Hiding in plain sight: Genome-wide recombination and a dynamic accessory genome drive diversity in Fusarium oxysporum f.sp. ciceris.

Understanding the origins of variation in agricultural pathogens is of fundamental interest and practical importance, especially for diseases that threaten food security. Fusarium oxysporum is among the most important of soil-borne pathogens, with a global distribution and an extensive host range. The pathogen is considered to be asexual, with horizontal transfer of chromosomes providing an analog of assortment by meiotic recombination. Here, we challenge those assumptions based on the results of population genomic analyses, describing the pathogen's diversity and inferring its origins and functional consequences in the context of a single, long-standing agricultural system. We identify simultaneously low nucleotide distance among strains, and unexpectedly high levels of genetic and genomic variability. We determine that these features arise from a combination of genome-scale recombination, best explained by widespread sexual reproduction, and presence-absence variation consistent with chromosomal rearrangement. Pangenome analyses document an accessory genome more than twice the size of the core genome, with contrasting evolutionary dynamics. The core genome is stable, with low diversity and high genetic differentiation across geographic space, while the accessory genome is paradoxically more diverse and unstable but with lower genetic differentiation and hallmarks of contemporary gene flow at local scales. We suggest a model in which episodic sexual reproduction generates haplotypes that are selected and then maintained through clone-like dynamics, followed by contemporary genomic rearrangements that reassort the accessory genome among sympatric strains. Taken together, these processes contribute unique genome content, including reassortment of virulence determinants that may explain observed variation in pathogenic potential.

A Commemorative Issue in Honor of 200th Anniversary of the Birth of Gregor Johann Mendel: The Genius of Genetics.

In celebration of the bicentennial of the birth of Gregor Johann Mendel, the genius of genetics, this Special Issue presents seven papers [...].

Global-level population genomics reveals differential effects of geography and phylogeny on horizontal gene transfer in soil bacteria.

Although microorganisms are known to dominate Earth's biospheres and drive biogeochemical cycling, little is known about the geographic distributions of microbial populations or the environmental factors that pattern those distributions. We used a global-level hierarchical sampling scheme to comprehensively characterize the evolutionary relationships and distributional limitations of the nitrogen-fixing bacterial symbionts of the crop chickpea, generating 1,027 draft whole-genome sequences at the level of bacterial populations, including 14 high-quality PacBio genomes from a phylogenetically representative subset. We find that diverse Mesorhizobium taxa perform symbiosis with chickpea and have largely overlapping global distributions. However, sampled locations cluster based on the phylogenetic diversity of Mesorhizobium populations, and diversity clusters correspond to edaphic and environmental factors, primarily soil type and latitude. Despite long-standing evolutionary divergence and geographic isolation, the diverse taxa observed to nodulate chickpea share a set of integrative conjugative elements (ICEs) that encode the major functions of the symbiosis. This symbiosis ICE takes 2 forms in the bacterial chromosome-tripartite and monopartite-with tripartite ICEs confined to a broadly distributed superspecies clade. The pairwise evolutionary relatedness of these elements is controlled as much by geographic distance as by the evolutionary relatedness of the background genome. In contrast, diversity in the broader gene content of Mesorhizobium genomes follows a tight linear relationship with core genome phylogenetic distance, with little detectable effect of geography. These results illustrate how geography and demography can operate differentially on the evolution of bacterial genomes and offer useful insights for the development of improved technologies for sustainable agriculture.

Ensuring Global Food Security by Improving Protein Content in Major Grain Legumes Using Breeding and 'Omics' Tools.

Grain legumes are a rich source of dietary protein for millions of people globally and thus a key driver for securing global food security. Legume plant-based 'dietary protein' biofortification is an economic strategy for alleviating the menace of rising malnutrition-related problems and hidden hunger. Malnutrition from protein deficiency is predominant in human populations with an insufficient daily intake of animal protein/dietary protein due to economic limitations, especially in developing countries. Therefore, enhancing grain legume protein content will help eradicate protein-related malnutrition problems in low-income and underprivileged countries. Here, we review the exploitable genetic variability for grain protein content in various major grain legumes for improving the protein content of high-yielding, low-protein genotypes. We highlight classical genetics-based inheritance of protein content in various legumes and discuss advances in molecular marker technology that have enabled us to underpin various quantitative trait loci controlling seed protein content (SPC) in biparental-based mapping populations and genome-wide association studies. We also review the progress of functional genomics in deciphering the underlying candidate gene(s) controlling SPC in various grain legumes and the role of proteomics and metabolomics in shedding light on the accumulation of various novel proteins and metabolites in high-protein legume genotypes. Lastly, we detail the scope of genomic selection, high-throughput phenotyping, emerging genome editing tools, and speed breeding protocols for enhancing SPC in grain legumes to achieve legume-based dietary protein security and thus reduce the global hunger risk.

