Copy number variation shapes genome diversity in Arabidopsis over immediate family generational scales

De novo genome assembly of white clover (Trifolium repens L.) reveals the role of copy number variation in rapid environmental adaptation

BACKGROUND

White clover (Trifolium repens) is a globally important perennial forage legume. This species also serves as an eco-evolutionary model system for studying within-species chemical defense variation; it features a well-studied polymorphism for cyanogenesis (HCN release following tissue damage), with higher frequencies of cyanogenic plants favored in warmer locations worldwide. Using a newly generated haplotype-resolved genome and two other long-read assemblies, we tested the hypothesis that copy number variants (CNVs) at cyanogenesis genes play a role in the ability of white clover to rapidly adapt to local environments. We also examined questions on subgenome evolution in this recently evolved allotetraploid species and on chromosomal rearrangements in the broader IRLC legume clade.

RESULTS

Integration of PacBio HiFi, Omni-C, Illumina, and linkage map data yielded a completely de novo genome assembly for white clover (created without a priori sequence assignment to subgenomes). We find that white clover has undergone extensive transposon diversification since its origin but otherwise shows highly conserved genome organization and composition with its diploid progenitors. Unlike some other clover species, its chromosomal structure is conserved with other IRLC legumes. We further find extensive evidence of CNVs at the major cyanogenesis loci; these contribute to quantitative variation in the cyanogenic phenotype and to local adaptation across wild North American populations.

CONCLUSIONS

This work provides a case study documenting the role of CNVs in local adaptation in a plant species, and it highlights the value of pan-genome data for identifying contributions of structural variants to adaptation in nature.

Characterization and genetic differences analysis in adventitious roots development of 38 Populus germplasm resources

To select poplar clones with excellent adventitious roots development (ARD) and deepen the understanding of its molecular mechanism, a comprehensive evaluation was conducted on 38 Populus germplasm resources with cuttings cultured in the greenhouse. Genetic differences between poplar clones with good ARD and with poor ARD were explored from the perspectives of genomics and transcriptomics. By cluster analysis of the seven adventitious roots (AR) traits, the materials were classified into three clusters, of which cluster I indicated excellent AR developmental capability and promising breeding potential, especially P.×canadensis 'Guariento', P. 'jingtong1', P. deltoides 'Zhongcheng5', P. deltoides 'Zhongcheng2'. At the genomic level, the cross-population composite likelihood ratio (XP-CLR) analysis identified 1944 positive selection regions related to ARD, and variation detection analysis identified 3426 specific SNPs and 687 specific Indels in the clones with good ARD, 3212 specific SNPs and 583 specific Indels in the clones with poor ARD, respectively. Through XP-CLR, variation detection, and weighted gene co-expression network analysis based on transcriptome data, eight major putative genes associated with poplar ARD were primary identified, and a co-expression network of eight genes was constructed, it was discovered that CSD1 and WRKY6 may be important in the ARD. Subsequently, we confirmed that SWEET17 had a non-synonymous mutation at the site of 928,404 in the clones with poor ARD, resulting in an alteration of the amino acid. After exploring phenotypic differences and the genetic variation of adventitious roots development in different poplar clones, this study provides valuable reference information for future poplar breeding and genetic improvement.

Genomic consequences associated with Agrobacterium-mediated transformation of plants

Attenuated strains of the naturally occurring plant pathogen Agrobacterium tumefaciens can transfer virtually any DNA sequence of interest to model plants and crops. This has made Agrobacterium-mediated transformation (AMT) one of the most commonly used tools in agricultural biotechnology. Understanding AMT, and its functional consequences, is of fundamental importance given that it sits at the intersection of many fundamental fields of study, including plant-microbe interactions, DNA repair/genome stability, and epigenetic regulation of gene expression. Despite extensive research and use of AMT over the last 40 years, the extent of genomic disruption associated with integrating exogenous DNA into plant genomes using this method remains underappreciated. However, new technologies like long-read sequencing make this disruption more apparent, complementing previous findings from multiple research groups that have tackled this question in the past. In this review, we cover progress on the molecular mechanisms involved in Agrobacterium-mediated DNA integration into plant genomes. We also discuss localized mutations at the site of insertion and describe the structure of these DNA insertions, which can range from single copy insertions to large concatemers, consisting of complex DNA originating from different sources. Finally, we discuss the prevalence of large-scale genomic rearrangements associated with the integration of DNA during AMT with examples. Understanding the intended and unintended effects of AMT on genome stability is critical to all plant researchers who use this methodology to generate new genetic variants.

A comprehensive framework for detecting copy number variants from single nucleotide polymorphism data: 'rCNV', a versatile r package for paralogue and CNV detection

Recent studies have highlighted the significant role of copy number variants (CNVs) in phenotypic diversity, environmental adaptation and species divergence across eukaryotes. The presence of CNVs also has the potential to introduce genotyping biases, which can pose challenges to accurate population and quantitative genetic analyses. However, detecting CNVs in genomes, particularly in non-model organisms, presents a formidable challenge. To address this issue, we have developed a statistical framework and an accompanying r software package that leverage allelic-read depth from single nucleotide polymorphism (SNP) data for accurate CNV detection. Our framework capitalises on two key principles. First, it exploits the distribution of allelic-read depth ratios in heterozygotes for individual SNPs by comparing it against an expected distribution based on binomial sampling. Second, it identifies SNPs exhibiting an apparent excess of heterozygotes under Hardy-Weinberg equilibrium. By employing multiple statistical tests, our method not only enhances sensitivity to sampling effects but also effectively addresses reference biases, resulting in optimised SNP classification. Our framework is compatible with various NGS technologies (e.g. RADseq, Exome-capture). This versatility enables CNV calling from genomes of diverse complexities. To streamline the analysis process, we have implemented our framework in the user-friendly r package 'rCNV', which automates the entire workflow seamlessly. We trained our models using simulated data and validated their performance on four datasets derived from different sequencing technologies, including RADseq (Chinook salmon-Oncorhynchus tshawytscha), Rapture (American lobster-Homarus americanus), Exome-capture (Norway spruce-Picea abies) and WGS (Malaria mosquito-Anopheles gambiae).

Subtelomeric 5-enolpyruvylshikimate-3-phosphate synthase copy number variation confers glyphosate resistance in Eleusine indica

Genomic structural variation (SV) has profound effects on organismal evolution; often serving as a source of novel genetic variation. Gene copy number variation (CNV), one type of SV, has repeatedly been associated with adaptive evolution in eukaryotes, especially with environmental stress. Resistance to the widely used herbicide, glyphosate, has evolved through target-site CNV in many weedy plant species, including the economically important grass, Eleusine indica (goosegrass); however, the origin and mechanism of these CNVs remain elusive in many weed species due to limited genetic and genomic resources. To study this CNV in goosegrass, we present high-quality reference genomes for glyphosate-susceptible and -resistant goosegrass lines and fine-assembles of the duplication of glyphosate's target site gene 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). We reveal a unique rearrangement of EPSPS involving chromosome subtelomeres. This discovery adds to the limited knowledge of the importance of subtelomeres as genetic variation generators and provides another unique example for herbicide resistance evolution.

Resequencing of a Pekin duck breeding population provides insights into the genomic response to short-term artificial selection

BACKGROUND

Short-term, intense artificial selection drives fast phenotypic changes in domestic animals and leaves imprints on their genomes. However, the genetic basis of this selection response is poorly understood. To better address this, we employed the Pekin duck Z2 pure line, in which the breast muscle weight was increased nearly 3-fold after 10 generations of breeding. We denovo assembled a high-quality reference genome of a female Pekin duck of this line (GCA_003850225.1) and identified 8.60 million genetic variants in 119 individuals among 10 generations of the breeding population.

RESULTS

We identified 53 selected regions between the first and tenth generations, and 93.8% of the identified variations were enriched in regulatory and noncoding regions. Integrating the selection signatures and genome-wide association approach, we found that 2 regions covering 0.36 Mb containing UTP25 and FBRSL1 were most likely to contribute to breast muscle weight improvement. The major allele frequencies of these 2 loci increased gradually with each generation following the same trend. Additionally, we found that a copy number variation region containing the entire EXOC4 gene could explain 1.9% of the variance in breast muscle weight, indicating that the nervous system may play a role in economic trait improvement.

CONCLUSIONS

Our study not only provides insights into genomic dynamics under intense artificial selection but also provides resources for genomics-enabled improvements in duck breeding.

