Next-generation sequencing of flow-sorted wheat chromosome 5D reveals lineage-specific translocations and widespread gene duplications

On the Unknown Proteins of Eukaryotic Proteomes

To study unknown proteins on a large scale, a reference system has been set up for the three better studied eukaryotic kingdoms, built with 36 proteomes as taxonomically diverse as possible. Proteins from 362 other eukaryotic proteomes with no known homologue in this set were then analyzed, focusing noteworthy on singletons, that is, on such proteins with no known homologue in their own proteome. Consistently, for a given species, no more than 12% of the singletons thus found are known at the protein level, according to Uniprot. In addition, since they rely on the information found in the alignment of homologous sequences, predictions of AlphaFold2 for their tridimensional structure are poor. In the case of metazoan species, the number of singletons rarely exceeds 1000 for the species the closest to the reference system (divergence times below 75 Myr). Interestingly, in the cases of viridiplantae and fungi, larger amounts of singletons are found for such species, as if the timescale on which singletons are added to proteomes were different in metazoa and in other eukaryotic kingdoms. In order to confirm this phenomenon, further studies of proteomes closer to those of the reference system are, however, needed.

Chromosome painting reveals inter-chromosomal rearrangements and evolution of subgenome D of wheat

Aegilops species represent the most important gene pool for breeding bread wheat (Triticum aestivum). Thus, understanding the genome evolution, including chromosomal structural rearrangements and syntenic relationships among Aegilops species or between Aegilops and wheat, is important for both basic genome research and practical breeding applications. In the present study, we attempted to develop subgenome D-specific fluorescence in situ hybridization (FISH) probes by selecting D-specific oligonucleotides based on the reference genome of Chinese Spring. The oligo-based chromosome painting probes consisted of approximately 26 000 oligos per chromosome and their specificity was confirmed in both diploid and polyploid species containing the D subgenome. Two previously reported translocations involving two D chromosomes have been confirmed in wheat varieties and their derived lines. We demonstrate that the oligo painting probes can be used not only to identify the translocations involving D subgenome chromosomes, but also to determine the precise positions of chromosomal breakpoints. Chromosome painting of 56 accessions of Ae. tauschii from different origins led us to identify two novel translocations: a reciprocal 3D-7D translocation in two accessions and a complex 4D-5D-7D translocation in one accession. Painting probes were also used to analyze chromosomes from more diverse Aegilops species. These probes produced FISH signals in four different genomes. Chromosome rearrangements were identified in Aegilops umbellulata, Aegilops markgrafii, and Aegilops uniaristata, thus providing syntenic information that will be valuable for the application of these wild species in wheat breeding.

Development of DNA Markers From Physically Mapped Loci in Aegilops comosa and Aegilops umbellulata Using Single-Gene FISH and Chromosome Sequences

Breeding of agricultural crops adapted to climate change and resistant to diseases and pests is hindered by a limited gene pool because of domestication and thousands of years of human selection. One way to increase genetic variation is chromosome-mediated gene transfer from wild relatives by cross hybridization. In the case of wheat (Triticum aestivum), the species of genus Aegilops are a particularly attractive source of new genes and alleles. However, during the evolution of the Aegilops and Triticum genera, diversification of the D-genome lineage resulted in the formation of diploid C, M, and U genomes of Aegilops. The extent of structural genome alterations, which accompanied their evolution and speciation, and the shortage of molecular tools to detect Aegilops chromatin hamper gene transfer into wheat. To investigate the chromosome structure and help develop molecular markers with a known physical position that could improve the efficiency of the selection of desired introgressions, we developed single-gene fluorescence in situ hybridization (FISH) maps for M- and U-genome progenitors, Aegilops comosa and Aegilops umbellulata, respectively. Forty-three ortholog genes were located on 47 loci in Ae. comosa and on 52 loci in Ae. umbellulata using wheat cDNA probes. The results obtained showed that M-genome chromosomes preserved collinearity with those of wheat, excluding 2 and 6M containing an intrachromosomal rearrangement and paracentric inversion of 6ML, respectively. While Ae. umbellulata chromosomes 1, 3, and 5U maintained collinearity with wheat, structural reorganizations in 2, 4, 6, and 7U suggested a similarity with the C genome of Aegilops markgrafii. To develop molecular markers with exact physical positions on chromosomes of Aegilops, the single-gene FISH data were validated in silico using DNA sequence assemblies from flow-sorted M- and U-genome chromosomes. The sequence similarity search of cDNA sequences confirmed 44 out of the 47 single-gene loci in Ae. comosa and 40 of the 52 map positions in Ae. umbellulata. Polymorphic regions, thus, identified enabled the development of molecular markers, which were PCR validated using wheat-Aegilops disomic chromosome addition lines. The single-gene FISH-based approach allowed the development of PCR markers specific for cytogenetically mapped positions on Aegilops chromosomes, substituting as yet unavailable segregating map. The new knowledge and resources will support the efforts for the introgression of Aegilops genes into wheat and their cloning.

