Tapping the promise of genomics in species with complex, nonmodel genomes

Haplotype-resolved genomes of wild octoploid progenitors illuminate genomic diversifications from wild relatives to cultivated strawberry

Strawberry is an emerging model for studying polyploid genome evolution and rapid domestication of fruit crops. Here we report haplotype-resolved genomes of two wild octoploids (Fragaria chiloensis and Fragaria virginiana), the progenitor species of cultivated strawberry. Substantial variation is identified between species and between haplotypes. We redefine the four subgenomes and track the genetic contributions of diploid species by additional sequencing of the diploid F. nipponica genome. We provide multiple lines of evidence that F. vesca and F. iinumae, rather than other described extant species, are the closest living relatives of these wild and cultivated octoploids. In response to coexistence with quadruplicate gene copies, the octoploid strawberries have experienced subgenome dominance, homoeologous exchanges and coordinated expression of homoeologous genes. However, some homoeologues have substantially altered expression bias after speciation and during domestication. These findings enhance our understanding of the origin, genome evolution and domestication of strawberries.

Insights into the Taxonomically Challenging Hexaploid Alpine Shrub Willows of Salix Sections Phylicifoliae and Nigricantes (Salicaceae)

The complex genomic composition of allopolyploid plants leads to morphologically diverse species. The traditional taxonomical treatment of the medium-sized, hexaploid shrub willows distributed in the Alps is difficult based on their variable morphological characters. In this study, RAD sequencing data, infrared-spectroscopy, and morphometric data are used to analyze the phylogenetic relationships of the hexaploid species of the sections Nigricantes and Phylicifoliae in a phylogenetic framework of 45 Eurasian Salix species. Both sections comprise local endemics as well as widespread species. Based on the molecular data, the described morphological species appeared as monophyletic lineages (except for S. phylicifolia s.str. and S. bicolor, which are intermingled). Both sections Phylicifoliae and Nigricantes are polyphyletic. Infrared-spectroscopy mostly confirmed the differentiation of hexaploid alpine species. The morphometric data confirmed the molecular results and supported the inclusion of S. bicolor into S. phylicifolia s.l., whereas the alpine endemic S. hegetschweileri is distinct and closely related to species of the section Nigricantes. The genomic structure and co-ancestry analyses of the hexaploid species revealed a geographical pattern for widespread S. myrsinifolia, separating the Scandinavian from the alpine populations. The newly described S. kaptarae is tetraploid and is grouped within S. cinerea. Our data reveal that both sections Phylicifoliae and Nigricantes need to be redefined.

The Current Developments in Medicinal Plant Genomics Enabled the Diversification of Secondary Metabolites' Biosynthesis

Medicinal plants produce important substrates for their adaptation and defenses against environmental factors and, at the same time, are used for traditional medicine and industrial additives. Plants have relatively little in the way of secondary metabolites via biosynthesis. Recently, the whole-genome sequencing of medicinal plants and the identification of secondary metabolite production were revolutionized by the rapid development and cheap cost of sequencing technology. Advances in functional genomics, such as transcriptomics, proteomics, and metabolomics, pave the way for discoveries in secondary metabolites and related key genes. The multi-omics approaches can offer tremendous insight into the variety, distribution, and development of biosynthetic gene clusters (BGCs). Although many reviews have reported on the plant and medicinal plant genome, chemistry, and pharmacology, there is no review giving a comprehensive report about the medicinal plant genome and multi-omics approaches to study the biosynthesis pathway of secondary metabolites. Here, we introduce the medicinal plant genome and the application of multi-omics tools for identifying genes related to the biosynthesis pathway of secondary metabolites. Moreover, we explore comparative genomics and polyploidy for gene family analysis in medicinal plants. This study promotes medicinal plant genomics, which contributes to the biosynthesis and screening of plant substrates and plant-based drugs and prompts the research efficiency of traditional medicine.

Karyon: a computational framework for the diagnosis of hybrids, aneuploids, and other nonstandard architectures in genome assemblies

BACKGROUND

Recent technological developments have made genome sequencing and assembly highly accessible and widely used. However, the presence in sequenced organisms of certain genomic features such as high heterozygosity, polyploidy, aneuploidy, heterokaryosis, or extreme compositional biases can challenge current standard assembly procedures and result in highly fragmented assemblies. Hence, we hypothesized that genome databases must contain a nonnegligible fraction of low-quality assemblies that result from such type of intrinsic genomic factors.

FINDINGS

Here we present Karyon, a Python-based toolkit that uses raw sequencing data and de novo genome assembly to assess several parameters and generate informative plots to assist in the identification of nonchanonical genomic traits. Karyon includes automated de novo genome assembly and variant calling pipelines. We tested Karyon by diagnosing 35 highly fragmented publicly available assemblies from 19 different Mucorales (Fungi) species.

