Divergent mitochondrial lineages arose within a large,panmictic population of the Savannah sparrow (Passerculus sandwichensis)

Unusual patterns of mtDNA diversity can reveal interesting aspects of a species’ biology. However, making such inferences requires discerning among the many alternative scenarios that could underlie any given mtDNA pattern. Next‐generation sequencing methods provide large, multilocus data sets with increased power to resolve unusual mtDNA patterns. A mtDNA‐based phylogeography of the Savannah sparrow (Passerculus sandwichensis) previously identified two sympatric, but divergent (~2%) clades within the nominate subspecies group and a third clade that consisted of birds sampled from northwest Mexico. We revisited the phylogeography of this species using a population genomic data set to resolve the processes leading to the evolution of sympatric and divergent mtDNA lineages. We identified two genetic clusters in the genomic data set corresponding to (a) the nominate subspecies group and (b) northwestern Mexico birds. Following divergence, the nominate clade maintained a large, stable population, indicating that divergent mitochondrial lineages arose within a panmictic population. Simulations based on parameter estimates from this model further confirmed that this demographic history could produce observed levels of mtDNA diversity. Patterns of divergent, sympatric mtDNA lineages are frequently interpreted as admixture of historically isolated lineages. Our analyses reject this interpretation for Savannah sparrows and underscore the need for genomic data sets to resolve the evolutionary mechanisms behind anomalous, locus‐specific patterns.     

GIbPSs: a toolkit for fast and accurate analyses of genotyping‐by‐sequencing data without a reference genome

Genotyping‐by‐sequencing (GBS) and related methods are increasingly used for studies of non‐model organisms from population genetic to phylogenetic scales. We present GIbPSs, a new genotyping toolkit for the analysis of data from various protocols such as RAD, double‐digest RAD, GBS, and two‐enzyme GBS without a reference genome. GIbPSs can handle paired‐end GBS data and is able to assign reads from both strands of a restriction fragment to the same locus. GIbPSs is most suitable for population genetic and phylogeographic analyses. It avoids genotyping errors due to indel variation by identifying and discarding affected loci. GIbPSs creates a genotype database that offers rich functionality for data filtering and export in numerous formats. We performed comparative analyses of simulated and real GBS data with GIbPSs and another program, pyRAD. This program accounts for indel variation by aligning homologous sequences. GIbPSs performed better than pyRAD in several aspects. It required much less computation time and displayed higher genotyping accuracy. GIbPSs retained smaller numbers of loci overall in analyses of real GBS data. It nevertheless delivered more complete genotype matrices with greater locus overlap between individuals and greater numbers of loci sampled in all individuals.     

Design of efficient simplified genomic DNA and bisulfite sequencing in large plant populations

The next generation sequencing enables generation of high resolution and high throughput data for structure sequence of any genome at a fast declining cost. This opens opportunity for population based genetic and genomic analyses. In many applications, whole genome sequencing or re-sequencing is unnecessary or prohibited by budget limits. The Reduced Representation Genome Sequencing (RRGS), which sequences only a small proportion of the genome of interest, has been proposed to deal with the situations. Several forms of RRGS are proposed and implemented in the literature. When applied to plant or crop species, the current RRGS protocols shared a key drawback that a significantly high proportion (up to 60%) of sequence reads to be generated may be of non-genomic origin but attributed to chloroplast DNA or rRNA genes, leaving an exceptional low efficiency of the sequencing experiment. We recommended and discussed here the design of optimized simplified genomic DNA and bisulfite sequencing strategies, which may greatly improves efficiency of the sequencing experiments by bringing down the presentation of the undesirable sequencing reads to less than 10% in the whole sequence reads. The optimized RAD-seq and RRBS-seq methods are potentially useful for sequence variant screening and genotyping in large plant/crop populations.     