Subtelomeric 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) copy number variation confers glyphosate resistance in Eleusine indica

Genomic structural variation (SV) can have profound effects on an organism’s evolution, often serving as a novel source of genetic variation. Gene copy number variation (CNV), a specific form of SV, has repeatedly been associated with adaptive evolution in eukaryotes, especially to biotic and abiotic stresses. Resistance to the most widely used herbicide, glyphosate, has evolved through target-site CNV in many weedy plant species, including the economically important cosmopolitan grass, Eleusine indica (goosegrass); however, the origin and mechanisms of these resistance CNVs remain elusive in many weed species due to limited genetic and genomics resources. In order to study the target site CNV in goosegrass, we generated high-quality reference genomes for both glyphosate-susceptible and -resistant individuals, fine assembled the duplication of glyphosate's target site gene enolpyruvylshikimate-3-phosphate synthase (EPSPS), and revealed a novel rearrangement of EPSPS into the subtelomeric region of the chromosomes, ultimately leading to herbicide resistance evolution. This discovery adds to the limited knowledge of the importance of subtelomeres as rearrangement hotspots and novel variation generators as well as provides an example of yet another unique pathway for the formation of CNVs in plants.

Gene abundance linked to climate zone: Parallel evolution of gene content along elevation gradients in lichenized fungi

Introduction

Intraspecific genomic variability affects a species' adaptive potential toward climatic conditions. Variation in gene content across populations and environments may point at genomic adaptations to specific environments. The lichen symbiosis, a stable association of fungal and photobiont partners, offers an excellent system to study environmentally driven gene content variation. Many of these species have remarkable environmental tolerances, and often form populations across different climate zones. Here, we combine comparative and population genomics to assess the presence and absence of genes in high and low elevation genomes of two lichenized fungi of the genus Umbilicaria.

Methods

The two species have non-overlapping ranges, but occupy similar climatic niches in North America (U. phaea) and Europe (U. pustulata): high elevation populations are located in the cold temperate zone and low elevation populations in the Mediterranean zone. We assessed gene content variation along replicated elevation gradients in each of the two species, based on a total of 2050 individuals across 26 populations. Specifically, we assessed shared orthologs across species within the same climate zone, and tracked, which genes increase or decrease in abundance within populations along elevation.

Results

In total, we found 16 orthogroups with shared orthologous genes in genomes at low elevation and 13 at high elevation. Coverage analysis revealed one ortholog that is exclusive to genomes at low elevation. Conserved domain search revealed domains common to the protein kinase superfamily. We traced the discovered ortholog in populations along five replicated elevation gradients on both continents and found that the number of this protein kinase gene linearly declined in abundance with increasing elevation, and was absent in the highest populations.

Discussion

We consider the parallel loss of an ortholog in two species and in two geographic settings a rare find, and a step forward in understanding the genomic underpinnings of climatic tolerances in lichenized fungi. In addition, the tracking of gene content variation provides a widely applicable framework for retrieving biogeographical determinants of gene presence/absence patterns. Our work provides insights into gene content variation of lichenized fungi in relation to climatic gradients, suggesting a new research direction with implications for understanding evolutionary trajectories of complex symbioses in relation to climatic change.

Genetic mechanisms underlying increased microalgal thermotolerance, maximal growth rate, and yield on light following adaptive laboratory evolution

BACKGROUND

Adaptive laboratory evolution (ALE) is a powerful method for strain optimization towards abiotic stress factors and for identifying adaptation mechanisms. In this study, the green microalga Picochlorum sp. BPE23 was cultured under supra-optimal temperature to force genetic adaptation. The robustness and adaptive capacity of Picochlorum strains turned them into an emerging model for evolutionary studies on abiotic stressors such as temperature, salinity, and light.

RESULTS

Mutant strains showed an expanded maximal growth temperature of 44.6 °C, whereas the maximal growth temperature of the wild-type strain was 42 °C. Moreover, at the optimal growth temperature of 38 °C, the biomass yield on light was 22.3% higher, and the maximal growth rate was 70.5% higher than the wild type. Genome sequencing and transcriptome analysis were performed to elucidate the mechanisms behind the improved phenotype. A de novo assembled phased reference genome allowed the identification of 21 genic mutations involved in various processes. Moreover, approximately half of the genome contigs were found to be duplicated or even triplicated in all mutants, suggesting a causal role in adaptation.

CONCLUSIONS

The developed tools and mutant strains provide a strong framework from whereupon Picochlorum sp. BPE23 can be further developed. Moreover, the extensive strain characterization provides evidence of how microalgae evolve to supra-optimal temperature and to photobioreactor growth conditions. With this study, microalgal evolutionary mechanisms were identified by combining ALE with genome sequencing.

Neural networks enable efficient and accurate simulation-based inference of evolutionary parameters from adaptation dynamics

The rate of adaptive evolution depends on the rate at which beneficial mutations are introduced into a population and the fitness effects of those mutations. The rate of beneficial mutations and their expected fitness effects is often difficult to empirically quantify. As these 2 parameters determine the pace of evolutionary change in a population, the dynamics of adaptive evolution may enable inference of their values. Copy number variants (CNVs) are a pervasive source of heritable variation that can facilitate rapid adaptive evolution. Previously, we developed a locus-specific fluorescent CNV reporter to quantify CNV dynamics in evolving populations maintained in nutrient-limiting conditions using chemostats. Here, we use CNV adaptation dynamics to estimate the rate at which beneficial CNVs are introduced through de novo mutation and their fitness effects using simulation-based likelihood-free inference approaches. We tested the suitability of 2 evolutionary models: a standard Wright-Fisher model and a chemostat model. We evaluated 2 likelihood-free inference algorithms: the well-established Approximate Bayesian Computation with Sequential Monte Carlo (ABC-SMC) algorithm, and the recently developed Neural Posterior Estimation (NPE) algorithm, which applies an artificial neural network to directly estimate the posterior distribution. By systematically evaluating the suitability of different inference methods and models, we show that NPE has several advantages over ABC-SMC and that a Wright-Fisher evolutionary model suffices in most cases. Using our validated inference framework, we estimate the CNV formation rate at the GAP1 locus in the yeast Saccharomyces cerevisiae to be 10-4.7 to 10-4 CNVs per cell division and a fitness coefficient of 0.04 to 0.1 per generation for GAP1 CNVs in glutamine-limited chemostats. We experimentally validated our inference-based estimates using 2 distinct experimental methods-barcode lineage tracking and pairwise fitness assays-which provide independent confirmation of the accuracy of our approach. Our results are consistent with a beneficial CNV supply rate that is 10-fold greater than the estimated rates of beneficial single-nucleotide mutations, explaining the outsized importance of CNVs in rapid adaptive evolution. More generally, our study demonstrates the utility of novel neural network-based likelihood-free inference methods for inferring the rates and effects of evolutionary processes from empirical data with possible applications ranging from tumor to viral evolution.

Genomic structural variation in tomato and its role in plant immunity

It is well known that large genomic variations can greatly impact the phenotype of an organism. Structural Variants (SVs) encompass any genomic variation larger than 30 base pairs, and include changes caused by deletions, inversions, duplications, transversions, and other genome modifications. Due to their size and complex nature, until recently, it has been difficult to truly capture these variations. Recent advances in sequencing technology and computational analyses now permit more extensive studies of SVs in plant genomes. In tomato, advances in sequencing technology have allowed researchers to sequence hundreds of genomes from tomatoes, and tomato relatives. These studies have identified SVs related to fruit size and flavor, as well as plant disease response, resistance/susceptibility, and the ability of plants to detect pathogens (immunity). In this review, we discuss the implications for genomic structural variation in plants with a focus on its role in tomato immunity. We also discuss how advances in sequencing technology have led to new discoveries of SVs in more complex genomes, the current evidence for the role of SVs in biotic and abiotic stress responses, and the outlook for genetic modification of SVs to advance plant breeding objectives.

