilitate the recovery of small DNA fragments from ancient specimens. Recently, these techniques have also been applied in forensics, where they have been used for the recovery of mitochondrial DNA sequences from samples where traditional PCR-based assays fail because of the very short length of endogenous DNA molecules. Here, we describe the biological sexing of a ~4000-year-old Egyptian mummy using shotgun sequencing and two established methods of biological sex determination (R_X and R_Y), by way of mitochondrial genome analysis as a means of sequence data authentication. This particular case of historical interest increases the potential utility of HTS techniques for forensic purposes by demonstrating that data from the more discriminatory nuclear genome can be recovered from the most damaged specimens, even in cases where mitochondrial DNA cannot be recovered with current PCR-based forensic technologies. Although additional work remains to be done before nuclear DNA recovered via these methods can be used routinely in operational casework for individual identification purposes, these results indicate substantial promise for the retrieval of probative individually identifying DNA data from the most limited and degraded forensic specimens.

Single-strand DNA library preparation improves sequencing of formalin-fixed and paraffin-embedded (FFPE) cancer DNA

DNA derived from formalin-fixed and paraffin-embedded (FFPE) tissue has been a challenge to large-scale genomic sequencing, due to its low quality and quantities. Improved techniques enabling the genome-wide analysis of FFPE material would be of great value, both from a research and clinical perspective.

Comparing a single-strand DNA library preparation method originally developed for ancient DNA to conventional protocols using double-stranded DNA derived from FFPE material we obtain on average 900-fold more library molecules and improved sequence complexity from as little as 5 ng input DNA. FFPE DNA is highly fragmented, usually below 100bp, and up to 60% of reads start after or end prior to adenine residues, suggesting that crosslinks predominate at adenine residues. Similar to ancient DNA, C > T substitutions are slightly increased with maximum rates up to 3% at the ends of molecules. In whole exome sequencing of single-strand libraries from lung, breast, colorectal, prostate and skin cancers we identify known cancer mutations. In summary, we show that single-strand library preparation enables genomic sequencing, even from low amounts of degraded FFPE DNA. This method provides a clear advantage both in research and clinical settings, where FFPE material (e.g. from biopsies) often is the only source of DNA available. Improving the genetic characterization that can be performed on conventional archived FFPE tissue, the single-strand library preparation allows scarce samples to be used in personalized medicine and enables larger sample sizes in future sequencing studies.

Genome skimming herbarium specimens for DNA barcoding and phylogenomics

BACKGROUND

The world's herbaria contain millions of specimens, collected and named by thousands of researchers, over hundreds of years. However, this treasure has remained largely inaccessible to genetic studies, because of both generally limited success of DNA extraction and the challenges associated with PCR-amplifying highly degraded DNA. In today's next-generation sequencing world, opportunities and prospects for historical DNA have changed dramatically, as most NGS methods are actually designed for taking short fragmented DNA molecules as templates.

RESULTS

As a practical test of routine recovery of rDNA and plastid genome sequences from herbarium specimens, we sequenced 25 herbarium specimens up to 80 years old from 16 different Angiosperm families. Paired-end reads were generated, yielding successful plastid genome assemblies for 23 species and nuclear rDNAs for 24 species, respectively. These data showed that genome skimming can be used to generate genomic information from herbarium specimens as old as 80 years and using as little as 500 pg of degraded starting DNA.

CONCLUSIONS

The routine plastome sequencing from herbarium specimens is feasible and cost-effective (compare with Sanger sequencing or plastome-enrichment approaches), and can be performed with limited sample destruction.

Sex Determination from Fragmented and Degenerated DNA by Amplified Product-Length Polymorphism Bidirectional SNP Analysis of Amelogenin and SRY Genes

Sex determination is important in archeology and anthropology for the study of past societies, cultures, and human activities. Sex determination is also one of the most important components of individual identification in criminal investigations. We developed a new method of sex determination by detecting a single-nucleotide polymorphism in the amelogenin gene using amplified product-length polymorphisms in combination with sex-determining region Y analysis. We particularly focused on the most common types of postmortem DNA damage in ancient and forensic samples: fragmentation and nucleotide modification resulting from deamination. Amplicon size was designed to be less than 60 bp to make the method more useful for analyzing degraded DNA samples. All DNA samples collected from eight Japanese individuals (four male, four female) were evaluated correctly using our method. The detection limit for accurate sex determination was determined to be 20 pg of DNA. We compared our new method with commercial short tandem repeat analysis kits using DNA samples artificially fragmented by ultraviolet irradiation. Our novel method was the most robust for highly fragmented DNA samples. To deal with allelic dropout resulting from deamination, we adopted “bidirectional analysis,” which analyzed samples from both sense and antisense strands. This new method was applied to 14 Jomon individuals (3500-year-old bone samples) whose sex had been identified morphologically. We could correctly identify the sex of 11 out of 14 individuals. These results show that our method is reliable for the sex determination of highly degenerated samples.