Legume Genetics and Biology: From Mendel's Pea to Legume Genomics.

Legumes have played an important part in cropping systems since the dawn of agriculture, both as human food and as animal feed. The legume family is arguably one of the most abundantly domesticated crop plant families. Their ability to symbiotically fix nitrogen and improve soil fertility has been rewarded since antiquity and makes them a key protein source. The pea was the original model organism used in Mendel's discovery of the laws of inheritance, making it the foundation of modern plant genetics. This Special Issue provides up-to-date information on legume biology, genetic advances, and the legacy of Mendel.

Population genomic analysis of mango (Mangifera indica) suggests a complex history of domestication.

Humans have domesticated diverse species from across the plant kingdom, yet much of our foundational knowledge of domestication has come from studies investigating relatively few of the most important annual food crops. Here, we examine the impacts of domestication on genetic diversity in a tropical perennial fruit species, mango (Mangifera indica). We used restriction site associated DNA sequencing to generate genomic single nucleotide polymorphism (SNP) data from 106 mango cultivars from seven geographical regions along with 52 samples of closely related species and unidentified cultivars to identify centers of mango genetic diversity and examine how post-domestication dispersal shaped the geographical distribution of diversity. We identify two gene pools of cultivated mango, representing Indian and Southeast Asian germplasm. We found no significant genetic bottleneck associated with the introduction of mango into new regions of the world. By contrast, we show that mango populations in introduced regions have elevated levels of diversity. Our results suggest that mango has a more complex history of domestication than previously supposed, perhaps including multiple domestication events, hybridization and regional selection. Our work has direct implications for mango breeding and genebank management, and also builds on recent efforts to understand how woody perennial crops respond to domestication.

Functional Dissection of the Chickpea (Cicer arietinum L.) Stay-Green Phenotype Associated with Molecular Variation at an Ortholog of Mendel's I Gene for Cotyledon Color: Implications for Crop Production and Carotenoid Biofortification.

"Stay-green" crop phenotypes have been shown to impact drought tolerance and nutritional content of several crops. We aimed to genetically describe and functionally dissect the particular stay-green phenomenon found in chickpeas with a green cotyledon color of mature dry seed and investigate its potential use for improvement of chickpea environmental adaptations and nutritional value. We examined 40 stay-green accessions and a set of 29 BC2F4-5 stay-green introgression lines using a stay-green donor parent ICC 16340 and two Indian elite cultivars (KAK2, JGK1) as recurrent parents. Genetic studies of segregating populations indicated that the green cotyledon trait is controlled by a single recessive gene that is invariantly associated with the delayed degreening (extended chlorophyll retention). We found that the chickpea ortholog of Mendel's I locus of garden pea, encoding a SGR protein as very likely to underlie the persistently green cotyledon color phenotype of chickpea. Further sequence characterization of this chickpea ortholog CaStGR1 (CaStGR1, for carietinum stay-green gene 1) revealed the presence of five different molecular variants (alleles), each of which is likely a loss-of-function of the chickpea protein (CaStGR1) involved in chlorophyll catabolism. We tested the wild type and green cotyledon lines for components of adaptations to dry environments and traits linked to agronomic performance in different experimental systems and different levels of water availability. We found that the plant processes linked to disrupted CaStGR1 gene did not functionality affect transpiration efficiency or water usage. Photosynthetic pigments in grains, including provitaminogenic carotenoids important for human nutrition, were 2-3-fold higher in the stay-green type. Agronomic performance did not appear to be correlated with the presence/absence of the stay-green allele. We conclude that allelic variation in chickpea CaStGR1 does not compromise traits linked to environmental adaptation and agronomic performance, and is a promising genetic technology for biofortification of provitaminogenic carotenoids in chickpea.

The complete chloroplast genome of Primulina and two novel strategies for development of high polymorphic loci for population genetic and phylogenetic studies.