Duplication and specialization of NUDX1 in Rosaceae led to geraniol production in rose petals

Nudix hydrolases are conserved enzymes ubiquitously present in all kingdoms of life. Recent research revealed that several Nudix hydrolases are involved in terpenoid metabolism in plants. In modern roses, RhNUDX1 is responsible for formation of geraniol, a major compound of rose scent. Nevertheless, this compound is produced by monoterpene synthases in many geraniol-producing plants. As a consequence, this raised the question about the origin of RhNUDX1 function and the NUDX1 gene evolution in Rosaceae, in wild roses or/and during the domestication process. Here, we showed that three distinct clades of NUDX1 emerged in the Rosoidae subfamily (Nudx1-1 to Nudx1-3 clades), and two subclades evolved in the Rosa genus (Nudx1-1a and Nudx1-1b subclades). We also showed that the Nudx1-1b subclade was more ancient than the Nudx1-1a subclade, and that the NUDX1-1a gene emerged by a trans-duplication of the more ancient NUDX1-1b gene. After the transposition, NUDX1-1a was cis-duplicated, leading to a gene dosage effect on the production of geraniol in different species. Furthermore, the NUDX1-1a appearance was accompanied by the evolution of its promoter, most likely from a Copia retrotransposon origin, leading to its petal-specific expression. Thus, our data strongly suggest that the unique function of NUDX1-1a in geraniol formation was evolved naturally in the genus Rosa before domestication.

Concerted genomic and epigenomic changes accompany stabilization of Arabidopsis allopolyploids

During evolution successful allopolyploids must overcome 'genome shock' between hybridizing species but the underlying process remains elusive. Here, we report concerted genomic and epigenomic changes in resynthesized and natural Arabidopsis suecica (TTAA) allotetraploids derived from Arabidopsis thaliana (TT) and Arabidopsis arenosa (AA). A. suecica shows conserved gene synteny and content with more gene family gain and loss in the A and T subgenomes than respective progenitors, although A. arenosa-derived subgenome has more structural variation and transposon distributions than A. thaliana-derived subgenome. These balanced genomic variations are accompanied by pervasive convergent and concerted changes in DNA methylation and gene expression among allotetraploids. The A subgenome is hypomethylated rapidly from F1 to resynthesized allotetraploids and convergently to the T-subgenome level in natural A. suecica, despite many other methylated loci being inherited from F1 to all allotetraploids. These changes in DNA methylation, including small RNAs, in allotetraploids may affect gene expression and phenotypic variation, including flowering, silencing of self-incompatibility and upregulation of meiosis- and mitosis-related genes. In conclusion, concerted genomic and epigenomic changes may improve stability and adaptation during polyploid evolution.

Transposition and duplication of MADS-domain transcription factor genes in annual and perennial Arabis species modulates flowering

Annual and perennial species differ in their timing and intensity of flowering, but the underlying mechanisms are poorly understood. We hybridized closely related annual and perennial plants and used genetics, transgenesis, and genomics to characterize differences in the activity and function of their flowering-time genes. We identify a gene encoding a transcription factor that moved between chromosomes and is retained in the annual but absent from the perennial. This gene strongly delays flowering, and we propose that it has been retained in the annual to compensate for reduced activity of closely related genes. This study highlights the value of using direct hybridization between closely related plant species to characterize functional differences in fast-evolving reproductive traits.

The timing of reproduction is an adaptive trait in many organisms. In plants, the timing, duration, and intensity of flowering differ between annual and perennial species. To identify interspecies variation in these traits, we studied introgression lines derived from hybridization of annual and perennial species, Arabis montbretiana and Arabis alpina, respectively. Recombination mapping identified two tandem A. montbretiana genes encoding MADS-domain transcription factors that confer extreme late flowering on A. alpina. These genes are related to the MADS AFFECTING FLOWERING (MAF) cluster of floral repressors of other Brassicaceae species and were named A. montbretiana (Am) MAF-RELATED (MAR) genes. AmMAR1 but not AmMAR2 prevented floral induction at the shoot apex of A. alpina, strongly enhancing the effect of the MAF cluster, and MAR1 is absent from the genomes of all A. alpina accessions analyzed. Exposure of plants to cold (vernalization) represses AmMAR1 transcription and overcomes its inhibition of flowering. Assembly of the tandem arrays of MAR and MAF genes of six A. alpina accessions and three related species using PacBio long-sequence reads demonstrated that the MARs arose within the Arabis genus by interchromosomal transposition of a MAF1-like gene followed by tandem duplication. Time-resolved comparative RNA-sequencing (RNA-seq) suggested that AmMAR1 may be retained in A. montbretiana to enhance the effect of the AmMAF cluster and extend the duration of vernalization required for flowering. Our results demonstrate that MAF genes transposed independently in different Brassicaceae lineages and suggest that they were retained to modulate adaptive flowering responses that differ even among closely related species.

Structural variations in papaya genomes

BACKGROUND

Structural variations (SVs) are a type of mutations that have not been widely detected in plant genomes and studies in animals have shown their role in the process of domestication. An in-depth study of SVs will help us to further understand the impact of SVs on the phenotype and environmental adaptability during papaya domestication and provide genomic resources for the development of molecular markers.

RESULTS

We detected a total of 8083 SVs, including 5260 deletions, 552 tandem duplications and 2271 insertions with deletion being the predominant, indicating the universality of deletion in the evolution of papaya genome. The distribution of these SVs is non-random in each chromosome. A total of 1794 genes overlaps with SV, of which 1350 genes are expressed in at least one tissue. The weighted correlation network analysis (WGCNA) of these expressed genes reveals co-expression relationship between SVs-genes and different tissues, and functional enrichment analysis shows their role in biological growth and environmental responses. We also identified some domesticated SVs genes related to environmental adaptability, sexual reproduction, and important agronomic traits during the domestication of papaya. Analysis of artificially selected copy number variant genes (CNV-genes) also revealed genes associated with plant growth and environmental stress.

CONCLUSIONS

SVs played an indispensable role in the process of papaya domestication, especially in the reproduction traits of hermaphrodite plants. The detection of genome-wide SVs and CNV-genes between cultivated gynodioecious populations and wild dioecious populations provides a reference for further understanding of the evolution process from male to hermaphrodite in papaya.

Adaptive evolution driving the young duplications in six Rosaceae species

Background

In plant genomes, high proportions of duplicate copies reveals that gene duplications play an important role in the evolutionary processes of plant species. A series of gene families under positive selection after recent duplication events in plant genomes indicated the evolution of duplicates driven by adaptive evolution. However, the genome-wide evolutionary features of young duplicate genes among closely related species are rarely reported.

Results

In this study, we conducted a systematic survey of young duplicate genes at genome-wide levels among six Rosaceae species, whose whole-genome sequencing data were successively released in recent years. A total of 35,936 gene families were detected among the six species, in which 60.25% were generated by young duplications. The 21,650 young duplicate gene families could be divided into two expansion types based on their duplication patterns, species-specific and lineage-specific expansions. Our results showed the species-specific expansions advantaging over the lineage-specific expansions. In the two types of expansions, high-frequency duplicate domains exhibited functional preference in response to environmental stresses.

Conclusions

The functional preference of the young duplicate genes in both the expansion types showed that they were inclined to respond to abiotic or biotic stimuli. Moreover, young duplicate genes under positive selection in both species-specific and lineage-specific expansions suggested that they were generated to adapt to the environmental factors in Rosaceae species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07422-7.

Segmental duplications are hot spots of copy number variants affecting barley gene content

Copy number variants (CNVs) are pervasive in several animal and plant genomes and contribute to shaping genetic diversity. In barley, there is evidence that changes in gene copy number underlie important agronomic traits. The recently released reference sequence of barley represents a valuable genomic resource for unveiling the incidence of CNVs that affect gene content and for identifying sequence features associated with CNV formation. Using exome sequencing and read count data, we detected 16 605 deletions and duplications that affect barley gene content by surveying a diverse panel of 172 cultivars, 171 landraces, 22 wild relatives and other 32 uncategorized domesticated accessions. The quest for segmental duplications (SDs) in the reference sequence revealed many low-copy repeats, most of which overlap predicted coding sequences. Statistical analyses revealed that the incidence of CNVs increases significantly in SD-rich regions, indicating that these sequence elements act as hot spots for the formation of CNVs. The present study delivers a comprehensive genome-wide study of CNVs affecting barley gene content and implicates SDs in the molecular mechanisms that lead to the formation of this class of CNVs.

Adaptation to novel environments during crop diversification

In the context of the global challenge of climate change, mitigation strategies are needed to adapt crops to novel environments. The main goal to address this is an understanding of the genetic basis of crop adaptation to different agro-ecological conditions. The movement of crops during the Colombian Exchange that started with the travels of Columbus in 1492 is an example of rapid adaptation to novel environments. Many diversification-related traits have been characterised in multiple crop species, and association-mapping analyses have identified loci involved in these. Here, we present an overview of current knowledge regarding the molecular basis related to the complex patterns of crop adaptation and dissemination, particularly outside their centres of origin. Investigation of the genomic basis of crop expansion offers a powerful contribution to the development of tools to identify and exploit valuable genetic diversity and to improve and design novel resilient crop varieties.