How does inflammation drive mutagenesis in colorectal cancer?

Colorectal cancer (CRC) is a major health challenge worldwide. Factors thought to be important in CRC etiology include diet, microbiome, exercise, obesity, a history of colon inflammation and family history. Interventions, including the use of non-steroidal anti-Inflammatory drugs (NSAIDs) and anti-inflammatory agents, have been shown to decrease incidence in some settings. However, our current understanding of the mechanistic details that drive CRC are insufficient to sort out the complex and interacting factors responsible for cancer-initiating events. It has been known for some time that the development of CRC involves mutations in key genes such as p53 and APC, and the sequence in which these mutations occur can determine tumor presentation. Observed recurrent mutations are dominated by C to T transitions at CpG sites, implicating the deamination of 5-methylcytosine (5mC) as a key initiating event in cancer-driving mutations. While it has been widely assumed that inflammation-mediated oxidation drives mutations in CRC, oxidative damage to DNA induces primarily G to T transversions, not C to T transitions. In this review, we discuss this unresolved conundrum, and specifically, we elucidate how the known nucleotide excision repair (NER) and base excision repair (BER) pathways, which are partially redundant and potentially competing, might provide a critical link between oxidative DNA damage and C to T mutations. Studies using recently developed next-generation DNA sequencing technologies have revealed the genetic heterogeneity in human tissues including tumors, as well as the presence of DNA damage. The capacity to follow DNA damage, repair and mutagenesis in human tissues using these emerging technologies could provide a mechanistic basis for understanding the role of oxidative damage in CRC tumor initiation. The application of these technologies could identify mechanism-based biomarkers useful in earlier diagnosis and aid in the development of cancer prevention strategies.

Considering DNA damage when interpreting mtDNA heteroplasmy in deep sequencing data

Resolution of mitochondrial (mt) DNA heteroplasmy is now possible when applying a massively parallel sequencing (MPS) approach, including minor components down to 1%. However, reporting thresholds and interpretation criteria will need to be established for calling heteroplasmic variants that address a number of important topics, one of which is DNA damage. We assessed the impact of increasing amounts of DNA damage on the interpretation of minor component sequence variants in the mtDNA control region, including low-level mixed sites. A passive approach was used to evaluate the impact of storage conditions, and an active approach was employed to accelerate the process of hydrolytic damage (for example, replication errors associated with depurination events). The patterns of damage were compared and assessed in relation to damage typically encountered in poor quality samples. As expected, the number of miscoding lesions increased as conditions worsened. Single nucleotide polymorphisms (SNPs) associated with miscoding lesions were indistinguishable from innate heteroplasmy and were most often observed as 1-2% of the total sequencing reads. Numerous examples of miscoding lesions above 2% were identified, including two complete changes in the nucleotide sequence, presenting a challenge when assessing the placement of reporting thresholds for heteroplasmy. To mitigate the impact, replication of miscoding lesions was not observed in stored samples, and was rarely seen in data associated with accelerated hydrolysis. In addition, a significant decrease in the expected transition:transversion ratio was observed, providing a useful tool for predicting the presence of damage-induced lesions. The results of this study directly impact MPS analysis of minor sequence variants from poorly preserved DNA extracts, and when biological samples have been exposed to agents that induce DNA damage. These findings are particularly relevant to clinical and forensic investigations.