CONCLUSIONS

Our results show that 10 (28.57%) of the assemblies presented signs of unusual genomic configurations, suggesting that these are common, at least for some lineages within the Fungi.

Twenty years of plant genome sequencing: achievements and challenges

Publication of the complete genome sequence of Arabidopsis thaliana, the first plant reference genome, in December 2000 heralded the beginning of the plant genome era. Over the past 20 years reference genomes have been generated for hundreds of plant species, spanning non-vascular to flowering plants. Releasing these plant genomes has dramatically advanced studies in all disciplines of plant biology. Importantly, multiple reference-level genomes have been generated for the major crops and their progenitors, enabling the creation of pan-genomes and exploration of domestication history and natural variations that can be adopted by modern crop breeding. We summarize the progress of plant genome sequencing and the challenges of sequencing more complex plant genomes and generating pan-genomes.

Novel Approaches for Species Concepts and Delimitation in Polyploids and Hybrids

Hybridization and polyploidization are important processes for plant evolution. However, classification of hybrid or polyploid species has been notoriously difficult because of the complexity of processes and different evolutionary scenarios that do not fit with classical species concepts. Polyploid complexes are formed via combinations of allopolyploidy, autopolyploidy and homoploid hybridization with persisting sexual reproduction, resulting in many discrete lineages that have been classified as species. Polyploid complexes with facultative apomixis result in complicated net-work like clusters, or rarely in agamospecies. Various case studies illustrate the problems that apply to traditional species concepts to hybrids and polyploids. Conceptual progress can be made if lineage formation is accepted as an inevitable consequence of meiotic sex, which is established already in the first eukaryotes as a DNA restoration tool. The turnaround of the viewpoint that sex forms species as lineages helps to overcome traditional thinking of species as "units". Lineage formation and self-sustainability is the prerequisite for speciation and can also be applied to hybrids and polyploids. Species delimitation is aided by the improved recognition of lineages via various novel -omics methods, by understanding meiosis functions, and by recognizing functional phenotypes by considering morphological-physiological-ecological adaptations.

How challenging RADseq data turned out to favor coalescent-based species tree inference. A case study in Aichryson (Crassulaceae)

Analysing multiple genomic regions while incorporating detection and qualification of discordance among regions has become standard for understanding phylogenetic relationships. In plants, which usually have comparatively large genomes, this is feasible by the combination of reduced-representation library (RRL) methods and high-throughput sequencing enabling the cost effective acquisition of genomic data for thousands of loci from hundreds of samples. One popular RRL method is RADseq. A major disadvantage of established RADseq approaches is the rather short fragment and sequencing range, leading to loci of little individual phylogenetic information. This issue hampers the application of coalescent-based species tree inference. The modified RADseq protocol presented here targets ca. 5,000 loci of 300-600nt length, sequenced with the latest short-read-sequencing (SRS) technology, has the potential to overcome this drawback. To illustrate the advantages of this approach we use the study group Aichryson Webb & Berthelott (Crassulaceae), a plant genus that diversified on the Canary Islands. The data analysis approach used here aims at a careful quality control of the long loci dataset. It involves an informed selection of thresholds for accurate clustering, a thorough exploration of locus properties, such as locus length, coverage and variability, to identify potential biased data and a comparative phylogenetic inference of filtered datasets, accompanied by an evaluation of resulting BS support, gene and site concordance factor values, to improve overall resolution of the resulting phylogenetic trees. The final dataset contains variable loci with an average length of 373nt and facilitates species tree estimation using a coalescent-based summary approach. Additional improvements brought by the approach are critically discussed.

An extreme-phenotype genome-wide association study identifies candidate cannabinoid pathway genes in Cannabis

Cannabis produces a class of isoprenylated resorcinyl polyketides known as cannabinoids, a subset of which are medically important and exclusive to this plant. The cannabinoid alkyl group is a critical structural feature that governs therapeutic activity. Genetic enhancement of the alkyl side-chain could lead to the development of novel chemical phenotypes (chemotypes) for pharmaceutical end-use. However, the genetic determinants underlying in planta variation of cannabinoid alkyl side-chain length remain uncharacterised. Using a diversity panel derived from the Ecofibre Cannabis germplasm collection, an extreme-phenotype genome-wide association study (XP-GWAS) was used to enrich for alkyl cannabinoid polymorphic regions. Resequencing of chemotypically extreme pools revealed a known cannabinoid synthesis pathway locus as well as a series of chemotype-associated genomic regions. One of these regions contained a candidate gene encoding a β-keto acyl carrier protein (ACP) reductase (BKR) putatively associated with polyketide fatty acid starter unit synthesis and alkyl side-chain length. Association analysis revealed twenty-two polymorphic variants spanning the length of this gene, including two nonsynonymous substitutions. The success of this first reported application of XP-GWAS for an obligate outcrossing and highly heterozygote plant genus suggests that this approach may have generic application for other plant species.