Deciphering the Impact of a Bacterial Infection on Meiotic Recombination in Arabidopsis with Fluorescence Tagged Lines

Plants are under strong evolutionary pressure to maintain surveillance against pathogens. One major disease resistance mechanism is based on NB-LRR (NLR) proteins that specifically recognize pathogen effectors. The cluster organization of the NLR gene family could favor sequence exchange between NLR genes via recombination, favoring their evolutionary dynamics. Increasing data, based on progeny analysis, suggest the existence of a link between the perception of biotic stress and the production of genetic diversity in the offspring. This could be driven by an increased rate of meiotic recombination in infected plants, but this has never been strictly demonstrated. In order to test if pathogen infection can increase DNA recombination in pollen meiotic cells, we infected Arabidopsis Fluorescent Tagged Lines (FTL) with the virulent bacteria Pseudomonas syringae. We measured the meiotic recombination rate in two regions of chromosome 5, containing or not an NLR gene cluster. In all tested intervals, no significant difference in genetic recombination frequency between infected and control plants was observed. Although it has been reported that pathogen exposure can sometimes increase the frequency of recombinant progeny in plants, our findings suggest that meiotic recombination rate in Arabidopsis may be resilient to at least some pathogen attack. Alternative mechanisms are discussed.

Gynoecy instability in cucumber (Cucumis sativus L.) is due to unequal crossover at the copy number variation-dependent Femaleness (F) locus

Cucumber, Cucumis sativus is an important vegetable crop, and gynoecy has played a critical role in yield increase of hybrid cucumber production. Cucumber has a unique genetic system for gynoecious sex expression, which is determined by the copy number variation (CNV)-based, dominant, and dosage-dependent femaleness (F) locus. However, this gynoecy expression system seems unstable since monecious plants could often be found in F-dependent gynoecious cucumber inbreds. We hypothesized that gynoecy instability (gynoecy loss) may be due to unequal crossing over (UCO) during meiosis among repeat units of the CNV. In this study, using high throughput genome resequencing, fiber-FISH and genomic qPCR analyses, we first confirmed and refined the structure of the F locus, which was a CNV of a 30.2-kb tandem repeat. Gynoecious plants contained three genes: CsACS1, CsACS1G, and CsMYB, of which CsACS1G is a duplication of CsACS1 but with a recombinant distal promoter that may contribute to gynoecy sex expression. In two large populations from self-pollinated gynoecious inbred lines, 'gynoecy loss' mutants were identified with similar mutation rates (~0.12%). We show that these monecious mutants have lost CsACS1G. In addition, we identified gynoecious lines in natural populations that carry two copies of CSACS1G. We proposed a model to explain gynoecy instability in F-dependent cucumbers, which is caused by UCO among CSACS1/G units during meiosis. The findings present a convincing case that the phenotypic variation of an economically important trait is associated with the dynamic changes of copy numbers at the F locus. This work also has important implications in cucumber breeding.

Tandem repeats lead to sequence assembly errors and impose multi-level challenges for genome and protein databases

The widespread occurrence of repetitive stretches of DNA in genomes of organisms across the tree of life imposes fundamental challenges for sequencing, genome assembly, and automated annotation of genes and proteins. This multi-level problem can lead to errors in genome and protein databases that are often not recognized or acknowledged. As a consequence, end users working with sequences with repetitive regions are faced with 'ready-to-use' deposited data whose trustworthiness is difficult to determine, let alone to quantify. Here, we provide a review of the problems associated with tandem repeat sequences that originate from different stages during the sequencing-assembly-annotation-deposition workflow, and that may proliferate in public database repositories affecting all downstream analyses. As a case study, we provide examples of the Atlantic cod genome, whose sequencing and assembly were hindered by a particularly high prevalence of tandem repeats. We complement this case study with examples from other species, where mis-annotations and sequencing errors have propagated into protein databases. With this review, we aim to raise the awareness level within the community of database users, and alert scientists working in the underlying workflow of database creation that the data they omit or improperly assemble may well contain important biological information valuable to others.

The genomic pool of standing structural variation outnumbers single nucleotide polymorphism by threefold in the marine teleost Chrysophrys auratus

Recent studies have highlighted an important role of structural variation (SV) in ecological and evolutionary processes, but few have studied nonmodel species in the wild. As part of our long-term research programme on the nonmodel teleost fish Australasian snapper (Chrysophrys auratus), we aim to build one of the first catalogues of genomic variants (SNPs and indels, and deletions, duplications and inversions) in fishes and evaluate overlap of genomic variants with regions under putative selection (Tajima's D and π), and coding sequences (genes). For this, we analysed six males and six females from three locations in New Zealand and generated a high-resolution genomic variation catalogue. We characterized 20,385 SVs and found they intersected with almost a third of all annotated genes. Together with small indels, SVs account for three times more variation in the genome in terms of bases affected compared to SNPs. We found that a sizeable portion of detected SVs was in the upper and lower genomic regions of Tajima's D and π, indicating that some of these have an effect on the phenotype. Together, these results shed light on the often neglected genomic variation that is produced by SVs and highlights the need to go beyond the mere measure of SNPs when investigating evolutionary processes, such as species diversification and adaptation.

Identification and characterization of a fast-neutron-induced mutant with elevated seed protein content in soybean

KEY MESSAGE

Protein content of soybean is critical for utility of soybean meal. A fast-neutron-induced deletion on chromosome 12 was found to be associated with increased protein content. Soybean seed composition affects the utility of soybean, and improving seed composition is an essential breeding goal. Fast neutron radiation introduces genomic mutations resulting in novel variation for traits of interest. Two elite soybean lines were irradiated with fast neutrons and screened for altered seed composition. Twenty-three lines with altered protein, oil, or sucrose content were selected based on near-infrared spectroscopy data from five environments and yield tested at five locations. Mutants with significantly increased protein averaged 19.1-36.8 g kg-1 more protein than the parents across 10 environments. Comparative genomic hybridization (CGH) identified putative mutations in a mutant, G15FN-12, that has 36.8 g kg-1 higher protein than the parent genotype, and whole genome sequencing (WGS) of the mutant has confirmed these mutations. An F2:3 population was developed from G15FN-12 to determine association between genomic changes and increased protein content. Bulked segregant analysis of the population using the SoySNP50K BeadChip identified a CGH- and WGS-confirmed deletion on chromosome 12 to be responsible for elevated protein content. The population was genotyped using a KASP marker designed at the mutation region, and significant association (P < 0.0001) between the deletion on chromosome 12 and elevated protein content was observed and confirmed in the F3:4 generation. The F2 segregants homozygous for the deletion averaged 27 g kg-1 higher seed protein and 8 g kg-1 lower oil than homozygous wild-type segregants. Mutants with altered seed composition are a new resource for gene function studies and provide elite materials for genetic improvement of seed composition.

The Draft Genome of Kochia scoparia and the Mechanism of Glyphosate Resistance via Transposon-Mediated EPSPS Tandem Gene Duplication

Increased copy number of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene confers resistance to glyphosate, the world's most-used herbicide. There are typically three to eight EPSPS copies arranged in tandem in glyphosate-resistant populations of the weed kochia (Kochia scoparia). Here, we report a draft genome assembly from a glyphosate-susceptible kochia individual. Additionally, we assembled the EPSPS locus from a glyphosate-resistant kochia plant by sequencing select bacterial artificial chromosomes from a kochia bacterial artificial chromosome library. Comparing the resistant and susceptible EPSPS locus allowed us to reconstruct the history of duplication in the structurally complex EPSPS locus and uncover the genes that are coduplicated with EPSPS, several of which have a corresponding change in transcription. The comparison between the susceptible and resistant assemblies revealed two dominant repeat types. Additionally, we discovered a mobile genetic element with a FHY3/FAR1-like gene predicted in its sequence that is associated with the duplicated EPSPS gene copies in the resistant line. We present a hypothetical model based on unequal crossing over that implicates this mobile element as responsible for the origin of the EPSPS gene duplication event and the evolution of herbicide resistance in this system. These findings add to our understanding of stress resistance evolution and provide an example of rapid resistance evolution to high levels of environmental stress.

