Whole-Genome Capture of Ancient DNA Using Homemade Baits

A Million Years of Mammoth Mitogenome Evolution

The genomic study of specimens dating to the Early and Middle Pleistocene (EP and MP), a period spanning from 2.6 million years ago (Ma) to 126 thousand years ago (ka), has the potential to elucidate the evolutionary processes that shaped present-day biodiversity. Obtaining genomic data from this period is challenging, but mitochondrial DNA, given its higher abundance compared to nuclear DNA, could play an important role to understand evolutionary processes at this time scale. In this study, we report 34 new mitogenomes, including two EP and nine MP mammoth (Mammuthus spp.) specimens from Siberia and North America and analyze them jointly with >200 publicly available mitogenomes to reconstruct a transect of mammoth mitogenome diversity throughout the last million years. We find that our EP mitogenomes fall outside the diversity of all Late Pleistocene (LP) mammoths, while those derived from MP mammoths are basal to LP mammoth Clades 2 and 3, supporting an ancient Siberian origin of these lineages. In contrast, the geographical origin of Clade 1 remains unresolved. With these new deep-time mitogenomes, we observe diversification events across all clades that appear consistent with previously hypothesized MP and LP demographic changes. Furthermore, we improve upon an existing methodology for molecular clock dating of specimens >50 ka, demonstrating that specimens need to be individually dated to avoid biases in their age estimates. Both the molecular and analytical improvements presented here highlight the importance of deep-time genomic data to discover long-lost genetic diversity, enabling better assessments of evolutionary histories.

Targeted enrichment of whole-genome SNPs from highly burned skeletal remains

Genetic assessment of highly incinerated and/or degraded human skeletal material is a persistent challenge in forensic DNA analysis, including identifying victims of mass disasters. Few studies have investigated the impact of thermal degradation on whole-genome single-nucleotide polymorphism (SNP) quality and quantity using next-generation sequencing (NGS). We present whole-genome SNP data obtained from the bones and teeth of 27 fire victims using two DNA extraction techniques. Extracts were converted to double-stranded DNA libraries then enriched for whole-genome SNPs using unpublished biotinylated RNA baits and sequenced on an Illumina NextSeq 550 platform. Raw reads were processed using the EAGER (Efficient Ancient Genome Reconstruction) pipeline, and the SNPs filtered and called using FreeBayes and GATK (v. 3.8). Mixed-effects modeling of the data suggest that SNP variability and preservation is predominantly determined by skeletal element and burn category, and not by extraction type. Whole-genome SNP data suggest that selecting long bones, hand and foot bones, and teeth subjected to temperatures <350°C are the most likely sources for higher genomic DNA yields. Furthermore, we observed an inverse correlation between the number of captured SNPs and the extent to which samples were burned, as well as a significant decrease in the total number of SNPs measured for samples subjected to temperatures >350°C. Our data complement previous analyses of burned human remains that compare extraction methods for downstream forensic applications and support the idea of adopting a modified Dabney extraction technique when traditional forensic methods fail to produce DNA yields sufficient for genetic identification.

Ancient mitochondrial genomes unveil the origins and evolutionary history of New Zealand's enigmatic takahē and moho

Many avian species endemic to Aotearoa New Zealand were driven to extinction or reduced to relict populations following successive waves of human arrival, due to hunting, habitat destruction and the introduction of mammalian predators. Among the affected species were the large flightless South Island takahē (Porphyrio hochstetteri) and the moho (North Island takahē; P. mantelli), with the latter rendered extinct and the former reduced to a single relictual population. Little is known about the evolutionary history of these species prior to their decline and/or extinction. Here we sequenced mitochondrial genomes from takahē and moho subfossils (12 takahē and 4 moho) and retrieved comparable sequence data from takahē museum skins (n = 5) and contemporary individuals (n = 17) to examine the phylogeny and recent evolutionary history of these species. Our analyses suggest that prehistoric takahē populations lacked deep phylogeographic structure, in contrast to moho, which exhibited significant spatial genetic structure, albeit based on limited sample sizes (n = 4). Temporal genetic comparisons show that takahē have lost much of their mitochondrial genetic diversity, likely due to a sudden demographic decline soon after human arrival (~750 years ago). Time-calibrated phylogenetic analyses strongly support a sister species relationship between takahē and moho, suggesting these flightless taxa diverged around 1.5 million years ago, following a single colonisation of New Zealand by a flighted Porphyrio ancestor approximately 4 million years ago. This study highlights the utility of palaeogenetic approaches for informing the conservation and systematic understanding of endangered species whose ranges have been severely restricted by anthropogenic impacts.