Phylogenomic Relationships and Evolution of Polyploid Salix Species Revealed by RAD Sequencing Data

Polyploidy is common in the genus Salix. However, little is known about the origin, parentage and genomic composition of polyploid species because of a lack of suitable molecular markers and analysis tools. We established a phylogenomic framework including species of all described sections of Eurasian shrub willows. We analyzed the genomic composition of seven polyploid willow species in comparison to putative diploid parental species to draw conclusions on their origin and the effects of backcrossing and post-origin evolution. We applied recently developed programs like SNAPP, HyDe, and SNiPloid to establish a bioinformatic pipeline for unravelling the complexity of polyploid genomes. RAD sequencing revealed 23,393 loci and 320,010 high quality SNPs for the analysis of relationships of 35 species of Eurasian shrub willows (Salix subg. Chamaetia/Vetrix). Polyploid willow species appear to be predominantly of allopolyploid origin. More ancient allopolyploidization events were observed for two hexaploid and one octoploid species, while our data suggested a more recent allopolyploid origin for the included tetraploids and identified putative parental taxa. SNiPloid analyses disentangled the different genomic signatures resulting from hybrid origin, backcrossing, and secondary post-origin evolution in the polyploid species. Our RAD sequencing data demonstrate that willow genomes are shaped by ancient and recent reticulate evolution, polyploidization, and post-origin divergence of species.

Genome Synteny Has Been Conserved Among the Octoploid Progenitors of Cultivated Strawberry Over Millions of Years of Evolution

Allo-octoploid cultivated strawberry (Fragaria × ananassa) originated through a combination of polyploid and homoploid hybridization, domestication of an interspecific hybrid lineage, and continued admixture of wild species over the last 300 years. While genes appear to flow freely between the octoploid progenitors, the genome structures and diversity of the octoploid species remain poorly understood. The complexity and absence of an octoploid genome frustrated early efforts to study chromosome evolution, resolve subgenomic structure, and develop a single coherent linkage group nomenclature. Here, we show that octoploid Fragaria species harbor millions of subgenome-specific DNA variants. Their diversity was sufficient to distinguish duplicated (homoeologous and paralogous) DNA sequences and develop 50K and 850K SNP genotyping arrays populated with co-dominant, disomic SNP markers distributed throughout the octoploid genome. Whole-genome shotgun genotyping of an interspecific segregating population yielded 1.9M genetically mapped subgenome variants in 5,521 haploblocks spanning 3,394 cM in F. chiloensis subsp. lucida, and 1.6M genetically mapped subgenome variants in 3,179 haploblocks spanning 2,017 cM in F. × ananassa. These studies provide a dense genomic framework of subgenome-specific DNA markers for seamlessly cross-referencing genetic and physical mapping information and unifying existing chromosome nomenclatures. Using comparative genomics, we show that geographically diverse wild octoploids are effectively diploidized, nearly completely collinear, and retain strong macro-synteny with diploid progenitor species. The preservation of genome structure among allo-octoploid taxa is a critical factor in the unique history of garden strawberry, where unimpeded gene flow supported its origin and domestication through repeated cycles of interspecific hybridization.

Cyberinfrastructure and resources to enable an integrative approach to studying forest trees

Sequencing technologies and bioinformatic approaches are now available to resolve the challenges associated with complex and heterozygous genomes. Increased access to less expensive and more effective instrumentation will contribute to a wealth of high-quality plant genomes in the next few years. In the meantime, more than 370 tree species are associated with public projects in primary repositories that are interrogating expression profiles, identifying variants, or analyzing targeted capture without a high-quality reference genome. Genomic data from these projects generates sequences that represent intermediate assemblies for transcriptomes and genomes. These data contribute to forest tree biology, but the associated sequence remains trapped in supplemental files that are poorly integrated in plant community databases and comparative genomic platforms. Successful implementation of life science cyberinfrastructure is improving data standards, ontologies, analytic workflows, and integrated database platforms for both model and non-model plant species. Unique to forest trees with large populations that are long-lived, outcrossing, and genetically diverse, the phenotypic and environmental metrics associated with georeferenced populations are just as important as the genomic data sampled for each individual. To address questions related to forest health and productivity, cyberinfrastructure must keep pace with the magnitude of genomic and phenomic sampling of larger populations. This review examines the current landscape of cyberinfrastructure, with an emphasis on best practices and resources to align community data with the Findable, Accessible, Interoperable, and Reusable (FAIR) guidelines.