High‐resolution mapping of the recombination landscape of the phytopathogen Fusarium graminearum suggests two‐speed genome evolution

Recombination is a major evolutionary force, increasing genetic diversity and permitting efficient coevolution of fungal pathogen(s) with their host(s). The ascomycete Fusarium graminearum is a devastating pathogen of cereal crops, and can contaminate food and feed with harmful mycotoxins. Previous studies have suggested a high adaptive potential of this pathogen, illustrated by an increase in pathogenicity and resistance to fungicides. In this study, we provide the first detailed picture of the crossover events occurring during meiosis and discuss the role of recombination in pathogen evolution. An experimental recombinant population (n = 88) was created and genotyped using 1306 polymorphic markers obtained from restriction site‐associated DNA sequencing (RAD‐seq) and aligned to the reference genome. The construction of a high‐density linkage map, anchoring 99% of the total length of the reference genome, allowed the identification of 1451 putative crossovers, positioned at a median resolution of 24 kb. The majority of crossovers (87.2%) occurred in a relatively small portion of the genome (30%). All chromosomes demonstrated recombination‐active sections, which had a near 15‐fold higher crossover rate than non‐active recombinant sections. The recombination rate showed a strong positive correlation with nucleotide diversity, and recombination‐active regions were enriched for genes with a putative role in host–pathogen interaction, as well as putative diversifying genes. Our results confirm the preliminary analysis observed in other F. graminearum strains and suggest a conserved ‘two‐speed’ recombination landscape. The consequences with regard to the evolutionary potential of this major fungal pathogen are also discussed.     

Targeted genotyping by sequencing: a new way to genome profile the cat

Targeted GBS is a recent approach for obtaining an effective characterization for hundreds to thousands of markers. The high throughput of next‐generation sequencing technologies, moreover, allows sample multiplexing. The aims of this study were to (i) define a panel of single nucleotide polymorphisms (SNPs) in the cat, (ii) use GBS for profiling 16 cats, and (iii) evaluate the performance with respect to the inference using standard approaches at different coverage thresholds, thereby providing useful information for designing similar experiments. Probes for sequencing 230 variants were designed based on the Felis_catus_8.0. 8.0 genome. The regions comprised anonymous and non‐anonymous SNPs. Sixteen cat samples were analysed, some of which had already been genotyped in a large group of loci and one having been whole‐genome sequenced in the 99_Lives Cat Genome Sequencing Project. The accuracy of the method was assessed by comparing the GBS results with the genotypes already available. Overall, GBS achieved good performance, with 92–96% correct assignments, depending on the coverage threshold used to define the set of trustable genotypes. Analyses confirmed that (i) the reliability of the inference of each genotype depends on the coverage at that locus and (ii) the fraction of target loci whose genotype can be inferred correctly is a function of the total coverage. GBS proves to be a valid alternative to other methods. Data suggested a depth of less than 11× is required for greater than 95% accuracy. However, sequencing depth must be adapted to the total size of the targets to ensure proper genotype inference.     

The future is now: Amplicon sequencing and sequence capture usher in the conservation genomics era

The genomics revolution has initiated a new era of population genetics where genome‐wide data are frequently used to understand complex patterns of population structure and selection. However, the application of genomic tools to inform management and conservation has been somewhat rare outside a few well studied species. Fortunately, two recently developed approaches, amplicon sequencing and sequence capture, have the potential to significantly advance the field of conservation genomics. Here, amplicon sequencing refers to highly multiplexed PCR followed by high‐throughput sequencing (e.g., GTseq), and sequence capture refers to using capture probes to isolate loci from reduced‐representation libraries (e.g., Rapture). Both approaches allow sequencing of thousands of individuals at relatively low costs, do not require any specialized equipment for library preparation, and generate data that can be analyzed without sophisticated computational infrastructure. Here, we discuss the advantages and disadvantages of each method and provide a decision framework for geneticists who are looking to integrate these methods into their research programme. While it will always be important to consider the specifics of the biological question and system, we believe that amplicon sequencing is best suited for projects aiming to genotype <500 loci on many individuals (>1,500) or for species where continued monitoring is anticipated (e.g., long‐term pedigrees). Sequence capture, on the other hand, is best applied to projects including fewer individuals or where >500 loci are required. Both of these techniques should smooth the transition from traditional genetic techniques to genomics, helping to usher in the conservation genomics era.     