Whole-genome re-sequencing reveals the impact of the interaction of copy number variants of the rhg1 and Rhg4 genes on broad-based resistance to soybean cyst nematode

Soybean cyst nematode (SCN) is the most devastating plant-parasitic nematode. Most commercial soybean varieties with SCN resistance are derived from PI88788. Resistance derived from PI88788 is breaking down due to narrow genetic background and SCN population shift. PI88788 requires mainly the rhg1-b locus, while 'Peking' requires rhg1-a and Rhg4 for SCN resistance. In the present study, whole genome re-sequencing of 106 soybean lines was used to define the Rhg haplotypes and investigate their responses to the SCN HG-Types. The analysis showed a comprehensive profile of SNPs and copy number variations (CNV) at these loci. CNV of rhg1 (GmSNAP18) only contributed towards resistance in lines derived from PI88788 and 'Cloud'. At least 5.6 copies of the PI88788-type rhg1 were required to confer SCN resistance, regardless of the Rhg4 (GmSHMT08) haplotype. However, when the GmSNAP18 copies dropped below 5.6, a 'Peking'-type GmSHMT08 haplotype was required to ensure SCN resistance. This points to a novel mechanism of epistasis between GmSNAP18 and GmSHMT08 involving minimum requirements for copy number. The presence of more Rhg4 copies confers resistance to multiple SCN races. Moreover, transcript abundance of the GmSHMT08 in root tissue correlates with more copies of the Rhg4 locus, reinforcing SCN resistance. Finally, haplotype analysis of the GmSHMT08 and GmSNAP18 promoters inferred additional levels of the resistance mechanism. This is the first report revealing the genetic basis of broad-based resistance to SCN and providing new insight into epistasis, haplotype-compatibility, CNV, promoter variation and its impact on broad-based disease resistance in plants.

Duplication of a domestication locus neutralized a cryptic variant that caused a breeding barrier in tomato

Genome editing technologies are being widely adopted in plant breeding¹. However, a looming challenge of engineering desirable genetic variation in diverse genotypes is poor predictability of phenotypic outcomes due to unforeseen interactions with pre-existing cryptic mutations^2-4. In tomato, breeding with a classical MADS-box gene mutation that improves harvesting by eliminating fruit stem abscission frequently results in excessive inflorescence branching, flowering and reduced fertility due to interaction with a cryptic variant that causes partial mis-splicing in a homologous gene^5-8. Here, we show that a recently evolved tandem duplication carrying the second-site variant achieves a threshold of functional transcripts to suppress branching, enabling breeders to neutralize negative epistasis on yield. By dissecting the dosage mechanisms by which this structural variant restored normal flowering and fertility, we devised strategies that use CRISPR-Cas9 genome editing to predictably improve harvesting. Our findings highlight the under-appreciated impact of epistasis in targeted trait breeding and underscore the need for a deeper characterization of cryptic variation to enable the full potential of genome editing in agriculture.

Structural variants in 3000 rice genomes

Investigation of large structural variants (SVs) is a challenging yet important task in understanding trait differences in highly repetitive genomes. Combining different bioinformatic approaches for SV detection, we analyzed whole-genome sequencing data from 3000 rice genomes and identified 63 million individual SV calls that grouped into 1.5 million allelic variants. We found enrichment of long SVs in promoters and an excess of shorter variants in 5′ UTRs. Across the rice genomes, we identified regions of high SV frequency enriched in stress response genes. We demonstrated how SVs may help in finding causative variants in genome-wide association analysis. These new insights into rice genome biology are valuable for understanding the effects SVs have on gene function, with the prospect of identifying novel agronomically important alleles that can be utilized to improve cultivated rice.

Copy Number Variation in Domestication

Domesticated plants have long served as excellent models for studying evolution. Many genes and mutations underlying important domestication traits have been identified, and most causal mutations appear to be SNPs. Copy number variation (CNV) is an important source of genetic variation that has been largely neglected in studies of domestication. Ongoing work demonstrates the importance of CNVs as a source of genetic variation during domestication, and during the diversification of domesticated taxa. Here, we review how CNVs contribute to evolutionary processes underlying domestication, and review examples of domestication traits caused by CNVs. We draw from examples in plant species, but also highlight cases in animal systems that could illuminate the roles of CNVs in the domestication process.

Embracing heterogeneity: coalescing the Tree of Life and the future of phylogenomics

Building the Tree of Life (ToL) is a major challenge of modern biology, requiring advances in cyberinfrastructure, data collection, theory, and more. Here, we argue that phylogenomics stands to benefit by embracing the many heterogeneous genomic signals emerging from the first decade of large-scale phylogenetic analysis spawned by high-throughput sequencing (HTS). Such signals include those most commonly encountered in phylogenomic datasets, such as incomplete lineage sorting, but also those reticulate processes emerging with greater frequency, such as recombination and introgression. Here we focus specifically on how phylogenetic methods can accommodate the heterogeneity incurred by such population genetic processes; we do not discuss phylogenetic methods that ignore such processes, such as concatenation or supermatrix approaches or supertrees. We suggest that methods of data acquisition and the types of markers used in phylogenomics will remain restricted until a posteriori methods of marker choice are made possible with routine whole-genome sequencing of taxa of interest. We discuss limitations and potential extensions of a model supporting innovation in phylogenomics today, the multispecies coalescent model (MSC). Macroevolutionary models that use phylogenies, such as character mapping, often ignore the heterogeneity on which building phylogenies increasingly rely and suggest that assimilating such heterogeneity is an important goal moving forward. Finally, we argue that an integrative cyberinfrastructure linking all steps of the process of building the ToL, from specimen acquisition in the field to publication and tracking of phylogenomic data, as well as a culture that values contributors at each step, are essential for progress.

The complex architecture and epigenomic impact of plant T-DNA insertions

The bacterium Agrobacterium tumefaciens has been the workhorse in plant genome engineering. Customized replacement of native tumor-inducing (Ti) plasmid elements enabled insertion of a sequence of interest called Transfer-DNA (T-DNA) into any plant genome. Although these transfer mechanisms are well understood, detailed understanding of structure and epigenomic status of insertion events was limited by current technologies. Here we applied two single-molecule technologies and analyzed Arabidopsis thaliana lines from three widely used T-DNA insertion collections (SALK, SAIL and WISC). Optical maps for four randomly selected T-DNA lines revealed between one and seven insertions/rearrangements, and the length of individual insertions from 27 to 236 kilobases. De novo nanopore sequencing-based assemblies for two segregating lines partially resolved T-DNA structures and revealed multiple translocations and exchange of chromosome arm ends. For the current TAIR10 reference genome, nanopore contigs corrected 83% of non-centromeric misassemblies. The unprecedented contiguous nucleotide-level resolution enabled an in-depth study of the epigenome at T-DNA insertion sites. SALK_059379 line T-DNA insertions were enriched for 24nt small interfering RNAs (siRNA) and dense cytosine DNA methylation, resulting in transgene silencing via the RNA-directed DNA methylation pathway. In contrast, SAIL_232 line T-DNA insertions are predominantly targeted by 21/22nt siRNAs, with DNA methylation and silencing limited to a reporter, but not the resistance gene. Additionally, we profiled the H3K4me3, H3K27me3 and H2A.Z chromatin environments around T-DNA insertions using ChIP-seq in SALK_059379, SAIL_232 and five additional T-DNA lines. We discovered various effect s ranging from complete loss of chromatin marks to the de novo incorporation of H2A.Z and trimethylation of H3K4 and H3K27 around the T-DNA integration sites. This study provides new insights into the structural impact of inserting foreign fragments into plant genomes and demonstrates the utility of state-of-the-art long-range sequencing technologies to rapidly identify unanticipated genomic changes.

Our routine ability to add or alter genes in plant genomes using transgenesis has proven to be a game changer to plant sciences. Transgenics not only enables the study of gene function but also allows the development of modern crop plants without the unwanted genetic baggage coming from natural crossing. A major tool to create transgenics is the Agrobacterium system which naturally shuttles and integrates pieces of foreign DNA into its host genome. While the position and number of integrations was relatively easy to track, molecular tools never allowed to see the integrated piece of DNA within a single “picture”. Here we have utilized state-of-the-art DNA sequencing technology to capture the size and structure of multiple DNA insertion events in a plant genome. We discovered that insertion of the anticipated DNA fragment occurred as multiple concatenated full and partial fragments that led in some cases to intra- and interchromosomal rearrangements. Our analysis of the epigenetic landscapes showed variable effects from silencing of the integrated foreign DNA to alterations of chromatin marks and thus chromatin structure and functionality.