The Boechera Genus as a Resource for Apomixis Research

The genera Boechera (A. Löve et D. Löve) and Arabidopsis, the latter containing the model plant Arabidopsis thaliana, belong to the same clade within the Brassicaceae family. Boechera is the only among the more than 370 genera in the Brassicaceae where apomixis is well documented. Apomixis refers to the asexual reproduction through seed, and a better understanding of the underlying mechanisms has great potential for applications in agriculture. The Boechera genus currently includes 110 species (of which 38 are reported to be triploid and thus apomictic), which are distributed mostly in the North America. The apomictic lineages of Boechera occur at both the diploid and triploid level and show signs of a hybridogenic origin, resulting in a modification of their chromosome structure, as reflected by alloploidy, aneuploidy, substitutions of homeologous chromosomes, and the presence of aberrant chromosomes. In this review, we discuss the advantages of the Boechera genus to study apomixis, consider its modes of reproduction as well as the inheritance and possible mechanisms controlling apomixis. We also consider population genetic aspects and a possible role of hybridization at the origin of apomixis in Boechera. The molecular tools available to study Boechera, such as transformation techniques, laser capture microdissection, analysis of transcriptomes etc. are also discussed. We survey available genome assemblies of Boechera spp. and point out the challenges to assemble the highly heterozygous genomes of apomictic species. Due to these challenges, we argue for the application of an alternative reference-free method for the comparative analysis of such genomes, provide an overview of genomic sequencing data in the genus Boechera suitable for such analysis, and provide examples of its application.

Plant genome and transcriptome annotations: from misconceptions to simple solutions

Next-generation sequencing has triggered an explosion of available genomic and transcriptomic resources in the plant sciences. Although genome and transcriptome sequencing has become orders of magnitudes cheaper and more efficient, often the functional annotation process is lagging behind. This might be hampered by the lack of a comprehensive enumeration of simple-to-use tools available to the plant researcher. In this comprehensive review, we present (i) typical ontologies to be used in the plant sciences, (ii) useful databases and resources used for functional annotation, (iii) what to expect from an annotated plant genome, (iv) an automated annotation pipeline and (v) a recipe and reference chart outlining typical steps used to annotate plant genomes/transcriptomes using publicly available resources.

Ecological plant epigenetics: Evidence from model and non-model species, and the way forward

Growing evidence shows that epigenetic mechanisms contribute to complex traits, with implications across many fields of biology. In plant ecology, recent studies have attempted to merge ecological experiments with epigenetic analyses to elucidate the contribution of epigenetics to plant phenotypes, stress responses, adaptation to habitat, and range distributions. While there has been some progress in revealing the role of epigenetics in ecological processes, studies with non-model species have so far been limited to describing broad patterns based on anonymous markers of DNA methylation. In contrast, studies with model species have benefited from powerful genomic resources, which contribute to a more mechanistic understanding but have limited ecological realism. Understanding the significance of epigenetics for plant ecology requires increased transfer of knowledge and methods from model species research to genomes of evolutionarily divergent species, and examination of responses to complex natural environments at a more mechanistic level. This requires transforming genomics tools specifically for studying non-model species, which is challenging given the large and often polyploid genomes of plants. Collaboration among molecular geneticists, ecologists and bioinformaticians promises to enhance our understanding of the mutual links between genome function and ecological processes.

Computational Tools for Genomic Studies in Plants

In recent years, the genomic sequence of numerous plant species including the main crop species has been determined. Computational tools have been developed to deal with the issue of which plant has been sequenced and where is the sequence hosted. In this mini-review, the databases for genome projects, the databases created to host species/clade projects and the databases developed to perform plant comparative genomics are revised. Because of their importance in modern research, an in-depth analysis of the plant comparative genomics databases has been performed. This comparative analysis is focused in the common and specific computational tools developed to achieve the particular objectives of each database. Besides, emerging high-performance bioinformatics tools specific for plant research are commented. What kind of computational approaches should be implemented in next years to efficiently analyze plant genomes is discussed.

Ascertainment bias from imputation methods evaluation in wheat

BACKGROUND

Whole-genome genotyping techniques like Genotyping-by-sequencing (GBS) are being used for genetic studies such as Genome-Wide Association (GWAS) and Genomewide Selection (GS), where different strategies for imputation have been developed. Nevertheless, imputation error may lead to poor performance (i.e. smaller power or higher false positive rate) when complete data is not required as it is for GWAS, and each marker is taken at a time. The aim of this study was to compare the performance of GWAS analysis for Quantitative Trait Loci (QTL) of major and minor effect using different imputation methods when no reference panel is available in a wheat GBS panel.