Chromosome genomics uncovers plant genome organization and function

The identification of causal genomic loci and their interactions underlying various traits in plants has been greatly aided by progress in understanding the organization of the nuclear genome. This provides clues to the responses of plants to environmental stimuli at the molecular level. Apart from other uses, these insights are needed to fully explore the potential of new breeding techniques that rely on genome editing. However, genome analysis and sequencing is not straightforward in the many agricultural crops and their wild relatives that possess large and complex genomes. Chromosome genomics streamlines this task by dissecting the genome to single chromosomes whose DNA is then used instead of nuclear DNA. This results in a massive and lossless reduction in DNA sample complexity, reduces the time and cost of the experiment, and simplifies data interpretation. Flow cytometric sorting of condensed mitotic chromosomes makes it possible to purify single chromosomes in large quantities, and as the DNA remains intact this process can be coupled successfully with many techniques in molecular biology and genomics. Since the first experiments with flow cytometric sorting in the late 1980s, numerous applications have been developed, and chromosome genomics has been having a significant impact in many areas of research, including the sequencing of complex genomes of important crops and gene cloning. This review discusses these applications, describes their contribution to advancements in plant genome analysis and gene cloning, and outlines future directions.

Highly efficient synchronization of sheep skin fibroblasts at G2/M phase and isolation of sheep Y chromosomes by flow cytometric sorting

At present, based on whole genome sequencing, sequences and genes annotation of the sheep (Ovis aries) Y chromosome are still absent. The isolation of Y chromosomes followed by sequencing has been approved as an effective approach to analyze this complex chromosome in other species. In this study, we established a highly efficient synchronization method for G2/M phase of sheep fibroblasts, which was successfully applied to flow-sorting chromosomes of sheep, with a focus on isolation and sequencing of the ovine Y chromosome. The isolated (~80,000) Y chromosomes were verified by fluorescence quantitative real-time polymerase chain reaction, further confirmed by fluorescence in situ hybridization, and amplified by the MALBAC method before next-generation sequencing. The sequence results indicated that 68.90% of reads were Y chromosome-related sequences as they are homologous to the bovine Y chromosome. The remaining 31.1% of reads were aligned to the sheep reference genome, including 13.57% reads to chromosome X and 6.68% to chromosome 17. Importantly, the paired-end reads that are properly aligned to the bovine Y sequence assembly accounted for 46.49%, indicating the success in the ovine Y chromosome isolation and the high quality of the Y chromosome sequences. This study not only set up a foundation for future sequencing, assembly and annotation of the ovine Y chromosome, but also provide a validated approach to overcoming difficulties in sequencing Y chromosome in other mammalian species.

Impact of arbuscular mycorrhizal fungi (AMF) on gene expression of some cell wall and membrane elements of wheat (Triticum aestivum L.) under water deficit using transcriptome analysis

Mycorrhizal symbiotic relationship is one of the most common collaborations between plant roots and the arbuscular mycorrhizal fungi (AMF). The first barrier for establishing this symbiosis is plant cell wall which strongly provides protection against biotic and abiotic stresses. The aim of this study was to investigate the gene expression changes in cell wall of wheat root cv. Chamran after inoculation with AMF, Funneliformis mosseae under two different irrigation regimes. To carry out this investigation, total RNA was extracted from the roots of mycorrhizal and non-mycorrhizal plants, and analyzed using RNA-Seq in an Illumina Next-Seq 500 platform. The results showed that symbiotic association between wheat and AMF and irrigation not only affect transcription profile of the plant growth, but also cell wall and membrane components. Of the 114428 genes expressed in wheat roots, the most differentially expressed genes were related to symbiotic plants under water stress. The most differentially expressed genes were observed in carbohydrate metabolic process, lipid metabolic process, cellulose synthase activity, membrane transports, nitrogen compound metabolic process and chitinase activity related genes. Our results indicated alteration in cell wall and membrane composition due to mycorrhization and irrigation regimes might have a noteworthy effect on the plant tolerance to water deficit.