Genomics of the divergence continuum in an African plant biodiversity hotspot,I: drivers of population divergence in Restio capensis (Restionaceae)

Understanding the drivers of population divergence, speciation and species persistence is of great interest to molecular ecology, especially for species‐rich radiations inhabiting the world's biodiversity hotspots. The toolbox of population genomics holds great promise for addressing these key issues, especially if genomic data are analysed within a spatially and ecologically explicit context. We have studied the earliest stages of the divergence continuum in the Restionaceae, a species‐rich and ecologically important plant family of the Cape Floristic Region (CFR) of South Africa, using the widespread CFR endemic Restio capensis (L.) H.P. Linder & C.R. Hardy as an example. We studied diverging populations of this morphotaxon for plastid DNA sequences and >14 400 nuclear DNA polymorphisms from Restriction site Associated DNA (RAD) sequencing and analysed the results jointly with spatial, climatic and phytogeographic data, using a Bayesian generalized linear mixed modelling (GLMM) approach. The results indicate that population divergence across the extreme environmental mosaic of the CFR is mostly driven by isolation by environment (IBE) rather than isolation by distance (IBD) for both neutral and non‐neutral markers, consistent with genome hitchhiking or coupling effects during early stages of divergence. Mixed modelling of plastid DNA and single divergent outlier loci from a Bayesian genome scan confirmed the predominant role of climate and pointed to additional drivers of divergence, such as drift and ecological agents of selection captured by phytogeographic zones. Our study demonstrates the usefulness of population genomics for disentangling the effects of IBD and IBE along the divergence continuum often found in species radiations across heterogeneous ecological landscapes.     

Intraspecific DNA contamination distorts subtle population structure in a marine fish: Decontamination of herring samples before restriction‐site associated sequencing and its effects on population genetic statistics

Wild specimens are often collected in challenging field conditions, where samples may be contaminated with the DNA of conspecific individuals. This contamination can result in false genotype calls, which are difficult to detect, but may also cause inaccurate estimates of heterozygosity, allele frequencies and genetic differentiation. Marine broadcast spawners are especially problematic, because population genetic differentiation is low and samples are often collected in bulk and sometimes from active spawning aggregations. Here, we used contaminated and clean Pacific herring (Clupea pallasi) samples to test (a) the efficacy of bleach decontamination, (b) the effect of decontamination on RAD genotypes and (c) the consequences of contaminated samples on population genetic analyses. We collected fin tissue samples from actively spawning (and thus contaminated) wild herring and nonspawning (uncontaminated) herring. Samples were soaked for 10 min in bleach or left untreated, and extracted DNA was used to prepare DNA libraries using a restriction site‐associated DNA (RAD) approach. Our results demonstrate that intraspecific DNA contamination affects patterns of individual and population variability, causes an excess of heterozygotes and biases estimates of population structure. Bleach decontamination was effective at removing intraspecific DNA contamination and compatible with RAD sequencing, producing high‐quality sequences, reproducible genotypes and low levels of missing data. Although sperm contamination may be specific to broadcast spawners, intraspecific contamination of samples may be common and difficult to detect from high‐throughput sequencing data and can impact downstream analyses.     

Speciation genomics and a role for the Z chromosome in the early stages of divergence between Mexican ducks and mallards