RESULTS

In this study, we compared the power and false positive rate of dissecting quantitative traits for imputed and not-imputed marker score matrices in: (1) a complete molecular marker barley panel array, and (2) a GBS wheat panel with missing data. We found that there is an ascertainment bias in imputation method comparisons. Simulating over a complete matrix and creating missing data at random proved that imputation methods have a poorer performance. Furthermore, we found that when QTL were simulated with imputed data, the imputation methods performed better than the not-imputed ones. On the other hand, when QTL were simulated with not-imputed data, the not-imputed method and one of the imputation methods performed better for dissecting quantitative traits. Moreover, larger differences between imputation methods were detected for QTL of major effect than QTL of minor effect. We also compared the different marker score matrices for GWAS analysis in a real wheat phenotype dataset, and we found minimal differences indicating that imputation did not improve the GWAS performance when a reference panel was not available.

CONCLUSIONS

Poorer performance was found in GWAS analysis when an imputed marker score matrix was used, no reference panel is available, in a wheat GBS panel.

Renaissance in phytomedicines: promising implications of NGS technologies

Medicinal plant research is growing significantly in faith to discover new and more biologically compatible phytomedicines. Deposition of huge genome/trancriptome sequence data assisted by NGS technologies has revealed the new possibilities for producing upgraded bioactive molecules in medicinal plants. Growing interest of investors and consumers in the herbal drugs raises the need for extensive research to open the facts and details of every inch of life canvas of medicinal plants to produce improved quality of phytomedicines. As in agriculture crops, knowledge emergence from medicinal plant's genome/transcriptome, can be used to assure their amended quality and these improved varieties are then transported to the fields for cultivation. Genome studies generate huge sequence data which can be exploited further for obtaining information regarding genes/gene clusters involved in biosynthesis as well as regulation. This can be achieved rapidly at a very large scale with NGS platforms. Identification of new RNA molecules has become possible, which can lead to the discovery of novel compounds. Sequence information can be combined with advanced phytochemical and bioinformatics tools to discover functional herbal drugs. Qualitative and quantitative analysis of small RNA species put a light on the regulatory aspect of biosynthetic pathways for phytomedicines. Inter or intra genomic as well as transcriptomic interactive processes for biosynthetic pathways can be elucidated in depth. Quality management of herbal material will also become rapid and high throughput. Enrichment of sequence information will be used to engineer the plants to get more efficient phytopharmaceuticals. The present review comprises of role of NGS technologies to boost genomic studies of pharmaceutically important plants and further, applications of sequence information aiming to produce enriched phytomedicines. Emerging knowledge from the medicinal plants genome/transcriptome can give birth to deep understanding of the processes responsible for biosynthesis of medicinally important compounds.

Genomics-based strategies for the use of natural variation in the improvement of crop metabolism

Next-generation genomics holds great potential in the study of plant phenotypic variation. With several crop reference genomes now available, the affordable costs of de novo genome assembly or target resequencing offer the opportunity to mine the enormous amount of genetic diversity hidden in crop wild relatives. Wide introgressions from these wild ancestors species or land races represent a possible strategy to improve cultivated varieties. In this review, we discuss the mechanisms underlying metabolic diversity within plant species and the possible strategies (and barriers) to introgress novel metabolic traits into cultivated varieties. We show how deep genomic surveys uncover various types of structural variants from extended gene pools of major crops and highlight how this variation may be used for the improvement of crop metabolism.

Genome-guided investigation of plant natural product biosynthesis

The medicinal plant Madagascar periwinkle, Catharanthus roseus (L.) G. Don, produces hundreds of biologically active monoterpene-derived indole alkaloid (MIA) metabolites and is the sole source of the potent, expensive anti-cancer compounds vinblastine and vincristine. Access to a genome sequence would enable insights into the biochemistry, control, and evolution of genes responsible for MIA biosynthesis. However, generation of a near-complete, scaffolded genome is prohibitive to small research communities due to the expense, time, and expertise required. In this study, we generated a genome assembly for C. roseus that provides a near-comprehensive representation of the genic space that revealed the genomic context of key points within the MIA biosynthetic pathway including physically clustered genes, tandem gene duplication, expression sub-functionalization, and putative neo-functionalization. The genome sequence also facilitated high resolution co-expression analyses that revealed three distinct clusters of co-expression within the components of the MIA pathway. Coordinated biosynthesis of precursors and intermediates throughout the pathway appear to be a feature of vinblastine/vincristine biosynthesis. The C. roseus genome also revealed localization of enzyme-rich genic regions and transporters near known biosynthetic enzymes, highlighting how even a draft genome sequence can empower the study of high-value specialized metabolites.