Chromosome transmission in BC4 progenies of intergeneric hybrids between Saccharum spp. and Erianthus arundinaceus (Retz.) Jeswiet

Intergeneric hybrids between Saccharum spp. and Erianthus arundinaceus and clones derived from these hybrids and backcrosses to Saccharum spp. were used to study the transmission of E. arundinaceus chromosomes by genomic in situ hybridization (GISH). True hybrid progenies were precisely identified using PCR with a primer pair, AGRP52/53. The results showed that AGRP52/53 was an E. arundinaceus-specific primer pair and could be used as molecular marker to assist breeding. EaHN92, a 364 bp E. arundinaceus-specific tandem repeat satellite DNA sequence, was cloned from the E. arundinaceus clone HN92-105 with AGRP52/53, and was localized on sub-telomeric regions of all E. arundinaceus chromosomes. YCE06-61, a BC₃ progeny, had 7 E. arundinaceus chromosomes and its progenies had approximately 1-6 E. arundinaceus chromosomes. The number of E. arundinaceus chromosomes in true hybrids appeared as Gaussian distribution in 3 cross combinations. In addition, GISH detected intergeneric chromosome translocation in a few progenies. Hence, screening clones containing approximately 1-2 E. arundinaceus chromosomes without translocation could be used for sorting and sequencing E. arundinaceus chromosomes. This study provides a method for breeders to select true hybrid progenies between Saccharum spp. and E. arundinaceus, which will accelerate this intergeneric hybridization breeding.

Chromosome-based survey sequencing reveals the genome organization of wild wheat progenitor Triticum dicoccoides

Wild emmer wheat (Triticum turgidum ssp. dicoccoides) is the progenitor of wheat. We performed chromosome-based survey sequencing of the 14 chromosomes, examining repetitive sequences, protein-coding genes, miRNA/target pairs and tRNA genes, as well as syntenic relationships with related grasses. We found considerable differences in the content and distribution of repetitive sequences between the A and B subgenomes. The gene contents of individual chromosomes varied widely, not necessarily correlating with chromosome size. We catalogued candidate agronomically important loci, along with new alleles and flanking sequences that can be used to design exome sequencing. Syntenic relationships and virtual gene orders revealed several small-scale evolutionary rearrangements, in addition to providing evidence for the 4AL-5AL-7BS translocation in wild emmer wheat. Chromosome-based sequence assemblies contained five novel miRNA families, among 59 families putatively encoded in the entire genome which provide insight into the domestication of wheat and an overview of the genome content and organization.

Complete chloroplast genome sequence and comparative analysis of loblolly pine (Pinus taeda L.) with related species

Pinaceae, the largest family of conifers, has a diversified organization of chloroplast (cp) genomes with two typical highly reduced inverted repeats (IRs). In the current study, we determined the complete sequence of the cp genome of an economically and ecologically important conifer tree, the loblolly pine (Pinus taeda L.), using Illumina paired-end sequencing and compared the sequence with those of other pine species. The results revealed a genome size of 121,531 base pairs (bp) containing a pair of 830-bp IR regions, distinguished by a small single copy (42,258 bp) and large single copy (77,614 bp) region. The chloroplast genome of P. taeda encodes 120 genes, comprising 81 protein-coding genes, four ribosomal RNA genes, and 35 tRNA genes, with 151 randomly distributed microsatellites. Approximately 6 palindromic, 34 forward, and 22 tandem repeats were found in the P. taeda cp genome. Whole cp genome comparison with those of other Pinus species exhibited an overall high degree of sequence similarity, with some divergence in intergenic spacers. Higher and lower numbers of indels and single-nucleotide polymorphism substitutions were observed relative to P. contorta and P. monophylla, respectively. Phylogenomic analyses based on the complete genome sequence revealed that 60 shared genes generated trees with the same topologies, and P. taeda was closely related to P. contorta in the subgenus Pinus. Thus, the complete P. taeda genome provided valuable resources for population and evolutionary studies of gymnosperms and can be used to identify related species.