BACKGROUND: Primulina Hance is an emerging model for studying evolutionary divergence, adaptation and speciation of the karst flora. However, phylogenetic relationships within the genus have not been resolved due to low variation detected in the cpDNA regions. Chloroplast genomes can provide important information for phylogenetic and population genetic studies. Recent advances in next-generation sequencing (NGS) techniques greatly facilitate sequencing whole chloroplast genomes for multiple individuals. Consequently, novel strategies for development of highly polymorphic loci for population genetic and phylogenetic studies based on NGS data are needed. METHODS: For development of high polymorphic loci for population genetic and phylogenetic studies, two novel strategies are proposed here. The first protocol develops lineage-specific highly variable markers from the true high variation regions (Con_Seas) across whole cp genomes, instead of traditional noncoding regions. The pipeline has been integrated into a single perl script, and named "Con_Sea_Identification_and_PIC_Calculation". The second method assembles chloroplast fragments (poTs) and sub-super-marker (CpContigs) through our "SACRing" pipeline. This approach can fundamentally alter the strategies used in phylogenetic and population genetic studies based on cp markers, facilitating a transition from traditional Sanger sequencing to RAD-Seq. Both of these scripts are available at https://github.com/scbgfengchao/ . RESULTS: Three complete Primulina chloroplast genomes were assembled from genome survey data, and then two novel strategies were developed to yield highly polymorphic markers. For experimental evaluation of the first protocol, a set of Primulina species were used for PCR amplification. The results showed that these newly developed markers are more variable than traditional ones, and seem to be a better choice for phylogenetic and population studies in Primulin a. The second method was also successfully applied in population genetic studies of 21 individuals from three natural populations of Primulina. CONCLUSIONS: These two novel strategies may provide a pathway for similar research in other non-model species. The newly developed high polymorphic loci in this study will promote further the phylogenetic and population genetic studies in Primulina and other genera of the family Gesneriaceae.

Multiple post-domestication origins of kabuli chickpea through allelic variation in a diversification-associated transcription factor.

Chickpea (Cicer arietinum) is among the founder crops domesticated in the Fertile Crescent. One of two major forms of chickpea, the so-called kabuli type, has white flowers and light-colored seed coats, properties not known to exist in the wild progenitor. The origin of the kabuli form has been enigmatic. We genotyped a collection of wild and cultivated chickpea genotypes with 538 single nucleotide polymorphisms (SNPs) and examined patterns of molecular diversity relative to geographical sources and market types. In addition, we examined sequence and expression variation in candidate anthocyanin biosynthetic pathway genes. A reduction in genetic diversity and extensive genetic admixture distinguish cultivated chickpea from its wild progenitor species. Among germplasm, the kabuli form is polyphyletic. We identified a basic helix-loop-helix (bHLH) transcription factor at chickpea's B locus that conditions flower and seed colors, orthologous to Mendel's A gene of garden pea, whose loss of function is associated invariantly with the kabuli type of chickpea. From the polyphyletic distribution of the kabuli form in germplasm, an absence of nested variation within the bHLH gene and invariant association of loss of function of bHLH among the kabuli type, we conclude that the kabuli form arose multiple times during the phase of phenotypic diversification after initial domestication of cultivated chickpea.

The ecological genomic basis of salinity adaptation in Tunisian Medicago truncatula.

BACKGROUND: As our world becomes warmer, agriculture is increasingly impacted by rising soil salinity and understanding plant adaptation to salt stress can help enable effective crop breeding. Salt tolerance is a complex plant phenotype and we know little about the pathways utilized by naturally tolerant plants. Legumes are important species in agricultural and natural ecosystems, since they engage in symbiotic nitrogen-fixation, but are especially vulnerable to salinity stress. RESULTS: Our studies of the model legume Medicago truncatula in field and greenhouse settings demonstrate that Tunisian populations are locally adapted to saline soils at the metapopulation level and that saline origin genotypes are less impacted by salt than non-saline origin genotypes; these populations thus likely contain adaptively diverged alleles. Whole genome resequencing of 39 wild accessions reveals ongoing migration and candidate genomic regions that assort non-randomly with soil salinity. Consistent with natural selection acting at these sites, saline alleles are typically rare in the range-wide species' gene pool and are also typically derived relative to the sister species M. littoralis. Candidate regions for adaptation contain genes that regulate physiological acclimation to salt stress, such as abscisic acid and jasmonic acid signaling, including a novel salt-tolerance candidate orthologous to the uncharacterized gene AtCIPK21. Unexpectedly, these regions also contain biotic stress genes and flowering time pathway genes. We show that flowering time is differentiated between saline and non-saline populations and may allow salt stress escape. CONCLUSIONS: This work nominates multiple potential pathways of adaptation to naturally stressful environments in a model legume. These candidates point to the importance of both tolerance and avoidance in natural legume populations. We have uncovered several promising targets that could be used to breed for enhanced salt tolerance in crop legumes to enhance food security in an era of increasing soil salinization.