Speciation is a continuous and dynamic process, and studying organisms during the early stages of this process can aid in identifying speciation mechanisms. The mallard (Anas platyrhynchos) and Mexican duck (A. [p.] diazi) are two recently diverged taxa with a history of hybridization and controversial taxonomy. To understand their evolutionary history, we conducted genomic scans to characterize patterns of genetic diversity and divergence across the mitochondrial DNA (mtDNA) control region, 3523 autosomal loci and 172 Z‐linked sex chromosome loci. Between the two taxa, Z‐linked loci (Φ_ST = 0.088) were 5.2 times more differentiated than autosomal DNA (Φ_ST = 0.017) but comparable to mtDNA (Φ_ST = 0.092). This elevated Z differentiation deviated from neutral expectations inferred from simulated data that incorporated demographic history and differences in effective population sizes between marker types. Furthermore, 3% of Z‐linked loci, compared to <0.1% of autosomal loci, were detected as outlier loci under divergent selection with elevated relative (Φ_ST) and absolute (d_XY) estimates of divergence. In contrast, the ratio of Z‐linked and autosomal differentiation among the seven Mexican duck sampling locations was close to 1:1 (Φ_ST = 0.018 for both markers). We conclude that between mallards and Mexican ducks, divergence at autosomal markers is largely neutral, whereas greater divergence on the Z chromosome (or some portions thereof) is likely the product of selection that has been important in speciation. Our results contribute to a growing body of literature indicating elevated divergence on the Z chromosome and its likely importance in avian speciation.     

Structural genomics: correlation blocks, population structure, and genome architecture

An integration of the pattern of genome-wide inter-site associations with evolutionary forces is important for gaining insights into the genomic evolution in natural or artificial populations. Here, we assess the inter-site correlation blocks and their distributions along chromosomes. A correlation block is broadly termed as the DNA segment within which strong correlations exist between genetic diversities at any two sites. We bring together the population genetic structure and the genomic diversity structure that have been independently built on different scales and synthesize the existing theories and methods for characterizing genomic structure at the population level. We discuss how population structure could shape correlation blocks and their patterns within and between populations. Effects of evolutionary forces (selection, migration, genetic drift, and mutation) on the pattern of genome-wide correlation blocks are discussed. In eukaryote organisms, we briefly discuss the associations between the pattern of correlation blocks and genome assembly features in eukaryote organisms, including the impacts of multigene family, the perturbation of transposable elements, and the repetitive nongenic sequences and GC-rich isochores. Our reviews suggest that the observable pattern of correlation blocks can refine our understanding of the ecological and evolutionary processes underlying the genomic evolution at the population level.     

Living in the city: urban environments shape the evolution of a native annual plant

Urban environments are warmer, have higher levels of atmospheric CO₂ and have altered patterns of disturbance and precipitation than nearby rural areas. These differences can be important for plant growth and are likely to create distinct selective environments. We planted a common garden experiment with seeds collected from natural populations of the native annual plant Lepidium virginicum, growing in five urban and nearby rural areas in the northern United States to determine whether and how urban populations differ from those from surrounding rural areas. When grown in a common environment, plants grown from seeds collected from urban areas bolted sooner, grew larger, had fewer leaves, had an extended time between bolting and flowering, and produced more seeds than plants grown from seeds collected from rural areas. Interestingly, the rural populations exhibited larger phenotypic differences from one another than urban populations. Surprisingly, genomic data revealed that the majority of individuals in each of the urban populations were more closely related to individuals from other urban populations than they were to geographically proximate rural areas – the one exception being urban and rural populations from New York which were nearly identical. Taken together, our results suggest that selection in urban environments favors different traits than selection in rural environments and that these differences can drive adaptation and shape population structure.     

Plastid genome sequencing,comparative genomics,and phylogenomics:Current status and prospects

More than 190 plastid genomes have been completely sequenced during the past two decades due to advances in DNA sequencing technologies.Based on this unprecedented abundance of data,extensive genomic changes have been revealed in the plastid genomes.Inversion is the most common mechanism that leads to gene order changes.Several inversion events have been recognized as informative phylogenetic markers,such as a 30-kb inversion found in all living vascular plants minus lycopsids and two short inversions putat...     