Ancient DNA is preserved in fish fossils from tropical lake sediments

Tropical freshwater lakes are well known for their high biodiversity, and particularly the East African Great Lakes are renowned for their adaptive radiation of cichlid fishes. While comparative phylogenetic analyses of extant species flocks have revealed patterns and processes of their diversification, little is known about evolutionary trajectories within lineages, the impacts of environmental drivers, or the scope and nature of now-extinct diversity. Time-structured palaeodata from geologically young fossil records, such as fossil counts and particularly ancient DNA (aDNA) data, would help fill this large knowledge gap. High ambient temperatures can be detrimental to the preservation of DNA, but refined methodology now allows data generation even from very poorly preserved samples. Here, we show for the first time that fish fossils from tropical lake sediments yield endogenous aDNA. Despite generally low endogenous content and high sample dropout, the application of high-throughput sequencing and, in some cases, sequence capture allowed taxonomic assignment and phylogenetic placement of 17% of analysed fish fossils to family or tribe level, including remains which are up to 2700 years old or weigh less than 1 mg. The relationship between aDNA degradation and the thermal age of samples is similar to that described for terrestrial samples from cold environments when adjusted for elevated temperature. Success rates and aDNA preservation differed between the investigated lakes Chala, Kivu and Victoria, possibly caused by differences in bottom water oxygenation. Our study demonstrates that the sediment records of tropical lakes can preserve genetic information on rapidly diversifying fish taxa over time scales of millennia.

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.

Ancient DNA from the extinct New Zealand grayling (Prototroctes oxyrhynchus) reveals evidence for Miocene marine dispersal

The evolutionary history of Southern Hemisphere graylings (Retropinnidae) in New Zealand (NZ), including their relationship to the Australian grayling, is poorly understood. The NZ grayling (Prototroctes oxyrhynchus) is the only known fish in NZ to have gone extinct since human arrival there. Despite its historical abundance, only 23 wet and dried, formalin-fixed specimens exist in museums. We used high-throughput DNA sequencing to generate mitogenomes from formalin-fixed P. oxyrhynchus specimens, and analysed these in a temporal phylogenetic framework of retropinnids and osmerids. We recovered a strong sister-relationship between NZ and Australian grayling (P. mareana), with a common ancestor ~13.8 Mya [95% highest posterior density (HPD): 6.1–23.2 Mya], after the height of Oligocene marine inundation in NZ. Our temporal phylogenetic analysis suggests a single marine dispersal between NZ and Australia, although the direction of dispersal is equivocal, followed by divergence into genetically and morphologically distinguishable species through isolation by distance. This study provides further insights into the possible extinction drivers of the NZ grayling, informs discussion regarding reintroduction of Prototroctes to NZ and highlights how advances in palaeogenetics can be used to test evolutionary hypotheses in fish, which, until relatively recently, have been comparatively neglected in ancient-DNA research.

Paleogenomes Reveal a Complex Evolutionary History of Late Pleistocene Bison in Northeastern China

Steppe bison are a typical representative of the Mid-Late Pleistocene steppes of the northern hemisphere. Despite the abundance of fossil remains, many questions related to their genetic diversity, population structure and dispersal route are still elusive. Here, we present both near-complete and partial mitochondrial genomes, as well as a partial nuclear genome from fossil bison samples excavated from Late Pleistocene strata in northeastern China. Maximum-likelihood and Bayesian trees both suggest the bison clade are divided into three maternal haplogroups (A, B and C), and Chinese individuals fall in two of them. Bayesian analysis shows that the split between haplogroup C and the ancestor of haplogroups A and B dates at 326 ky BP (95% HPD: 397-264 ky BP). In addition, our nuclear phylogenomic tree also supports a basal position for the individual carrying haplogroup C. Admixture analyses suggest that CADG467 (haplogroup C) has a similar genetic structure to steppe bison from Siberia (haplogroup B). Our new findings indicate that the genetic diversity of Pleistocene bison was probably even higher than previously thought and that northeastern Chinese populations of several mammalian species, including Pleistocene bison, were genetically distinct.