Accelerated rates of large-scale mutations in the presence of copper and nickel

Mutation rate variation has been under intense investigation for decades. Despite these efforts, little is known about the extent to which environmental stressors accelerate mutation rates and influence the genetic load of populations. Moreover, most studies on stressors have focused on unicellular organisms and point mutations rather than large-scale deletions and duplications (copy number variations [CNVs]). We estimated mutation rates in Daphnia pulex exposed to low levels of environmental stressors as well as the effect of selection on de novo mutations. We conducted a mutation accumulation (MA) experiment in which selection was minimized, coupled with an experiment in which a population was propagated under competitive conditions in a benign environment. After an average of 103 generations of MA propagation, we sequenced 60 genomes and found significantly accelerated rates of deletions and duplications in MA lines exposed to ecologically relevant concentrations of metals. Whereas control lines had gene deletion and duplication rates comparable to other multicellular eukaryotes (1.8 × 10-6 per gene per generation), the presence of nickel and copper increased these rates fourfold. The realized mutation rate under selection was reduced to 0.4× that of control MA lines, providing evidence that CNVs contribute to mutational load. Our CNV breakpoint analysis revealed that nonhomologous recombination associated with regions of DNA fragility is the primary source of CNVs, plausibly linking metal-induced DNA strand breaks with higher CNV rates. Our findings suggest that environmental stress, in particular multiple stressors, can have profound effects on large-scale mutation rates and mutational load of multicellular organisms.

Extreme copy number variation at a tRNA ligase gene affecting phenology and fitness in yellow monkeyflowers

Copy number variation (CNV) is a major part of the genetic diversity segregating within populations, but remains poorly understood relative to single nucleotide variation. Here, we report on a tRNA ligase gene (Migut.N02091; RLG1a) exhibiting unprecedented, and fitness-relevant, CNV within an annual population of the yellow monkeyflower Mimulus guttatus. RLG1a variation was associated with multiple traits in pooled population sequencing (PoolSeq) scans of phenotypic and phenological cohorts. Resequencing of inbred lines revealed intermediate-frequency three-copy variants of RLG1a (trip+; 5/35 = 14%), and trip+ lines exhibited elevated RLG1a expression under multiple conditions. trip+ carriers, in addition to being over-represented in late-flowering and large-flowered PoolSeq populations, flowered later under stressful conditions in a greenhouse experiment (p < 0.05). In wild population samples, we discovered an additional rare RLG1a variant (high+) that carries 250-300 copies of RLG1a totalling ~5.7 Mb (20-40% of a chromosome). In the progeny of a high+ carrier, Mendelian segregation of diagnostic alleles and qPCR-based copy counts indicate that high+ is a single tandem array unlinked to the single-copy RLG1a locus. In the wild, high+ carriers had highest fitness in two particularly dry and/or hot years (2015 and 2017; both p < 0.01), while single-copy individuals were twice as fecund as either CNV type in a lush year (2016: p < 0.005). Our results demonstrate fluctuating selection on CNVs affecting phenological traits in a wild population, suggest that plant tRNA ligases mediate stress-responsive life-history traits, and introduce a novel system for investigating the molecular mechanisms of gene amplification.

Halophytism: What Have We Learnt From Arabidopsis thaliana Relative Model Systems?

Halophytes are able to thrive in salt concentrations that would kill 99% of other plant species, and identifying their salt-adaptive mechanisms has great potential for improving the tolerance of crop plants to salinized soils. Much research has focused on the physiological basis of halophyte salt tolerance, whereas the elucidation of molecular mechanisms has traditionally lagged behind due to the absence of a model halophyte system. However, over the last decade and a half, two Arabidopsis (Arabidopsis thaliana) relatives, Eutrema salsugineum and Schrenkiella parvula, have been established as transformation-competent models with various genetic resources including high-quality genome assemblies. These models have facilitated powerful comparative analyses with salt-sensitive Arabidopsis to unravel the genetic adaptations that enable a halophytic lifestyle. The aim of this review is to explore what has been learned about halophytism using E. salsugineum and S. parvula We consider evidence from physiological and molecular studies suggesting that differences in salt tolerance between related halophytes and salt-sensitive plants are associated with alterations in the regulation of basic physiological, biochemical, and molecular processes. Furthermore, we discuss how salt tolerance mechanisms of the halophytic models are reflected at the level of their genomes, where evolutionary processes such as subfunctionalization and/or neofunctionalization have altered the expression and/or functions of genes to facilitate adaptation to saline conditions. Lastly, we summarize the many areas of research still to be addressed with E. salsugineum and S. parvula as well as obstacles hindering further progress in understanding halophytism.

Systems, variation, individuality and plant hormones

Inter-individual variation in plants and particularly in hormone content, figures strongly in evolution and behaviour. Homo sapiens and Arabidopsis exhibit similar and substantial phenotypic and molecular variation. Whereas there is a very substantial degree of hormone variation in mankind, reports of inter-individual variation in plant hormone content are virtually absent but are likely to be as large if not larger than that in mankind. Reasons for this absence are discussed. Using an example of inter-individual variation in ethylene content in ripening, the article shows how biological time is compressed by hormones. It further resolves an old issue of very wide hormone dose response that result directly from negative regulation in hormone (and light) transduction. Negative regulation is used because of inter-individual variability in hormone synthesis, receptors and ancillary proteins, a consequence of substantial genomic and environmental variation. Somatic mosaics have been reported for several plant tissues and these too contribute to tissue variation and wide variation in hormone response. The article concludes by examining what variation exists in gravitropic responses. There are multiple sensing systems of gravity vectors and multiple routes towards curvature. These are an aspect of the need for reliability in both inter-individual variation and unpredictable environments. Plant hormone inter-individuality is a new area for research and is likely to change appreciation of the mechanisms that underpin individual behaviour.

Gene copy number variations involved in balsam poplar (Populus balsamifera L.) adaptive variations

Gene copy number variations (CNVs) involved in phenotypic variations have already been shown in plants, but genomewide testing of CNVs for adaptive variation was not doable until recent technological developments. Thus, reports of the genomic architecture of adaptation involving CNVs remain scarce to date. Here, we investigated F₁ progenies of an intraprovenance cross (north-north cross, 58th parallel) and an interprovenances cross (north-south cross, 58th/49th parallels) for CNVs using comparative genomic hybridization on arrays of probes targeting gene sequences in balsam poplar (Populus balsamifera L.), a widespread North American forest tree. A total of 1,721 genes were found in varying copy numbers over the set of 19,823 tested genes. These gene CNVs presented an estimated average size of 8.3 kb and were distributed over poplar's 19 chromosomes including 22 hotspot regions. Gene CNVs number was higher for the interprovenance progeny in accordance with an expected higher genetic diversity related to the composite origin of this family. Regression analyses between gene CNVs and seven adaptive trait variations resulted in 23 significant links; among these adaptive gene CNVs, 30% were located in hotspots. One-to-five gene CNVs were found related to each of the measured adaptive traits and annotated for both biotic and abiotic stress responses. These annotations can be related to the occurrence of a higher pathogenic pressure in the southern parts of balsam poplar's distribution, and higher photosynthetic assimilation rates and water-use efficiency at high latitudes. Overall, our findings suggest that gene CNVs typically having higher mutation rates than SNPs may in fact represent efficient adaptive variations against fast-evolving pathogens.

The power and potential of genomics in weed biology and management

There have been previous calls for, and efforts focused on, realizing the power and potential of weed genomics for better understanding of weeds. Sustained advances in genome sequencing and assembly technologies now make it possible for individual research groups to generate reference genomes for multiple weed species at reasonable costs. Here, we present the outcomes from several meetings, discussions, and workshops focused on establishing an International Weed Genomics Consortium (IWGC) for a coordinated international effort in weed genomics. We review the 'state of the art' in genomics and weed genomics, including technologies, applications, and on-going weed genome projects. We also report the outcomes from a workshop and a global survey of the weed science community to identify priority species, key biological questions, and weed management applications that can be addressed through greater availability of, and access to, genomic resources. Major focus areas include the evolution of herbicide resistance and weedy traits, the development of molecular diagnostics, and the identification of novel targets and approaches for weed management. There is increasing interest in, and need for, weed genomics, and the establishment of the IWGC will provide the necessary global platform for communication and coordination of weed genomics research. © 2018 Society of Chemical Industry.