Combined hybridization capture and shotgun sequencing for ancient DNA analysis of extinct wild and domestic dromedary camel

The performance of hybridization capture combined with next‐generation sequencing (NGS) has seen limited investigation with samples from hot and arid regions until now. We applied hybridization capture and shotgun sequencing to recover DNA sequences from bone specimens of ancient‐domestic dromedary (Camelus dromedarius) and its extinct ancestor, the wild dromedary from Jordan, Syria, Turkey and the Arabian Peninsula, respectively. Our results show that hybridization capture increased the percentage of mitochondrial DNA (mtDNA) recovery by an average 187‐fold and in some cases yielded virtually complete mitochondrial (mt) genomes at multifold coverage in a single capture experiment. Furthermore, we tested the effect of hybridization temperature and time by using a touchdown approach on a limited number of samples. We observed no significant difference in the number of unique dromedary mtDNA reads retrieved with the standard capture compared to the touchdown method. In total, we obtained 14 partial mitochondrial genomes from ancient‐domestic dromedaries with 17–95% length coverage and 1.27–47.1‐fold read depths for the covered regions. Using whole‐genome shotgun sequencing, we successfully recovered endogenous dromedary nuclear DNA (nuDNA) from domestic and wild dromedary specimens with 1–1.06‐fold read depths for covered regions. Our results highlight that despite recent methodological advances, obtaining ancient DNA (aDNA) from specimens recovered from hot, arid environments is still problematic. Hybridization protocols require specific optimization, and samples at the limit of DNA preservation need multiple replications of DNA extraction and hybridization capture as has been shown previously for Middle Pleistocene specimens.

Artifactual mutations resulting from DNA lesions limit detection levels in ultrasensitive sequencing applications

The need in cancer research or evolutionary biology to detect rare mutations or variants present at very low frequencies (<10⁻⁵) poses an increasing demand on lowering the detection limits of available methods. Here we demonstrated that amplifiable DNA lesions introduce important error sources in ultrasensitive technologies such as single molecule PCR (smPCR) applications (e.g. droplet-digital PCR), or next-generation sequencing (NGS) based methods. Using templates with known amplifiable lesions (8-oxoguanine, deaminated 5-methylcytosine, uracil, and DNA heteroduplexes), we assessed with smPCR and duplex sequencing that templates with these lesions were amplified very efficiently by proofreading polymerases (except uracil), leading to G->T, and to a lesser extent, to unreported G->C substitutions at 8-oxoguanine lesions, and C->T transitions in amplified uracil containing templates. Long heat incubations common in many DNA extraction protocols significantly increased the number of G->T substitutions. Moreover, in ∼50-80% smPCR reactions we observed the random amplification preference of only one of both DNA strands explaining the known ‘PCR jackpot effect’, with the result that a lesion became indistinguishable from a true mutation or variant. Finally, we showed that artifactual mutations derived from uracil and 8-oxoguanine could be significantly reduced by DNA repair enzymes.

Hybridization Capture Using RAD Probes (hyRAD), a New Tool for Performing Genomic Analyses on Collection Specimens

In the recent years, many protocols aimed at reproducibly sequencing reduced-genome subsets in non-model organisms have been published. Among them, RAD-sequencing is one of the most widely used. It relies on digesting DNA with specific restriction enzymes and performing size selection on the resulting fragments. Despite its acknowledged utility, this method is of limited use with degraded DNA samples, such as those isolated from museum specimens, as these samples are less likely to harbor fragments long enough to comprise two restriction sites making possible ligation of the adapter sequences (in the case of double-digest RAD) or performing size selection of the resulting fragments (in the case of single-digest RAD). Here, we address these limitations by presenting a novel method called hybridization RAD (hyRAD). In this approach, biotinylated RAD fragments, covering a random fraction of the genome, are used as baits for capturing homologous fragments from genomic shotgun sequencing libraries. This simple and cost-effective approach allows sequencing of orthologous loci even from highly degraded DNA samples, opening new avenues of research in the field of museum genomics. Not relying on the restriction site presence, it improves among-sample loci coverage. In a trial study, hyRAD allowed us to obtain a large set of orthologous loci from fresh and museum samples from a non-model butterfly species, with a high proportion of single nucleotide polymorphisms present in all eight analyzed specimens, including 58-year-old museum samples. The utility of the method was further validated using 49 museum and fresh samples of a Palearctic grasshopper species for which the spatial genetic structure was previously assessed using mtDNA amplicons. The application of the method is eventually discussed in a wider context. As it does not rely on the restriction site presence, it is therefore not sensitive to among-sample loci polymorphisms in the restriction sites that usually causes loci dropout. This should enable the application of hyRAD to analyses at broader evolutionary scales.

Ultra-precise detection of mutations by droplet-based amplification of circularized DNA

Background

NGS (next generation sequencing) has been widely used in studies of biological processes, ranging from microbial evolution to cancer genomics. However, the error rate of NGS (0.1 % ~ 1 %) is still remaining a great challenge for comprehensively investigating the low frequency variations, and the current solution methods have suffered severe amplification bias or low efficiency.