High-throughput SNP genotyping of modern and wild emmer wheat for yield and root morphology using a combined association and linkage analysis

Durum wheat (Triticum turgidum var. durum Desf.) is a major world crop that is grown primarily in areas of the world that experience periodic drought, and therefore, breeding climate-resilient durum wheat is a priority. High-throughput single nucleotide polymorphism (SNP) genotyping techniques have greatly increased the power of linkage and association mapping analyses for bread wheat, but as yet there is no durum wheat-specific platform available. In this study, we evaluate the new 384HT Wheat Breeders Array for its usefulness in tetraploid wheat breeding by genotyping a breeding population of F₆ hybrids, derived from multiple crosses between T. durum cultivars and wild and cultivated emmer wheat accessions. Using a combined linkage and association mapping approach, we generated a genetic map including 1345 SNP markers, and identified markers linked to 6 QTLs for coleoptile length (2), heading date (1), anthocyanin accumulation (1) and osmotic stress tolerance (2). We also developed a straightforward approach for combining genetic data from multiple families of limited size that will be useful for evaluating and mapping pre-existing breeding material.

Contrasting evolutionary patterns of multiple loci uncover new aspects in the genome origin and evolutionary history of Leymus (Triticeae; Poaceae)

Leymus Hochst. (Triticeae: Poaceae), a group of allopolyploid species with the NsXm genomes, is a perennial genus with diversity in morphology, cytology, ecology, and distribution in the Triticeae. To investigate the genome origin and evolutionary history of Leymus, three unlinked low-copy nuclear genes (Acc1, Pgk1, and GBSSI) and three chloroplast regions (trnL-F, matK, and rbcL) of 32 Leymus species were analyzed with those of 36 diploid species representing 18 basic genomes in the Triticeae. The phylogenetic relationships were reconstructed using Bayesian inference, Maximum parsimony, and NeighborNet methods. A time-calibrated phylogeny was generated to estimate the evolutionary history of Leymus. The results suggest that reticulate evolution has occurred in Leymus species, with several distinct progenitors contributing to the Leymus. The molecular data in resolution of the Xm-genome lineage resulted in two apparently contradictory results, with one placing the Xm-genome lineage as closely related to the P/F genome and the other splitting the Xm-genome lineage as sister to the Ns-genome donor. Our results suggested that (1) the Ns genome of Leymus was donated by Psathyrostachys, and additional Ns-containing alleles may be introgressed into some Leymus polyploids by recurrent hybridization; (2) The phylogenetic incongruence regarding the resolution of the Xm-genome lineage suggested that the Xm genome of Leymus was closely related to the P genome of Agropyron; (3) Both Ns- and Xm-genome lineages served as the maternal donor during the speciation of Leymus species; (4) The Pseudoroegneria, Lophopyrum and Australopyrum genomes contributed to some Leymus species.

DNApod: DNA polymorphism annotation database from next-generation sequence read archives

With the rapid advances in next-generation sequencing (NGS), datasets for DNA polymorphisms among various species and strains have been produced, stored, and distributed. However, reliability varies among these datasets because the experimental and analytical conditions used differ among assays. Furthermore, such datasets have been frequently distributed from the websites of individual sequencing projects. It is desirable to integrate DNA polymorphism data into one database featuring uniform quality control that is distributed from a single platform at a single place. DNA polymorphism annotation database (DNApod; http://tga.nig.ac.jp/dnapod/) is an integrated database that stores genome-wide DNA polymorphism datasets acquired under uniform analytical conditions, and this includes uniformity in the quality of the raw data, the reference genome version, and evaluation algorithms. DNApod genotypic data are re-analyzed whole-genome shotgun datasets extracted from sequence read archives, and DNApod distributes genome-wide DNA polymorphism datasets and known-gene annotations for each DNA polymorphism. This new database was developed for storing genome-wide DNA polymorphism datasets of plants, with crops being the first priority. Here, we describe our analyzed data for 679, 404, and 66 strains of rice, maize, and sorghum, respectively. The analytical methods are available as a DNApod workflow in an NGS annotation system of the DNA Data Bank of Japan and a virtual machine image. Furthermore, DNApod provides tables of links of identifiers between DNApod genotypic data and public phenotypic data. To advance the sharing of organism knowledge, DNApod offers basic and ubiquitous functions for multiple alignment and phylogenetic tree construction by using orthologous gene information.