Array of MADS-Box Genes: Facilitator for Rapid Adaptation?

In a world of global warming, the question emerges whether all plants have suitable mechanisms to keep pace with the rapidly changing environment. Most previous studies have focused on either the ability of plants to rapidly acclimatize via physiological and developmental plasticity, or long-term adaptation over thousands of years. However, we wonder whether plants can also adapt to changes in the environment within only a few generations. We hypothesize that rapidly evolving clusters of tandemly duplicated developmental control genes represent a source for fast adaptation. Specifically, we propose that a tandem cluster of FLC-like MADS-box genes involved in the transition to flowering in Arabidopsis functions as a facilitator for rapid adaptation to changes in ambient temperature.

Characterization of photosynthetic ferredoxin from the Antarctic alga Chlamydomonas sp. UWO241 reveals novel features of cold adaptation

The objective of this work was to characterize photosynthetic ferredoxin from the Antarctic green alga Chlamydomonas sp. UWO241, a key enzyme involved in distributing photosynthetic reducing power. We hypothesize that ferredoxin possesses characteristics typical of cold-adapted enzymes, namely increased structural flexibility and high activity at low temperatures, accompanied by low stability at moderate temperatures. To address this objective, we purified ferredoxin from UWO241 and characterized the temperature dependence of its enzymatic activity and protein conformation. The UWO241 ferredoxin protein, RNA, and DNA sequences were compared with homologous sequences from related organisms. We provide evidence for the duplication of the main ferredoxin gene in the UWO241 nuclear genome and the presence of two highly similar proteins. Ferredoxin from UWO241 has both high activity at low temperatures and high stability at moderate temperatures, representing a novel class of cold-adapted enzymes. Our study reveals novel insights into how photosynthesis functions in the cold. The presence of two distinct ferredoxin proteins in UWO241 could provide an adaptive advantage for survival at cold temperatures. The primary amino acid sequence of ferredoxin is highly conserved among photosynthetic species, and we suggest that subtle differences in sequence can lead to significant changes in activity at low temperatures.

Glyphosate Resistance and EPSPS Gene Duplication: Convergent Evolution in Multiple Plant Species

One of the increasingly widespread mechanisms of resistance to the herbicide glyphosate is copy number variation (CNV) of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene. EPSPS gene duplication has been reported in 8 weed species, ranging from 3 to 5 extra copies to more than 150 extra copies. In the case of Palmer amaranth (Amaranthus palmeri), a section of >300 kb containing EPSPS and many other genes has been replicated and inserted at new loci throughout the genome, resulting in significant increase in total genome size. The replicated sequence contains several classes of mobile genetic elements including helitrons, raising the intriguing possibility of extra-chromosomal replication of the EPSPS-containing sequence. In kochia (Kochia scoparia), from 3 to more than 10 extra EPSPS copies are arranged as a tandem gene duplication at one locus. In the remaining 6 weed species that exhibit EPSPS gene duplication, little is known about the underlying mechanisms of gene duplication or their entire sequence. There is mounting evidence that adaptive gene amplification is an important mode of evolution in the face of intense human-mediated selection pressure. The convergent evolution of CNVs for glyphosate resistance in weeds, through at least 2 different mechanisms, may be indicative of a more general importance for this mechanism of adaptation in plants. CNVs warrant further investigation across plant functional genomics for adaptation to biotic and abiotic stresses, particularly for adaptive evolution on rapid time scales.

Copy number variation and disease resistance in plants

Plant genome diversity varies from single nucleotide polymorphisms to large-scale deletions, insertions, duplications, or re-arrangements. These re-arrangements of sequences resulting from duplication, gains or losses of DNA segments are termed copy number variations (CNVs). During the last decade, numerous studies have emphasized the importance of CNVs as a factor affecting human phenotype; in particular, CNVs have been associated with risks for several severe diseases. In plants, the exploration of the extent and role of CNVs in resistance against pathogens and pests is just beginning. Since CNVs are likely to be associated with disease resistance in plants, an understanding of the distribution of CNVs could assist in the identification of novel plant disease-resistance genes. In this paper, we review existing information about CNVs; their importance, role and function, as well as their association with disease resistance in plants.

Gene copy number variations in adaptive evolution: The genomic distribution of gene copy number variations revealed by genetic mapping and their adaptive role in an undomesticated species, white spruce (Picea glauca)

Gene copy number variation (CNV) has been associated with phenotypic variability in animals and plants, but a genomewide understanding of their impacts on phenotypes is largely restricted to human and agricultural systems. As such, CNVs have rarely been considered in investigations of the genomic architecture of adaptation in wild species. Here, we report on the genetic mapping of gene CNVs in white spruce, which lacks a contiguous assembly of its large genome (~20 Gb), and their relationships with adaptive phenotypic variation. We detected 3,911 gene CNVs including de novo structural variations using comparative genome hybridization on arrays (aCGH) in a large progeny set. We inferred the heterozygosity at CNV loci within parents by comparing haploid and diploid tissues and genetically mapped 82 gene CNVs. Our analysis showed that CNVs were distributed over 10 linkage groups and identified four CNV hotspots that we predict to occur in other species of the Pinaceae. Significant relationships were found between 29 of the gene CNVs and adaptive traits based on regression analyses with timings of bud set and bud flush, and height growth, suggesting a role for CNVs in climate adaptation. The importance of CNVs in adaptive evolution of white spruce was also indicated by functional gene annotations and the clustering of 31% of the mapped adaptive gene CNVs in CNV hotspots. Taken together, these results illustrate the feasibility of studying CNVs in undomesticated species and represent a major step towards a better understanding of the roles of CNVs in adaptive evolution.

Hybrid assembly with long and short reads improves discovery of gene family expansions

Background

Long-read and short-read sequencing technologies offer competing advantages for eukaryotic genome sequencing projects. Combinations of both may be appropriate for surveys of within-species genomic variation.

Methods

We developed a hybrid assembly pipeline called “Alpaca” that can operate on 20X long-read coverage plus about 50X short-insert and 50X long-insert short-read coverage. To preclude collapse of tandem repeats, Alpaca relies on base-call-corrected long reads for contig formation.

Results

Compared to two other assembly protocols, Alpaca demonstrated the most reference agreement and repeat capture on the rice genome. On three accessions of the model legume Medicago truncatula, Alpaca generated the most agreement to a conspecific reference and predicted tandemly repeated genes absent from the other assemblies.

Conclusion

Our results suggest Alpaca is a useful tool for investigating structural and copy number variation within de novo assemblies of sampled populations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3927-8) contains supplementary material, which is available to authorized users.

Brief temperature stress during reproductive stages alters meiotic recombination and somatic mutation rates in the progeny of Arabidopsis

BACKGROUND

Plants exposed to environmental stresses draw upon many genetic and epigenetic strategies, with the former sometimes modulated by the latter. This can help the plant, and its immediate progeny, at least, to better endure the stress. Some evidence has led to proposals that (epi) genetic changes can be both selective and sustainably heritable, while other evidence suggests that changes are effectively stochastic, and important only because they induce genetic variation. One type of stress with an arguably high level of stochasticity in its effects is temperature stress. Studies of how heat and cold affect the rates of meiotic recombination (MR) and somatic mutations (SMs, which are potentially heritable in plants) report increases, decreases, or no effect. Collectively, they do not point to any consistent patterns. Some of this variability, however, might arise from the stress being applied for such an extended time, typically days or weeks. Here, we adopted a targeted approach by (1) limiting exposure to one hour; and (2) timing it to coincide with (a) gamete, and early gametophyte, development, a period of high stress sensitivity; and (b) a late stage of vegetative development.

RESULTS

For plants (Arabidopsis thaliana) otherwise grown at 22 °C, we measured the effects of a 1 h exposure to cold (12 °C) or heat (32 °C) on the rates of MR, and four types of SMs (frameshift mutations; intrachromosomal recombination; base substitutions; transpositions) in the F1 progeny. One parent (wild type) was stressed, the other (unstressed) carried a genetic event detector. When rates were compared to those in progeny of control (both parents unstressed) two patterns emerged. In the progeny of younger plants (stressed at 36 days; pollinated at 40 days) heat and cold either had no effect (on MR) or (for SMs) had effects that were rare and stochastic. In the progeny of older plants (stressed at 41 days; pollinated at 45 days), while effects were also infrequent, those that were seen followed a consistent pattern: rates of all five genetic events were lowest at 12 °C and highest at 32 °C, i.e. they varied in a "dose-response" manner. This pattern was strongest (or, in the case of MR, only apparent) in progeny whose stressed parent was female.