The Park Grass Experiment and next-generation approaches: local adaptation of sweet vernal grass revisited.

Long-term ecological experiments provide unique opportunities to observe the effects of natural selection. The Park Grass Experiment at Rothamsted Experiment Station in Hertfordshire, UK, is the longest running ecological experiment that incorporates fertilization treatments and has been ongoing since 1856. In the 1970s, local adaptation was observed in the grass Anthoxanthum odoratum to the elevated soil aluminium levels of the fertilized plots. Gould et al. (2014) have utilized this system to reevaluate the extent of local adaptation, first documented nearly 45 years ago (Snaydon), and to use emerging molecular approaches to identify candidate genes for the adaptation. From their work, they identify several plausible candidate loci for aluminium tolerance. This work shows the power of long-term field-based trials in a scientific age concentrated on rapidly emerging molecular techniques often utilized in short, narrowly focused laboratory or controlled environment experiments. The current study clearly illustrates the benefits gained by combining these molecular approaches within long-term monitoring experiments that can be regularly revisited in a changing world and used to address questions on evolutionary scales.

Back to the wilds: tapping evolutionary adaptations for resilient crops through systematic hybridization with crop wild relatives.

The genetic diversity of our crop plants has been substantially reduced during the process of domestication and breeding. This reduction in diversity necessarily constrains our ability to expand a crop's range of cultivation into environments that are more extreme than those in which it was domesticated, including into "sustainable" agricultural systems with reduced inputs of pesticides, water, and fertilizers. Conversely, the wild progenitors of crop plants typically possess high levels of genetic diversity, which underlie an expanded (relative to domesticates) range of adaptive traits that may be of agricultural relevance, including resistance to pests and pathogens, tolerance to abiotic extremes, and reduced dependence on inputs. Despite their clear potential for crop improvement, wild relatives have rarely been used systematically for crop improvement, and in no cases, have full sets of wild diversity been introgressed into a crop. Instead, most breeding efforts have focused on specific traits and dealt with wild species in a limited and typically ad hoc manner. Although expedient, this approach misses the opportunity to test a large suite of traits and deploy the full potential of crop wild relatives in breeding for the looming challenges of the 21st century. Here we review examples of hybridization in several species, both intentionally produced and naturally occurring, to illustrate the gains that are possible. We start with naturally occurring hybrids, and then examine a range of examples of hybridization in agricultural settings.

Alternative ground covers and strip-tilling in CBD hemp production.

OBJECTIVE: Little research has been done on managing soil health for large-scale, outdoor hemp production, contributing to the possible overuse of black plastic for weed suppression. Our experiment aimed to understand the performance of alternative ground covers including forage crops and hay as well as a less disruptive tilling method called strip-tilling compared to black plastic. RESULTS: Yield and soil health data were taken from three experimental plantings from two different outdoor CBD hemp farms in Vermont, USA. We find that hay may be a competitive alternative to black plastic in terms of producing heavier plants. Our research also found that clover seed and hay are both more cost-effective options than black plastic which may sway some farmers to adopt these alternative ground cover options.

Progress of Genomics-Driven Approaches for Sustaining Underutilized Legume Crops in the Post-Genomic Era.

Legume crops, belonging to the Fabaceae family, are of immense importance for sustaining global food security. Many legumes are profitable crops for smallholder farmers due to their unique ability to fix atmospheric nitrogen and their intrinsic ability to thrive on marginal land with minimum inputs and low cultivation costs. Recent progress in genomics shows promise for future genetic gains in major grain legumes. Still it remains limited in minor legumes/underutilized legumes, including adzuki bean, cluster bean, horse gram, lathyrus, red clover, urd bean, and winged bean. In the last decade, unprecedented progress in completing genome assemblies of various legume crops and resequencing efforts of large germplasm collections has helped to identify the underlying gene(s) for various traits of breeding importance for enhancing genetic gain and contributing to developing climate-resilient cultivars. This review discusses the progress of genomic resource development, including genome-wide molecular markers, key breakthroughs in genome sequencing, genetic linkage maps, and trait mapping for facilitating yield improvement in underutilized legumes. We focus on 1) the progress in genomic-assisted breeding, 2) the role of whole-genome resequencing, pangenomes for underpinning the novel genomic variants underlying trait gene(s), 3) how adaptive traits of wild underutilized legumes could be harnessed to develop climate-resilient cultivars, 4) the progress and status of functional genomics resources, deciphering the underlying trait candidate genes with putative function in underutilized legumes 5) and prospects of novel breeding technologies, such as speed breeding, genomic selection, and genome editing. We conclude the review by discussing the scope for genomic resources developed in underutilized legumes to enhance their production and play a critical role in achieving the "zero hunger" sustainable development goal by 2030 set by the United Nations.