Application of palaeogenetic techniques to historic mollusc shells reveals phylogeographic structure in a New Zealand abalone

Natural history collections worldwide contain a plethora of mollusc shells. Recent studies have detailed the sequencing of DNA extracted from shells up to thousands of years old and from various taphonomic and preservational contexts. However, previous approaches have largely addressed methodological rather than evolutionary research questions. Here we report the generation of DNA sequence data from mollusc shells using such techniques, applied to Haliotis virginea Gmelin, 1791, a New Zealand abalone, in which morphological variation has led to the recognition of several forms and subspecies. We successfully recovered near-complete mitogenomes from 22 specimens including 12 dry-preserved shells up to 60 years old. We used a combination of palaeogenetic techniques that have not previously been applied to shell, including DNA extraction optimized for ultra-short fragments and hybridization-capture of single-stranded DNA libraries. Phylogenetic analyses revealed three major, well-supported clades comprising samples from: 1) the Three Kings Islands; 2) the Auckland, Chatham and Antipodes Islands; and 3) mainland New Zealand and Campbell Island. This phylogeographic structure does not correspond to the currently recognized forms. Critically, our non-reliance on freshly collected or ethanol-preserved samples enabled inclusion of topotypes of all recognized subspecies as well as additional difficult-to-sample populations. Broader application of these comparatively cost-effective and reliable methods to modern, historical, archaeological and palaeontological shell samples has the potential to revolutionize invertebrate genetic research.

Simultaneous Barcode Sequencing of Diverse Museum Collection Specimens Using a Mixed RNA Bait Set

A growing number of publications presenting results from sequencing natural history collection specimens reflect the importance of DNA sequence information from such samples. Ancient DNA extraction and library preparation methods in combination with target gene capture are a way of unlocking archival DNA, including from formalin-fixed wet-collection material. Here we report on an experiment, in which we used an RNA bait set containing baits from a wide taxonomic range of species for DNA hybridisation capture of nuclear and mitochondrial targets for analysing natural history collection specimens. The bait set used consists of 2,492 mitochondrial and 530 nuclear RNA baits and comprises specific barcode loci of diverse animal groups including both invertebrates and vertebrates. The baits allowed to capture DNA sequence information of target barcode loci from 84% of the 37 samples tested, with nuclear markers being captured more frequently and consensus sequences of these being more complete compared to mitochondrial markers. Samples from dry material had a higher rate of success than wet-collection specimens, although target sequence information could be captured from 50% of formalin-fixed samples. Our study illustrates how efforts to obtain barcode sequence information from natural history collection specimens may be combined and are a way of implementing barcoding inventories of scientific collection material.

Ancient Mitogenomes Provide New Insights into the Origin and Early Introduction of Chinese Domestic Donkeys

Both molecular data and archaeological evidence strongly support an African origin for the domestic donkey. Recent genetic studies further suggest that there were two distinct maternal lineages involved in its initial domestication. However, the exact introduction time and the dispersal process of domestic donkeys into ancient China are still unresolved. To address these questions, we retrieved three near-complete mitochondrial genomes from donkey specimens excavated from Gaoling County, Shaanxi Province, and Linxia Basin, Gansu Province, China, dated at 2,349-2,301, 469-311, and 2,160-2,004 cal. BP, respectively. Maximum-likelihood and Bayesian phylogenetic analyses reveal that the two older samples fall into the two different main lineages (i.e., clade Ⅰ and clade Ⅱ) of the domestic donkey, suggesting that the two donkey maternal lineages had been introduced into Midwestern China at least at the opening of Silk Road (approximately the first century BC). Bayesian analysis shows that the split of the two donkey maternal lineages is dated at 0.323 Ma (95% CI: 0.583–0.191 Ma) using root-tip dating calibrations based on near-complete mitogenomes, supporting the hypothesis that modern domestic donkeys go back to at least two independent domestication events. Moreover, Bayesian skyline plot analyses indicate an apparent female population increase between 5,000 and 2,500 years ago for clade I followed by a stable population size to the present day. In contrast, clade II keeps a relatively stable population size over the past 5,000 years. Overall, our study provides new insights into the early domestication history of Chinese domestic donkeys.