Transcriptome-based SNP discovery by GBS and the construction of a genetic map for olive

Molecular markers located in the genic regions of plants are valuable tools for the identification of candidate genes of economically important traits and consequent use in marker-assisted selection (MAS). In the past, simple sequence repeat markers (SSRs) and single-nucleotide polymorphisms (SNPs) located in expressed sequence tags (ESTs) were developed by sequencing RNA derived from different plant tissues, which involves laborious RNA extraction, mRNA isolation, and cDNA synthesis. In order to develop SNP markers located in olive transcriptomes, we used the recently developed genotyping-by-sequencing (GBS) technique. An analysis was done for 125 olive DNA samples (123 DNA samples from a cross-pollinated F₁ mapping population, and two samples from parents). From 45 to 66% of Illumina reads from GBS analysis were aligned to the olive transcriptome. A total of 22,033 transcriptome-based SNP markers were identified, and 3384 of these were mapped in the olive genome. The genetic linkage map constructed in this study consists of 1 cleaved amplified polymorphic sequence (CAPS), 19 SSR, and 3384 transcriptome-based SNP markers. The map covers 3340.8 cM of the olive genome in 23 linkage groups, with the length of the linkage groups ranging from 55.6 to 248.7 cM. Average map distance between flanking markers was 0.98 cM. This genetic linkage map is a saturated genetic map and will be a useful tool for the localization of quantitative trait loci (QTLs) and gene(s) of interest and for the identification of candidate genes for economically important traits.

Wheat Landrace Genome Diversity

Understanding the genomic complexity of bread wheat is important for unraveling domestication processes, environmental adaptation, and for future of...

Understanding the genomic complexity of bread wheat (Triticum aestivum L.) is a cornerstone in the quest to unravel the processes of domestication and the following adaptation of domesticated wheat to a wide variety of environments across the globe. Additionally, it is of importance for future improvement of the crop, particularly in the light of climate change. Focusing on the adaptation after domestication, a nested association mapping (NAM) panel of 60 segregating biparental populations was developed, mainly involving landrace accessions from the core set of the Watkins hexaploid wheat collection optimized for genetic diversity. A modern spring elite variety, “Paragon,” was used as common reference parent. Genetic maps were constructed following identical rules to make them comparable. In total, 1611 linkage groups were identified, based on recombination from an estimated 126,300 crossover events over the whole NAM panel. A consensus map, named landrace consensus map (LRC), was constructed and contained 2498 genetic loci. These newly developed genetics tools were used to investigate the rules underlying genome fluidity or rigidity, e.g., by comparing marker distances and marker orders. In general, marker order was highly correlated, which provides support for strong synteny between bread wheat accessions. However, many exceptional cases of incongruent linkage groups and increased marker distances were also found. Segregation distortion was detected for many markers, sometimes as hot spots present in different populations. Furthermore, evidence for translocations in at least 36 of the maps was found. These translocations fell, in general, into many different translocation classes, but a few translocation classes were found in several accessions, the most frequent one being the well-known T5B:7B translocation. Loci involved in recombination rate, which is an interesting trait for plant breeding, were identified by QTL analyses using the crossover counts as a trait. In total, 114 significant QTL were detected, nearly half of them with increasing effect from the nonreference parents.

A large-scale chromosome-specific SNP discovery guideline

Single-nucleotide polymorphisms (SNPs) are the most prevalent type of variation in genomes that are increasingly being used as molecular markers in diversity analyses, mapping and cloning of genes, and germplasm characterization. However, only a few studies reported large-scale SNP discovery in Aegilops tauschii, restricting their potential use as markers for the low-polymorphic D genome. Here, we report 68,592 SNPs found on the gene-related sequences of the 5D chromosome of Ae. tauschii genotype MvGB589 using genomic and transcriptomic sequences from seven Ae. tauschii accessions, including AL8/78, the only genotype for which a draft genome sequence is available at present. We also suggest a workflow to compare SNP positions in homologous regions on the 5D chromosome of Triticum aestivum, bread wheat, to mark single nucleotide variations between these closely related species. Overall, the identified SNPs define a density of 4.49 SNPs per kilobyte, among the highest reported for the genic regions of Ae. tauschii so far. To our knowledge, this study also presents the first chromosome-specific SNP catalog in Ae. tauschii that should facilitate the association of these SNPs with morphological traits on chromosome 5D to be ultimately targeted for wheat improvement.