Results

We creatively developed Droplet-CirSeq for relatively efficient, low-bias and ultra-sensitive identification of variations by combining millions of picoliter uniform-sized droplets with Cir-seq. Droplet-CirSeq is entitled with an incredibly low error rate of 3 ~ 5 X 10^-6. To systematically evaluate the performances of amplification uniformity and capability of mutation identification for Droplet-CirSeq, we took the mixtures of two E. coli strains as specific instances to simulate the circumstances of mutations with different frequencies. Compared with Cir-seq, the coefficient of variance of read depth for Droplet-CirSeq was 10 times less (p = 2.6 X 10^-3), and the identified allele frequency presented more concentrated to the authentic frequency of mixtures (p = 4.8 X 10^-3), illustrating a significant improvement of amplification bias and accuracy in allele frequency determination. Additionally, Droplet-CirSeq detected 2.5 times genuine SNPs (p < 0.001), achieved a 2.8 times lower false positive rate (p < 0.05) and a 1.5 times lower false negative rate (p < 0.001), in the case of a 3 pg DNA input. Intriguingly, the false positive sites predominantly represented in two types of base substitutions (G- > A, C- > T). Our findings indicated that 30 pg DNA input accommodated in 5 ~ 10 million droplets resulted in maximal detection of authentic mutations compared to 3 pg (p = 1.2 X 10^-8) and 300 pg input (p = 2.2 X 10^-3).

Conclusions

We developed a method namely Droplet-CirSeq to significantly improve the amplification bias, which presents obvious superiority over the currently prevalent methods in exploitation of ultra-low frequency mutations. Droplet-CirSeq would be promisingly used in the identification of low frequency mutations initiated from extremely low input DNA, such as DNA of uncultured microorganisms, captured DNA of target region, circulation DNA of plasma et al, and its creative conception of rolling circle amplification in droplets would also be used in other low input DNA amplification fields.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2480-1) contains supplementary material, which is available to authorized users.

The First Attested Extraction of Ancient DNA in Legumes (Fabaceae)

Ancient DNA (aDNA) is any DNA extracted from ancient specimens, important for diverse evolutionary researches. The major obstacles in aDNA studies are mutations, contamination and fragmentation. Its studies may be crucial for crop history if integrated with human aDNA research and historical linguistics, both general and relating to agriculture. Legumes (Fabaceae) are one of the richest end economically most important plant families, not only from Neolithic onwards, since they were used as food by Neanderthals and Paleolithic modern man. The idea of extracting and analyzing legume aDNA was considered beneficial for both basic science and applied research, with an emphasis on genetic resources and plant breeding. The first reported successful and attested extraction of the legume aDNA was done from the sample of charred seeds of pea (Pisum sativum) and bitter vetch (Vicia ervilia) from Hissar, southeast Serbia, dated to 1,350-1,000 Before Christ. A modified version of cetyltrimethylammonium bromide (CTAB) method and the commercial kit for DNA extraction QIAGEN DNAesy yielded several ng μl(-1) of aDNA of both species and, after the whole genome amplification and with a fragment of nuclear ribosomal DNA gene 26S rDNA, resulted in the detection of the aDNA among the PCR products. A comparative analysis of four informative chloroplast DNA regions (trnSG, trnK, matK, and rbcL) among the modern wild and cultivated pea taxa demonstrated not only that the extracted aDNA was genuine, on the basis of mutation rate, but also that the ancient Hissar pea was most likely an early domesticated crop, related to the modern wild pea of a neighboring region. It is anticipated that this premier extraction of legume aDNA may provide taxonomists with the answers to diverse questions, such as leaf development in legumes, as well as with novel data on the single steps in domesticating legume crops worldwide.

Partial uracil-DNA-glycosylase treatment for screening of ancient DNA

The challenge of sequencing ancient DNA has led to the development of specialized laboratory protocols that have focused on reducing contamination and maximizing the number of molecules that are extracted from ancient remains. Despite the fact that success in ancient DNA studies is typically obtained by screening many samples to identify a promising subset, ancient DNA protocols have not, in general, focused on reducing the time required to screen samples. We present an adaptation of a popular ancient library preparation method that makes screening more efficient. First, the DNA extract is treated using a protocol that causes characteristic ancient DNA damage to be restricted to the terminal nucleotides, while nearly eliminating it in the interior of the DNA molecules, allowing a single library to be used both to test for ancient DNA authenticity and to carry out population genetic analysis. Second, the DNA molecules are ligated to a unique pair of barcodes, which eliminates undetected cross-contamination from this step onwards. Third, the barcoded library molecules include incomplete adapters of short length that can increase the specificity of hybridization-based genomic target enrichment. The adapters are completed just before sequencing, so the same DNA library can be used in multiple experiments, and the sequences distinguished. We demonstrate this protocol on 60 ancient human samples.

