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April 21, 2020  |  

Genetic map-guided genome assembly reveals a virulence-governing minichromosome in the lentil anthracnose pathogen Colletotrichum lentis.

Colletotrichum lentis causes anthracnose, which is a serious disease on lentil and can account for up to 70% crop loss. Two pathogenic races, 0 and 1, have been described in the C. lentis population from lentil. To unravel the genetic control of virulence, an isolate of the virulent race 0 was sequenced at 1481-fold genomic coverage. The 56.10-Mb genome assembly consists of 50 scaffolds with N50 scaffold length of 4.89 Mb. A total of 11 436 protein-coding gene models was predicted in the genome with 237 coding candidate effectors, 43 secondary metabolite biosynthetic enzymes and 229 carbohydrate-active enzymes (CAZymes), suggesting a contraction of the virulence gene repertoire in C. lentis. Scaffolds were assigned to 10 core and two minichromosomes using a population (race 0 × race 1, n = 94 progeny isolates) sequencing-based, high-density (14 312 single nucleotide polymorphisms) genetic map. Composite interval mapping revealed a single quantitative trait locus (QTL), qClVIR-11, located on minichromosome 11, explaining 85% of the variability in virulence of the C. lentis population. The QTL covers a physical distance of 0.84 Mb with 98 genes, including seven candidate effector and two secondary metabolite genes. Taken together, the study provides genetic and physical evidence for the existence of a minichromosome controlling the C. lentis virulence on lentil. © 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.

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April 21, 2020  |  

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.

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April 21, 2020  |  

A reference genome for pea provides insight into legume genome evolution.

We report the first annotated chromosome-level reference genome assembly for pea, Gregor Mendel’s original genetic model. Phylogenetics and paleogenomics show genomic rearrangements across legumes and suggest a major role for repetitive elements in pea genome evolution. Compared to other sequenced Leguminosae genomes, the pea genome shows intense gene dynamics, most likely associated with genome size expansion when the Fabeae diverged from its sister tribes. During Pisum evolution, translocation and transposition differentially occurred across lineages. This reference sequence will accelerate our understanding of the molecular basis of agronomically important traits and support crop improvement.

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April 21, 2020  |  

The role of genomic structural variation in the genetic improvement of polyploid crops

Many of our major crop species are polyploids, containing more than one genome or set of chromosomes. Polyploid crops present unique challenges, including difficulties in genome assembly, in discriminating between multiple gene and sequence copies, and in genetic mapping, hindering use of genomic data for genetics and breeding. Polyploid genomes may also be more prone to containing structural variation, such as loss of gene copies or sequences (presence–absence variation) and the presence of genes or sequences in multiple copies (copy-number variation). Although the two main types of genomic structural variation commonly identified are presence–absence variation and copy-number variation, we propose that homeologous exchanges constitute a third major form of genomic structural variation in polyploids. Homeologous exchanges involve the replacement of one genomic segment by a similar copy from another genome or ancestrally duplicated region, and are known to be extremely common in polyploids. Detecting all kinds of genomic structural variation is challenging, but recent advances such as optical mapping and long-read sequencing offer potential strategies to help identify structural variants even in complex polyploid genomes. All three major types of genomic structural variation (presence–absence, copy-number, and homeologous exchange) are now known to influence phenotypes in crop plants, with examples of flowering time, frost tolerance, and adaptive and agronomic traits. In this review, we summarize the challenges of genome analysis in polyploid crops, describe the various types of genomic structural variation and the genomics technologies and data that can be used to detect them, and collate information produced to date related to the impact of genomic structural variation on crop phenotypes. We highlight the importance of genomic structural variation for the future genetic improvement of polyploid crops.

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April 21, 2020  |  

A global survey of full-length transcriptome of Ginkgo biloba reveals transcript variants involved in flavonoid biosynthesis

Ginkgo biloba, which contains flavonoids as bioactive components, is widely used in traditional Chinese medicine. Increasing the flavonoid production of medicinal plants through genetic engineering generally focuses on the key genes involved in flavonoid biosynthesis. However, the molecular mechanisms underlying such biosynthesis are not yet well understood. To understand these mechanisms, a combination of second-generation sequencing (SGS) and single-molecule real-time (SMRT) sequencing was applied to G. biloba. Eight tissues were sampled for SMRT sequencing to generate a high-quality, full-length transcriptome database. From 23.36 Gb clean reads, 12,954 alternative polyadenylation events, 12,290 alternative splicing events, 929 fusion transcripts, 2,286 novel transcripts, and 1,270 lncRNAs were predicted by removing redundant reads. Further studies reveal that 7 AS, 5 lncRNA, and 6 fusion gene events were identified in flavonoid biosynthesis. A total of 12 gene modules were revealed to be involved in flavonoid metabolism structural genes and transcription factors by constructing co-expression networks. Weighted gene coexpression network analysis (WGCNA) analysis reveals that some hub genes operate during the biosynthesis by identifying transcription factors (TFs) and structure genes. Seven key hub genes were also identified by analyzing the correlation between gene expression level and flavonoids content. The results highlight the importance of SMRT sequencing of the full-length transcriptome in improving genome annotation and elucidating the gene regulation of flavonoid biosynthesis in G. biloba by providing a comprehensive set of reference transcripts.

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April 21, 2020  |  

A draft genome for Spatholobus suberectus.

Spatholobus suberectus Dunn (S. suberectus), which belongs to the Leguminosae, is an important medicinal plant in China. Owing to its long growth cycle and increased use in human medicine, wild resources of S. suberectus have decreased rapidly and may be on the verge of extinction. De novo assembly of the whole S. suberectus genome provides us a critical potential resource towards biosynthesis of the main bioactive components and seed development regulation mechanism of this plant. Utilizing several sequencing technologies such as Illumina HiSeq X Ten, single-molecule real-time sequencing, 10x Genomics, as well as new assembly techniques such as FALCON and chromatin interaction mapping (Hi-C), we assembled a chromosome-scale genome about 798?Mb in size. In total, 748?Mb (93.73%) of the contig sequences were anchored onto nine chromosomes with the longest scaffold being 103.57?Mb. Further annotation analyses predicted 31,634 protein-coding genes, of which 93.9% have been functionally annotated. All data generated in this study is available in public databases.

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