Dissecting miRNAs in Wheat D Genome Progenitor, Aegilops tauschii

As the post-transcriptional regulators of gene expression, microRNAs or miRNAs comprise an integral part of understanding how genomes function. Although miRNAs have been a major focus of recent efforts, miRNA research is still in its infancy in most plant species. Aegilops tauschii, the D genome progenitor of bread wheat, is a wild diploid grass exhibiting remarkable population diversity. Due to the direct ancestry and the diverse gene pool, A. tauschii is a promising source for bread wheat improvement. In this study, a total of 87 Aegilops miRNA families, including 51 previously unknown, were computationally identified both at the subgenomic level, using flow-sorted A. tauschii 5D chromosome, and at the whole genome level. Predictions at the genomic and subgenomic levels suggested A. tauschii 5D chromosome as rich in pre-miRNAs that are highly associated with Class II DNA transposons. In order to gain insights into miRNA evolution, putative 5D chromosome miRNAs were compared to its modern ortholog, Triticum aestivum 5D chromosome, revealing that 48 of the 58 A. tauschii 5D miRNAs were conserved in orthologous T. aestivum 5D chromosome. The expression profiles of selected miRNAs (miR167, miR5205, miR5175, miR5523) provided the first experimental evidence for miR5175, miR5205 and miR5523, and revealed differential expressional changes in response to drought in different genetic backgrounds for miR167 and miR5175. Interestingly, while miR5523 coding regions were present and expressed as pre-miR5523 in both T. aestivum and A. tauschii, the expression of mature miR5523 was observed only in A. tauschii under normal conditions, pointing out to an interference at the downstream processing of pre-miR5523 in T. aestivum. Overall, this study expands our knowledge on the miRNA catalog of A. tauschii, locating a subset specifically to the 5D chromosome, with ample functional and comparative insight which should contribute to and complement efforts to develop drought tolerant wheat varieties.

Single-molecule real-time transcript sequencing facilitates common wheat genome annotation and grain transcriptome research

Background

The large and complex hexaploid genome has greatly hindered genomics studies of common wheat (Triticum aestivum, AABBDD). Here, we investigated transcripts in common wheat developing caryopses using the emerging single-molecule real-time (SMRT) sequencing technology PacBio RSII, and assessed the resultant data for improving common wheat genome annotation and grain transcriptome research.

Results

We obtained 197,709 full-length non-chimeric (FLNC) reads, 74.6 % of which were estimated to carry complete open reading frame. A total of 91,881 high-quality FLNC reads were identified and mapped to 16,188 chromosomal loci, corresponding to 13,162 known genes and 3026 new genes not annotated previously. Although some FLNC reads could not be unambiguously mapped to the current draft genome sequence, many of them are likely useful for studying highly similar homoeologous or paralogous loci or for improving chromosomal contig assembly in further research. The 91,881 high-quality FLNC reads represented 22,768 unique transcripts, 9591 of which were newly discovered. We found 180 transcripts each spanning two or three previously annotated adjacent loci, suggesting that they should be merged to form correct gene models. Finally, our data facilitated the identification of 6030 genes differentially regulated during caryopsis development, and full-length transcripts for 72 transcribed gluten gene members that are important for the end-use quality control of common wheat.

Conclusions

Our work demonstrated the value of PacBio transcript sequencing for improving common wheat genome annotation through uncovering the loci and full-length transcripts not discovered previously. The resource obtained may aid further structural genomics and grain transcriptome studies of common wheat.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2257-y) contains supplementary material, which is available to authorized users.

Exploring the tertiary gene pool of bread wheat: sequence assembly and analysis of chromosome 5M(g) of Aegilops geniculata