Transposable elements as agents of rapid adaptation may explain the genetic paradox of invasive species

Rapid adaptation of invasive species to novel habitats has puzzled evolutionary biologists for decades, especially as this often occurs in the face of limited genetic variability. Although some ecological traits common to invasive species have been identified, little is known about the possible genomic/genetic mechanisms that may underlie their success. A common scenario in many introductions is that small founder population sizes will often lead to reduced genetic diversity, but that invading populations experience large environmental perturbations, such as changes in habitat and environmental stress. Although sudden and intense stress is usually considered in a negative context, these perturbations may actually facilitate rapid adaptation by affecting genome structure, organization and function via interactions with transposable elements (TEs), especially in populations with low genetic diversity. Stress-induced changes in TE activity can alter gene action and can promote structural variation that may facilitate the rapid adaptation observed in new environments. We focus here on the adaptive potential of TEs in relation to invasive species and highlight their role as powerful mutational forces that can rapidly create genetic diversity. We hypothesize that activity of transposable elements can explain rapid adaptation despite low genetic variation (the genetic paradox of invasive species), and provide a framework under which this hypothesis can be tested using recently developed and emerging genomic technologies.

Next Generation Sequencing Technologies: The Doorway to the Unexplored Genomics of Non-Model Plants

Non-model plants i.e., the species which have one or all of the characters such as long life cycle, difficulty to grow in the laboratory or poor fecundity, have been schemed out of sequencing projects earlier, due to high running cost of Sanger sequencing. Consequently, the information about their genomics and key biological processes are inadequate. However, the advent of fast and cost effective next generation sequencing (NGS) platforms in the recent past has enabled the unearthing of certain characteristic gene structures unique to these species. It has also aided in gaining insight about mechanisms underlying processes of gene expression and secondary metabolism as well as facilitated development of genomic resources for diversity characterization, evolutionary analysis and marker assisted breeding even without prior availability of genomic sequence information. In this review we explore how different Next Gen Sequencing platforms, as well as recent advances in NGS based high throughput genotyping technologies are rewarding efforts on de-novo whole genome/transcriptome sequencing, development of genome wide sequence based markers resources for improvement of non-model crops that are less costly than phenotyping.

Evolutionary origins and dynamics of octoploid strawberry subgenomes revealed by dense targeted capture linkage maps

Whole-genome duplications are radical evolutionary events that have driven speciation and adaptation in many taxa. Higher-order polyploids have complex histories often including interspecific hybridization and dynamic genomic changes. This chromosomal reshuffling is poorly understood for most polyploid species, despite their evolutionary and agricultural importance, due to the challenge of distinguishing homologous sequences from each other. Here, we use dense linkage maps generated with targeted sequence capture to improve the diploid strawberry (Fragaria vesca) reference genome and to disentangle the subgenomes of the wild octoploid progenitors of cultivated strawberry, Fragaria virginiana and Fragaria chiloensis. Our novel approach, POLiMAPS (Phylogenetics Of Linkage-Map-Anchored Polyploid Subgenomes), leverages sequence reads to associate informative interhomeolog phylogenetic markers with linkage groups and reference genome positions. In contrast to a widely accepted model, we find that one of the four subgenomes originates with the diploid cytoplasm donor F. vesca, one with the diploid Fragaria iinumae, and two with an unknown ancestor close to F. iinumae. Extensive unidirectional introgression has converted F. iinumae-like subgenomes to be more F. vesca-like, but never the reverse, due either to homoploid hybridization in the F. iinumae-like diploid ancestors or else strong selection spreading F. vesca-like sequence among subgenomes through homeologous exchange. In addition, divergence between homeologous chromosomes has been substantially augmented by interchromosomal rearrangements. Our phylogenetic approach reveals novel aspects of the complicated web of genetic exchanges that occur during polyploid evolution and suggests a path forward for unraveling other agriculturally and ecologically important polyploid genomes.

The utility of PacBio circular consensus sequencing for characterizing complex gene families in non-model organisms

BACKGROUND

Molecular characterization of highly diverse gene families can be time consuming, expensive, and difficult, especially when considering the potential for relatively large numbers of paralogs and/or pseudogenes. Here we investigate the utility of Pacific Biosciences single molecule real-time (SMRT) circular consensus sequencing (CCS) as an alternative to traditional cloning and Sanger sequencing PCR amplicons for gene family characterization. We target vomeronasal gene receptors, one of the most diverse gene families in mammals, with the goal of better understanding intra-specific V1R diversity of the gray mouse lemur (Microcebus murinus). Our study compares intragenomic variation for two V1R subfamilies found in the mouse lemur. Specifically, we compare gene copy variation within and between two individuals of M. murinus as characterized by different methods for nucleotide sequencing. By including the same individual animal from which the M. murinus draft genome was derived, we are able to cross-validate gene copy estimates from Sanger sequencing versus CCS methods.