Plastid genome sequencing,comparative genomics,and phylogenomics: Current status and prospects

Abstract More than 190 plastid genomes have been completely sequenced during the past two decades due to advances in DNA sequencing technologies. Based on this unprecedented abundance of data, extensive genomic changes have been revealed in the plastid genomes. Inversion is the most common mechanism that leads to gene order changes. Several inversion events have been recognized as informative phylogenetic markers, such as a 30‐kb inversion found in all living vascular plants minus lycopsids and two short inversions putatively shared by all ferns. Gene loss is a common event throughout plastid genome evolution. Many genes were independently lost or transferred to the nuclear genome in multiple plant lineages. The trnR‐CCG gene was lost in some clades of lycophytes, ferns, and seed plants, and all the ndh genes were absent in parasitic plants, gnetophytes, Pinaceae, and the Taiwan moth orchid. Certain parasitic plants have, in particular, lost plastid genes related to photosynthesis because of the relaxation of functional constraint. The dramatic growth of plastid genome sequences has also promoted the use of whole plastid sequences and genomic features to solve phylogenetic problems. Chloroplast phylogenomics has provided additional evidence for deep‐level phylogenetic relationships as well as increased phylogenetic resolutions at low taxonomic levels. However, chloroplast phylogenomics is still in its infant stage and rigorous analysis methodology has yet to be developed.     

Multiple introductions and population structure during the rapid expansion of the invasive Sahara mustard (Brassica tournefortii)

The specific mechanisms that result in the success of any species invasion case are difficult to document. Reproductive strategies are often cited as a primary driver of invasive success, with human activities further facilitating invasions by, for example, acting as seed vectors for dispersal via road, train, air, and marine traffic, and by producing efficient corridors for movement including canals, drainages, and roadways. Sahara mustard (Brassica tournefortii) is a facultative autogamous annual native to Eurasia that has rapidly invaded the southwestern United States within the past century, displacing natives, and altering water‐limited landscapes in the southwest. We used a genotyping‐by‐sequencing approach to study the population structure and spatial geography of Sahara mustard from 744 individuals from 52 sites across the range of the species’ invasion. We also used herbaria records to model range expansion since its initial introduction in the 1920s. We found that Sahara mustard occurs as three populations in the United States unstructured by geography, identified three introduction sites, and combined herbaria records with genomic analyses to map the spread of the species. Low genetic diversity and linkage disequilibrium are consistent with self‐fertilization, which likely promoted rapid invasive spread. Overall, we found that Sahara mustard experienced atypical expansion patterns, with a relatively constant rate of expansion and without the lag phase that is typical of many invasive species.     

Repetitive sequences: cause for variation in genome size and chromosome morphology in the genus Oryza

Large variation in genome size as determined by the nuclear DNA content and the mitotic chromosome size among diploid rice species is revealed using flow cytometry and image analyses. Both the total chromosomal length (r_0.939) and the total chromosomal area (r_0.927) correlated well with the nuclear DNA content. Among all the species examined, Oryza australiensis (E genome) and O. brachyantha (F genome), respectively, were the largest and smallest in genome size. O. sativa (A genome) involving all the cultivated species showed the intermediate genome size between them. The distribution patterns of genome-specific repetitive DNA sequences were physically determined using fluorescence in situ hybridization (FISH). O. brachyantha had limited sites of the repetitive DNA sequences specific to the F genome. O. australiensis showed overall amplification of genome-specific DNA sequences throughout the chromosomes. The amplification of the repetitive DNA sequences causes the variation in the chromosome morphology and thus the genome size among diploid species in the genus Oryza.     

Molecular ecology studies of species radiations: current research gaps,opportunities and challenges