Legume Pangenome: Status and Scope for Crop Improvement.

In the last decade, legume genomics research has seen a paradigm shift due to advances in genome sequencing technologies, assembly algorithms, and computational genomics that enabled the construction of high-quality reference genome assemblies of major legume crops. These advances have certainly facilitated the identification of novel genetic variants underlying the traits of agronomic importance in many legume crops. Furthermore, these robust sequencing technologies have allowed us to study structural variations across the whole genome in multiple individuals and at the species level using 'pangenome analysis.' This review updates the progress of constructing pangenome assemblies for various legume crops and discusses the prospects for these pangenomes and how to harness the information to improve various traits of economic importance through molecular breeding to increase genetic gain in legumes and tackle the increasing global food crisis.

Genetic diversity of Chamaecrista fasciculata (Fabaceae) from the USDA germplasm collection.

OBJECTIVE: Chamaecrista fasciculata is a widespread annual legume across Eastern North America, with potential as a restoration planting, biofuel crop, and genetic model for non-papillinoid legumes. As a non-Papilinoid, C. fasciculata, belongs to the Caesalpiniod group in which nodulation likely arose independently of the nodulation in Papilinoid and Mimosoid legumes. Thus, C. fasciculata is an attractive model system for legume evolution. In this study, we describe population structure and genetic diversity among 32 USDA germplasm accessions of C. fasciculata using 317 AFLP markers developed from 12 primer pairs, to assess where geographically there is the most genetic variation. RESULTS: We found that the C. fasciculata germplasm collection fall into four clusters with admixture among them. After correcting for outliers, our analysis shows two primary groups across Eastern and Central North America. To better understand the population biology of this species, further sampling of the full range of this widespread species is needed across North America, as well as the development of a larger set of markers providing denser coverage of the genome. Further sampling will help clarify geographical relationships in this widespread temperate species.

Tracking microhabitat temperature variation with iButton data loggers.

PREMISE OF THE STUDY: Fine-scale variation in temperature and soil moisture contribute to microhabitats across the landscape, affecting plant phenology, distribution, and fitness. The recent availability of compact and inexpensive temperature and humidity data loggers such as iButtons has facilitated research on microclimates. METHODS AND RESULTS: Here, we highlight the use of iButtons in three distinct settings: comparisons of empirical data to modeled climate data for rare rock ferns in the genus Asplenium in eastern North America; generation of fine-scale data to predict flowering time and vernalization responsiveness of crop wild relatives of chickpea from southeastern Anatolia; and measurements of extreme thermal variation of solar array installations in Vermont. DISCUSSION: We highlight a range of challenges with iButtons, including serious limitations of the Hygrochron function that affect their utility for measuring soil moisture, and methods for protecting them from the elements and from human interference. Finally, we provide MATLAB code to facilitate the processing of raw iButton data.

Ecology and genomics of an important crop wild relative as a prelude to agricultural innovation.

Domesticated species are impacted in unintended ways during domestication and breeding. Changes in the nature and intensity of selection impart genetic drift, reduce diversity, and increase the frequency of deleterious alleles. Such outcomes constrain our ability to expand the cultivation of crops into environments that differ from those under which domestication occurred. We address this need in chickpea, an important pulse legume, by harnessing the diversity of wild crop relatives. We document an extreme domestication-related genetic bottleneck and decipher the genetic history of wild populations. We provide evidence of ancestral adaptations for seed coat color crypsis, estimate the impact of environment on genetic structure and trait values, and demonstrate variation between wild and cultivated accessions for agronomic properties. A resource of genotyped, association mapping progeny functionally links the wild and cultivated gene pools and is an essential resource chickpea for improvement, while our methods inform collection of other wild crop progenitor species.