RESULTS

We generated 34,088 high quality circular consensus sequences of two diverse V1R subfamilies (here referred to as V1RI and V1RIX) from two individuals of Microcebus murinus. Using a minimum threshold of 7× coverage, we recovered approximately 90% of V1RI sequences previously identified in the draft M. murinus genome (59% being identical at all nucleotide positions). When low coverage sequences were considered (i.e. < 7× coverage) 100% of V1RI sequences identified in the draft genome were recovered. At least 13 putatively novel V1R loci were also identified using CCS technology.

CONCLUSIONS

Recent upgrades to the Pacific Biosciences RS instrument have improved the CCS technology and offer an alternative to traditional sequencing approaches. Our results suggest that the Microcebus murinus V1R repertoire has been underestimated in the draft genome. In addition to providing an improved understanding of V1R diversity in the mouse lemur, this study demonstrates the utility of CCS technology for characterizing complex regions of the genome. We anticipate that long-read sequencing technologies such as PacBio SMRT will allow for the assembly of multigene family clusters and serve to more accurately characterize patterns of gene copy variation in large gene families, thus revealing novel micro-evolutionary patterns within non-model organisms.

Next generation restoration genetics: applications and opportunities

Restoration ecology is a young scientific discipline underpinning improvements in the rapid global expansion of ecological restoration. The application of molecular tools over the past 20 years has made an important contribution to understanding genetic factors influencing ecological restoration success. Here we illustrate how recent advances in next generation sequencing (NGS) methods are revolutionising the practical contribution of genetics to restoration. Novel applications include a dramatically enhanced capacity to measure adaptive variation for optimal seed sourcing, high-throughput assessment and monitoring of natural and restored biological communities aboveground and belowground, and gene expression analysis as a measure of genetic resilience of restored populations. Challenges remain in data generation, handling and analysis, and how best to apply NGS for practical outcomes in restoration.

Intergenomic single nucleotide polymorphisms as a tool for bacterial artificial chromosome contig building of homoeologous Brassica napus regions

BACKGROUND

Homoeologous sequences pose a particular challenge if bacterial artificial chromosome (BAC) contigs shall be established for specific regions of an allopolyploid genome. Single nucleotide polymorphisms (SNPs) differentiating between homoeologous genomes (intergenomic SNPs) may represent a suitable screening tool for such purposes, since they do not only identify homoeologous sequences but also differentiate between them.

RESULTS

Sequence alignments between Brassica rapa (AA) and Brassica oleracea (CC) sequences mapping to corresponding regions on chromosomes A1 and C1, respectively were used to identify single nucleotide polymorphisms between the A and C genomes. A large fraction of these polymorphisms was also present in Brassica napus (AACC), an allopolyploid species that originated from hybridisation of A and C genome species. Intergenomic SNPs mapping throughout homoeologous chromosome segments spanning approximately one Mbp each were included in Illumina's GoldenGate® Genotyping Assay and used to screen multidimensional pools of a Brassica napus bacterial artificial chromosome library with tenfold genome coverage. Based on the results of 50 SNP assays, a BAC contig for the Brassica napus A subgenome was established that spanned the entire region of interest. The C subgenome region was represented in three BAC contigs.

CONCLUSIONS

This proof-of-concept study shows that sequence resources of diploid progenitor genomes can be used to deduce intergenomic SNPs suitable for multiplex polymerase chain reaction (PCR)-based screening of multidimensional BAC pools of a polyploid organism. Owing to their high abundance and ease of identification, intergenomic SNPs represent a versatile tool to establish BAC contigs for homoeologous regions of a polyploid genome.

Comparative genomic analysis of the R2R3 MYB secondary cell wall regulators of Arabidopsis, poplar, rice, maize, and switchgrass

BACKGROUND

R2R3 MYB proteins constitute one of the largest plant transcription factor clades and regulate diverse plant-specific processes. Several R2R3 MYB proteins act as regulators of secondary cell wall (SCW) biosynthesis in Arabidopsis thaliana (At), a dicotyledenous plant. Relatively few studies have examined SCW R2R3 MYB function in grasses, which may have diverged from dicots in terms of SCW regulatory mechanisms, as they have in cell wall composition and patterning. Understanding cell wall regulation is especially important for improving lignocellulosic bioenergy crops, such as switchgrass.