Reconstructing ancient genomes and epigenomes

Research involving ancient DNA (aDNA) has experienced a true technological revolution in recent years through advances in the recovery of aDNA and, particularly, through applications of high-throughput sequencing. Formerly restricted to the analysis of only limited amounts of genetic information, aDNA studies have now progressed to whole-genome sequencing for an increasing number of ancient individuals and extinct species, as well as to epigenomic characterization. Such advances have enabled the sequencing of specimens of up to 1 million years old, which, owing to their extensive DNA damage and contamination, were previously not amenable to genetic analyses. In this Review, we discuss these varied technical challenges and solutions for sequencing ancient genomes and epigenomes.

High-throughput sequencing in mutation detection: A new generation of genotoxicity tests?

The advent of next generation sequencing (NGS) technology has provided the means to directly analyze the genetic material in primary cells or tissues of any species in a high throughput manner for mutagenic effects of potential genotoxic agents. In principle, direct, genome-wide sequencing of human primary cells and/or tissue biopsies would open up opportunities to identify individuals possibly exposed to mutagenic agents, thereby replacing current risk assessment procedures based on surrogate markers and extrapolations from animal studies. NGS-based tests can also precisely characterize the mutation spectra induced by genotoxic agents, improving our knowledge of their mechanism of action. Thus far, NGS has not been widely employed in genetic toxicology due to the difficulties in measuring low-abundant somatic mutations. Here, we review different strategies to employ NGS for the detection of somatic mutations in a cost-effective manner and discuss the potential applicability of these methods in testing the mutagenicity of genotoxic agents.

Ancient human genomics: the methodology behind reconstructing evolutionary pathways

High-throughput sequencing (HTS) has radically altered approaches to human evolutionary research. Recent contributions highlight that HTS is able to reach depths of the human lineage previously thought to be impossible. In this paper, we outline the methodological advances afforded by recent developments in DNA recovery, data output, scalability, speed, and resolution of the current sequencing technology. We review and critically evaluate the 'DNA pipeline' for ancient samples: from DNA extraction, to constructing immortalized sequence libraries, to enrichment strategies (e.g., polymerase chain reaction [PCR] and hybridization capture), and finally, to bioinformatic analyses of sequence data. We argue that continued evaluations and improvements to this process are essential to ensure sequence data validity. Also, we highlight the role of contamination and authentication in ancient DNA-HTS, which is particularly relevant to ancient human genomics, since sequencing the genomes of hominins such as Homo erectus and Homo heidelbergensis may soon be within the realm of possibility.

Ancient DNA reveals matrilineal continuity in present-day Poland over the last two millennia

While numerous ancient human DNA datasets from across Europe have been published till date, modern-day Poland in particular, remains uninvestigated. Besides application in the reconstruction of continent-wide human history, data from this region would also contribute towards our understanding of the history of the Slavs, whose origin is hypothesized to be in East or Central Europe. Here, we present the first population-scale ancient human DNA study from the region of modern-day Poland by establishing mitochondrial DNA profiles for 23 samples dated to 200 BC – 500 AD (Roman Iron Age) and for 20 samples dated to 1000–1400 AD (Medieval Age). Our results show that mitochondrial DNA sequences from both periods belong to haplogroups that are characteristic of contemporary West Eurasia. Haplotype sharing analysis indicates that majority of the ancient haplotypes are widespread in some modern Europeans, including Poles. Notably, the Roman Iron Age samples share more rare haplotypes with Central and Northeast Europeans, whereas the Medieval Age samples share more rare haplotypes with East-Central and South-East Europeans, primarily Slavic populations. Our data demonstrates genetic continuity of certain matrilineages (H5a1 and N1a1a2) in the area of present-day Poland from at least the Roman Iron Age until present. As such, the maternal gene pool of present-day Poles, Czechs and Slovaks, categorized as Western Slavs, is likely to have descended from inhabitants of East-Central Europe during the Roman Iron Age.