Understanding the drivers and limits of species radiations is a crucial goal of evolutionary genetics and molecular ecology, yet research on this topic has been hampered by the notorious difficulty of connecting micro‐ and macroevolutionary approaches to studying the drivers of diversification. To chart the current research gaps, opportunities and challenges of molecular ecology approaches to studying radiations, we examine the literature in the journal Molecular Ecology and revisit recent high‐profile examples of evolutionary genomic research on radiations. We find that available studies of radiations are highly unevenly distributed among taxa, with many ecologically important and species‐rich organismal groups remaining severely understudied, including arthropods, plants and fungi. Most studies employed molecular methods suitable over either short or long evolutionary time scales, such as microsatellites or restriction site‐associated DNA sequencing (RAD‐seq) in the former case and conventional amplicon sequencing of organellar DNA in the latter. The potential of molecular ecology studies to address and resolve patterns and processes around the species level in radiating groups of taxa is currently limited primarily by sample size and a dearth of information on radiating nuclear genomes as opposed to organellar ones. Based on our literature survey and personal experience, we suggest possible ways forward in the coming years. We touch on the potential and current limitations of whole‐genome sequencing (WGS) in studies of radiations. We suggest that WGS and targeted (‘capture’) resequencing emerge as the methods of choice for scaling up the sampling of populations, species and genomes, including currently understudied organismal groups and the genes or regulatory elements expected to matter most to species radiations.     

The first genome size estimates for six species of krill (Malacostraca, Euphausiidae): large genomes at the north and south poles

Krill (family Euphausiidae) represent some of the most abundant organisms in the both northern and southern oceanic environments and provide food for various animals including humans. Despite their importance, little is known about krill from a genomic standpoint, even with regard to basic properties such as total genome size. This study provides genome size estimates for six species of krill from both the North Atlantic and Southern Oceans which are the first such estimates for any species of euphausiid. Genome size estimates were obtained using both flow cytometry and Feulgen image analysis densitometry with chicken and trout blood as internal standards. Haploid genome sizes ranged from 12.77 to 48.53 pg, providing roughly fourfold variation within these six species alone. With such large estimates, sequencing of a krill genome will currently be costly and laborious, but further studies should be conducted to determine the composition of these exceptionally large genomes.     

Genomics and telemetry suggest a role for migration harshness in determining overwintering habitat choice,but not gene flow,in anadromous Arctic Char

Migration is a ubiquitous life history trait with profound evolutionary and ecological consequences. Recent developments in telemetry and genomics, when combined, can bring significant insights on the migratory ecology of nonmodel organisms in the wild. Here, we used this integrative approach to document dispersal, gene flow and potential for local adaptation in anadromous Arctic Char from six rivers in the Canadian Arctic. Acoustic telemetry data from 124 tracked individuals indicated asymmetric dispersal, with a large proportion of fish (72%) tagged in three different rivers migrating up the same short river in the fall. Population genomics data from 6,136 SNP markers revealed weak, albeit significant, population differentiation (average pairwise F_ST = 0.011) and asymmetric dispersal was also revealed by population assignments. Approximate Bayesian computation simulations suggested the presence of asymmetric gene flow, although in the opposite direction to that observed from the telemetry data, suggesting that dispersal does not necessarily lead to gene flow. These observations suggested that Arctic Char home to their natal river to spawn, but may overwinter in rivers with the shortest migratory route to minimize the costs of migration in nonbreeding years. Genome scans and genetic–environment associations identified 90 outlier markers putatively under selection, 23 of which were in or near a gene. Of these, at least four were involved in muscle and cardiac function, consistent with the hypothesis that migratory harshness could drive local adaptation. Our study illustrates the power of integrating genomics and telemetry to study migrations in nonmodel organisms in logistically challenging environments such as the Arctic.     

Size and complexity of the nuclear genome of the ectomycorrhizal fungus Paxillus involutus

The basidiomycete Paxillus involutus is forming ectomycorrhizal symbiosis with a broad range of forest trees. Reassociation kinetics on P. involutus nuclear DNA indicated a haploid genome size of 23 Mb including 11% of repetitive DNA. A similar genome size (20 Mb) was estimated by genomic reconstruction analysis using three single copy genes. To assess the gene density in the P. involutus genome, a cosmid containing a 33-kb fragment of genomic DNA was sequenced and used to identify putative open reading frames (ORFs). Twelve potential ORFs were predicted, eight displayed significant sequence similarities to known proteins found in other organisms and notably, several homologues to the Podospora anserina vegetative incompatibility protein (HetE1) were found. By extrapolation, we estimate the total number of genes in the P. involutus haploid genome to approximately 7700.