RESULTS

Here, we describe the results of applying phylogenic, OrthoMCL, and sequence identity analyses to classify the R2R3 MYB family proteins from the annotated proteomes of Arabidposis, poplar, rice, maize and the initial genome (v0.0) and translated transcriptome of switchgrass (Panicum virgatum). We find that the R2R3 MYB proteins of the five species fall into 48 subgroups, including three dicot-specific, six grass-specific, and two panicoid grass-expanded subgroups. We observe four classes of phylogenetic relationships within the subgroups of known SCW-regulating MYB proteins between Arabidopsis and rice, ranging from likely one-to-one orthology (for AtMYB26, AtMYB103, AtMYB69) to no homologs identifiable (for AtMYB75). Microarray data for putative switchgrass SCW MYBs indicate that many maintain similar expression patterns with the Arabidopsis SCW regulators. However, some of the switchgrass-expanded candidate SCW MYBs exhibit differences in gene expression patterns among paralogs, consistent with subfunctionalization. Furthermore, some switchgrass representatives of grass-expanded clades have gene expression patterns consistent with regulating SCW development.

CONCLUSIONS

Our analysis suggests that no single comparative genomics tool is able to provide a complete picture of the R2R3 MYB protein family without leaving ambiguities, and establishing likely false-negative and -positive relationships, but that used together a relatively clear view emerges. Generally, we find that most R2R3 MYBs that regulate SCW in Arabidopsis are likely conserved in the grasses. This comparative analysis of the R2R3 MYB family will facilitate transfer of understanding of regulatory mechanisms among species and enable control of SCW biosynthesis in switchgrass toward improving its biomass quality.

Dihaploidy yields diploid apomicts and parthenogens in Erigeron (Asteraceae)

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PREMISE OF THE STUDY

Dihaploids result when tetraploids produce reduced eggs that develop without fertilization into diploid progeny. This process is useful for reducing genome complexity and studying trait expression at different ploidal levels. In this study we evaluated genetic inheritance and expression of diplospory (D) and parthenogenesis (P) in a population of dihaploids produced by tetraploid apomictic Erigeron•

METHODS

From 400 mostly maternal (tetraploid) progeny, we identified 64 (16%) dihaploids with 2n = 18 (53 plants) or 2n = 19 (11 plants). Differential interference contrast (DIC) imaging of ovules was used to evaluate megasporogenesis (meiosis vs. diplospory) and capacity for parthenogenetic embryo development. Seed production was estimated as the proportion of filled seeds.•

KEY RESULTS

For 60 analyzed dihaploids, diplospory vs. meiosis segregated approximately 1: 1 (P = 0.44) while all exhibited parthenogenetic embryo development. Parthenogenesis for meiotic progeny (n = 27) was observed in approximately 50% of ovules. Apomictic dihaploids (combining D and P; n = 33) produced seeds with mean 24.8% (range 1.3-74.4%) of total flowers.•

CONCLUSIONS

The dihaploid population consisted of half apomicts (D + P) and half parthenogens (P only). We infer that formation of dihaploid seeds requires the parthenogenesis locus. The highest seed values obtained for diploid apomicts are comparable to those recorded for wild type polyploid apomicts. This is one of the first reports of diploid apomixis in the Asteraceae and it demonstrates that both diplospory and parthenogenesis can be transmitted and expressed at a high level in the diploid condition.

Timber! Felling the loblolly pine genome

Conventional short read sequences derived from haploid DNA were extended into long super-reads enabling assembly of the massive 22 Gbp loblolly pine, Pinus taeda, genome.

See related research http://genomebiology.com/2014/15/3/R59

Unlocking Triticeae genomics to sustainably feed the future

The tribe Triticeae includes the major crops wheat and barley. Within the last few years, the whole genomes of four Triticeae species-barley, wheat, Tausch's goatgrass (Aegilops tauschii) and wild einkorn wheat (Triticum urartu)-have been sequenced. The availability of these genomic resources for Triticeae plants and innovative analytical applications using next-generation sequencing technologies are helping to revitalize our approaches in genetic work and to accelerate improvement of the Triticeae crops. Comparative genomics and integration of genomic resources from Triticeae plants and the model grass Brachypodium distachyon are aiding the discovery of new genes and functional analyses of genes in Triticeae crops. Innovative approaches and tools such as analysis of next-generation populations, evolutionary genomics and systems approaches with mathematical modeling are new strategies that will help us discover alleles for adaptive traits to future agronomic environments. In this review, we provide an update on genomic tools for use with Triticeae plants and Brachypodium and describe emerging approaches toward crop improvements in Triticeae.

From plant genomes to protein families: computational tools

The development of new high-throughput sequencing technologies has increased dramatically the number of successful genomic projects. Thus, draft genomic sequences of more than 60 plant species are currently available. Suitable bioinformatics tools are being developed to assemble, annotate and analyze the enormous number of sequences produced. In this context, specific plant comparative genomic databases are become powerful tools for gene family annotation in plant clades. In this mini-review, the current state-of-art of genomic projects is glossed. Besides, the computational tools developed to compare genomic data are compiled.