CONCLUSION

While the infrequency of effects suggests the need for cautious inference, the consistency of responses in the progeny of older plants, indicate that in some circumstances the level of stochasticity in inherited genetic responses to heat or cold stress can be context-dependent, possibly reflecting life-cycle stages in the parental generation that are variably stress sensitive.

Dosage sensitivity is a major determinant of human copy number variant pathogenicity

Human copy number variants (CNVs) account for genome variation an order of magnitude larger than single-nucleotide polymorphisms. Although much of this variation has no phenotypic consequences, some variants have been associated with disease, in particular neurodevelopmental disorders. Pathogenic CNVs are typically very large and contain multiple genes, and understanding the cause of the pathogenicity remains a major challenge. Here we show that pathogenic CNVs are significantly enriched for genes involved in development and genes that have greater evolutionary copy number conservation across mammals, indicative of functional constraints. Conversely, genes found in benign CNV regions have more variable copy number. These evolutionary constraints are characteristic of genes in pathogenic CNVs and can only be explained by dosage sensitivity of those genes. These results implicate dosage sensitivity of individual genes as a common cause of CNV pathogenicity. These evolutionary metrics suggest a path to identifying disease genes in pathogenic CNVs.

Copy number variants (CNVs) cause significant genomic variation in humans and may be benign or may cause disease. Here, the authors show that pathogenic CNVs are evolutionarily constrained compared with benign, pointing to dosage sensitivity as a potential cause of disease.

Structure and Origin of the White Cap Locus and Its Role in Evolution of Grain Color in Maize

Selection for yellow- and white-grain types has been central to postdomestication improvement of maize. While genetic control of carotenoid biosynthesis in endosperm is attributed primarily to the Yellow1 (Y1) phytoene synthase gene, less is known about the role of the dominant white endosperm factor White Cap (Wc). We show that the Wc locus contains multiple, tandem copies of a Carotenoid cleavage dioxygenase 1 (Ccd1) gene that encodes a carotenoid-degrading enzyme. A survey of 111 maize inbreds and landraces, together with 22 teosinte accessions, reveals that Wc is exclusive to maize, where it is prevalent in white-grain (y1) varieties. Moreover, Ccd1 copy number varies extensively among Wc alleles (from 1 to 23 copies), and confers a proportional range of Ccd1 expression in diverse organs. We propose that this dynamic source of quantitative variation in Ccd1 expression was created in maize shortly after domestication by a two-step, Tam3L transposon-mediated process. First, a chromosome segment containing Ccd1 and several nearby genes duplicated at a position 1.9 Mb proximal to the progenitor Ccd1r locus on chromosome 9. Second, a subsequent interaction of Tam3L transposons at the new locus created a 28-kb tandem duplication, setting up expansion of Ccd1 copy number by unequal crossing over. In this way, transposon-mediated variation in copy number at the Wc locus generated phenotypic variation that provided a foundation for breeding and selection of white-grain color in maize.

hsp90 and hsp47 appear to play an important role in minnow Puntius sophore for surviving in the hot spring run-off aquatic ecosystem

Changes in the expression of a number of hsp genes in minnow Puntius sophore collected from a hot spring run-off (Atri hot spring in Odisha, India; 20^o09'N 85^°18'E, 36-38 °C) were investigated to study the upper thermal acclimation response under heat stress, using same species from aquaculture ponds (water temperature 27 °C) as control. Expression of hsp genes was analyzed in both groups using RT-qPCR, which showed up-regulation of hsp90 (2.1-fold) and hsp47 (2.5-fold) in hot spring run-off fishes, whereas there was no alteration in expression of other hsps. As the fish inhabit the hot spring run-off area for very long duration, they could have adapted to the environment. To test this hypothesis, fishes collected from hot spring run-off were divided into two groups; one was heat-shocked at 41 °C/24 h, and the other was acclimatized at 27 °C/24 h. Up-regulation of all the hsps (except hsp78) was observed in the heat-shocked fishes, whereas expression of all hsps was found to be down-regulated to the basal level in fishes maintained at 27 °C/24 h. Pathway analysis showed that the expressions of all the hsps except hsp90 are regulated by the transcription factor heat shock factor 1 (Hsf1). This study showed that hsp90 and hsp47 play an important role in Puntius sophore for surviving in the high-temperature environment of the hot spring run-off. Additionally, we show that plasticity in hsp gene expression is not lost in the hot spring run-off population.

CNVs into the wild: screening the genomes of conifer trees (Picea spp.) reveals fewer gene copy number variations in hybrids and links to adaptation

Background

Copy number variations (CNVs) have been linked to different phenotypes in human, including many diseases. A genome-scale understanding of CNVs is available in a few plants but none are wild species, leaving a knowledge gap regarding their genome biology and evolutionary role. We developed a reliable CNV detection method for species lacking contiguous reference genome. We selected multiple probes within 14,078 gene sequences and developed comparative genome hybridization on arrays. Gene CNVs were assessed in three full-sib families from species with 20 Gb genomes, i.e., white and black spruce, and interior spruce - a natural hybrid.

Results

We discovered hundreds of gene CNVs in each species, 3612 in total, which were enriched in functions related to stress and defense responses and narrow expression profiles, indicating a potential role in adaptation. The number of shared CNVs was in accordance with the degree of relatedness between individuals and species. The genetically mapped subset of these genes showed a wide distribution across the genome, implying numerous structural variations. The hybrid family presented significantly fewer CNVs, suggesting that the admixture of two species within one genome reduces the occurrence of CNVs.

Conclusions

The approach we developed is of particular interest in non-model species lacking a reference genome. Our findings point to a role for CNVs in adaptation. Their reduced abundance in the hybrid may limit genetic variability and evolvability of hybrids. We propose that CNVs make a qualitatively distinct contribution to adaptation which could be important for short term change.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3458-8) contains supplementary material, which is available to authorized users.

Between-species differences in gene copy number are enriched among functions critical for adaptive evolution in Arabidopsis halleri

Background

Gene copy number divergence between species is a form of genetic polymorphism that contributes significantly to both genome size and phenotypic variation. In plants, copy number expansions of single genes were implicated in cultivar- or species-specific tolerance of high levels of soil boron, aluminium or calamine-type heavy metals, respectively. Arabidopsis halleri is a zinc- and cadmium-hyperaccumulating extremophile species capable of growing on heavy-metal contaminated, toxic soils. In contrast, its non-accumulating sister species A. lyrata and the closely related reference model species A. thaliana exhibit merely basal metal tolerance.

Results

For a genome-wide assessment of the role of copy number divergence (CND) in lineage-specific environmental adaptation, we conducted cross-species array comparative genome hybridizations of three plant species and developed a global signal scaling procedure to adjust for sequence divergence. In A. halleri, transition metal homeostasis functions are enriched twofold among the genes detected as copy number expanded. Moreover, biotic stress functions including mostly disease Resistance (R) gene-related genes are enriched twofold among genes detected as copy number reduced, when compared to the abundance of these functions among all genes.

Conclusions

Our results provide genome-wide support for a link between evolutionary adaptation and CND in A. halleri as shown previously for Heavy metal ATPase4. Moreover our results support the hypothesis that elemental defences, which result from the hyperaccumulation of toxic metals, allow the reduction of classical defences against biotic stress as a trade-off.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3319-5) contains supplementary material, which is available to authorized users.

Transcript Polymorphism Rates in Soybean Seed Tissue Are Increased in a Single Transformant of Glycine max

Transgenic crops have been utilized for decades to enhance agriculture and more recently have been applied as bioreactors for manufacturing pharmaceuticals. Recently, we investigated the gene expression profiles of several in-house transgenic soybean events, finding one transformant group to be consistently different from our controls. In the present study, we examined polymorphisms and sequence variations in the exomes of the same transgenic soybean events. We found that the previously dissimilar soybean line also exhibited markedly increased levels of polymorphisms within mRNA transcripts from seed tissue, many of which are classified as gene expression modifiers. The results from this work will direct future investigations to examine novel SNPs controlling traits of great interest for breeding and improving transgenic soybean crops. Further, this study marks the first work to investigate SNP rates in transgenic soybean seed tissues and demonstrates that while transgenesis may induce abundant unanticipated changes in gene expression and nucleotide variation, phenotypes and overall health of the plants examined remained unaltered.