Estimating the dwarfing rate of an extinct Sicilian elephant

Evolution on islands, together with the often extreme phenotypic changes associated with it, has attracted much interest from evolutionary biologists. However, measuring the rate of change of phenotypic traits of extinct animals can be challenging, in part due to the incompleteness of the fossil record. Here, we use combined molecular and fossil evidence to define the minimum and maximum rate of dwarfing in an extinct Mediterranean dwarf elephant from Puntali Cave (Sicily).¹ Despite the challenges associated with recovering ancient DNA from warm climates,² we successfully retrieved a mitogenome from a sample with an estimated age between 175,500 and 50,000 years. Our results suggest that this specific Sicilian elephant lineage evolved from one of the largest terrestrial mammals that ever lived³ to an island species weighing less than 20% of its original mass with an estimated mass reduction between 0.74 and 200.95 kg and height reduction between 0.15 and 41.49 mm per generation. We show that combining ancient DNA with paleontological and geochronological evidence can constrain the timing of phenotypic changes with greater accuracy than could be achieved using any source of evidence in isolation.

Successful application of ancient DNA extraction and library construction protocols to museum wet collection specimens

Millions of scientific specimens are housed in museum collections, a large part of which are fluid preserved. The use of formaldehyde as fixative and subsequent storage in ethanol is especially common in ichthyology and herpetology. This type of preservation damages DNA and reduces the chance of successful retrieval of genetic data. We applied ancient DNA extraction and single stranded library construction protocols to a variety of vertebrate samples obtained from wet collections and of different ages. Our results show that almost all samples tested yielded endogenous DNA. Archival DNA extraction was successful across different tissue types as well as using small amounts of tissue. Conversion of archival DNA fragments into single-stranded libraries resulted in usable data even for samples with initially undetectable DNA amounts. Subsequent target capture approaches for mitochondrial DNA using homemade baits on a subset of 30 samples resulted in almost complete mitochondrial genome sequences in several instances. Thus, application of ancient DNA methodology makes wet collection specimens, including type material as well as rare, old or extinct species, accessible for genetic and genomic analyses. Our results, accompanied by detailed step-by-step protocols, are a large step forward to open the DNA archive of museum wet collections for scientific studies.

High-throughput DNA sequencing of museum specimens sheds light on the long-missing species of the Bokermannohyla claresignata group (Anura: Hylidae: Cophomantini)

The two species of the Bokermannohyla claresignata species group (Anura: Hylidae) have not been collected for the last four decades. It is the only species group of the hyline tribe Cophomantini that has not yet been analysed genetically. Its phylogenetic position is thus uncertain, and it has a combination of adult and larval character states that make this group a crucial missing piece that hinders our understanding of Cophomantini phylogenetics and character evolution. We obtained DNA sequences from a museum larval specimen of Bok. claresignata, using specialized extraction methods and high-throughput DNA sequencing, and combined the molecular phylogenetic results with available phenotypic information to provide new insights into the taxonomy and phylogenetic relationships of its species group. Our phylogenetic results place Bok. claresignata as sister to the Boana pulchella group, supporting its inclusion in Boana, together with Bokermannohyla clepsydra. In light of this new finding, we recognize a newly defined Boana claresignata group to accommodate these species, thus resolving both the polyphyly of Bokermannohyla and the paraphyly of Boana. Considering the phylogenetic relationships of the Boana claresignata group, we also discuss the evolution of suctorial tadpoles and mature oocyte/egg pigmentation in Cophomantini.