Next-generation sequencing (NGS) provides a powerful tool for the discovery of important genes and alleles in crop plants and their wild relatives. Despite great advances in NGS technologies, whole-genome shotgun sequencing is cost-prohibitive for species with complex genomes. An attractive option is to reduce genome complexity to a single chromosome prior to sequencing. This work describes a strategy for studying the genomes of distant wild relatives of wheat by isolating single chromosomes from addition or substitution lines, followed by chromosome sorting using flow cytometry and sequencing of chromosomal DNA by NGS technology. We flow-sorted chromosome 5M(g) from a wheat/Aegilops geniculata disomic substitution line [DS5M(g) (5D)] and sequenced it using an Illumina HiSeq 2000 system at approximately 50 × coverage. Paired-end sequences were assembled and used for structural and functional annotation. A total of 4236 genes were annotated on 5M(g) , in close agreement with the predicted number of genes on wheat chromosome 5D (4286). Single-gene FISH indicated no major chromosomal rearrangements between chromosomes 5M(g) and 5D. Comparing chromosome 5M(g) with model grass genomes identified synteny blocks in Brachypodium distachyon, rice (Oryza sativa), sorghum (Sorghum bicolor) and barley (Hordeum vulgare). Chromosome 5M(g) -specific SNPs and cytogenetic probe-based resources were developed and validated. Deletion bin-mapped and ordered 5M(g) SNP markers will be useful to track 5M-specific introgressions and translocations. This study provides a detailed sequence-based analysis of the composition of a chromosome from a distant wild relative of bread wheat, and opens up opportunities to develop genomic resources for wild germplasm to facilitate crop improvement.

What's in a genome? The C-value enigma and the evolution of eukaryotic genome content

Some notable exceptions aside, eukaryotic genomes are distinguished from those of Bacteria and Archaea in a number of ways, including chromosome structure and number, repetitive DNA content, and the presence of introns in protein-coding regions. One of the most notable differences between eukaryotic and prokaryotic genomes is in size. Unlike their prokaryotic counterparts, eukaryotes exhibit enormous (more than 60,000-fold) variability in genome size which is not explained by differences in gene number. Genome size is known to correlate with cell size and division rate, and by extension with numerous organism-level traits such as metabolism, developmental rate or body size. Less well described are the relationships between genome size and other properties of the genome, such as gene content, transposable element content, base pair composition and related features. The rapid expansion of 'complete' genome sequencing projects has, for the first time, made it possible to examine these relationships across a wide range of eukaryotes in order to shed new light on the causes and correlates of genome size diversity. This study presents the results of phylogenetically informed comparisons of genome data for more than 500 species of eukaryotes. Several relationships are described between genome size and other genomic parameters, and some recommendations are presented for how these insights can be extended even more broadly in the future.

Molecular organization and comparative analysis of chromosome 5B of the wild wheat ancestor Triticum dicoccoides

Wild emmer wheat, Triticum turgidum ssp. dicoccoides is the wild relative of Triticum turgidum, the progenitor of durum and bread wheat, and maintains a rich allelic diversity among its wild populations. The lack of adequate genetic and genomic resources, however, restricts its exploitation in wheat improvement. Here, we report next-generation sequencing of the flow-sorted chromosome 5B of T. dicoccoides to shed light into its genome structure, function and organization by exploring the repetitive elements, protein-encoding genes and putative microRNA and tRNA coding sequences. Comparative analyses with its counterparts in modern and wild wheats suggest clues into the B-genome evolution. Syntenic relationships of chromosome 5B with the model grasses can facilitate further efforts for fine-mapping of traits of interest. Mapping of 5B sequences onto the root transcriptomes of two additional T. dicoccoides genotypes, with contrasting drought tolerances, revealed several thousands of single nucleotide polymorphisms, of which 584 shared polymorphisms on 228 transcripts were specific to the drought-tolerant genotype. To our knowledge, this study presents the largest genomics resource currently available for T. dicoccoides, which, we believe, will encourage the exploitation of its genetic and genomic potential for wheat improvement to meet the increasing demand to feed the world.

The physical map of wheat chromosome 5DS revealed gene duplications and small rearrangements

BACKGROUND

The substantially large bread wheat genome, organized into highly similar three sub-genomes, renders genomic research challenging. The construction of BAC-based physical maps of individual chromosomes reduces the complexity of this allohexaploid genome, enables elucidation of gene space and evolutionary relationships, provides tools for map-based cloning, and serves as a framework for reference sequencing efforts. In this study, we constructed the first comprehensive physical map of wheat chromosome arm 5DS, thereby exploring its gene space organization and evolution.

RESULTS

The physical map of 5DS was comprised of 164 contigs, of which 45 were organized into 21 supercontigs, covering 176 Mb with an N50 value of 2,173 kb. Fifty-eight of the contigs were larger than 1 Mb, with the largest contig spanning 6,649 kb. A total of 1,864 molecular markers were assigned to the map at a density of 10.5 markers/Mb, anchoring 100 of the 120 contigs (>5 clones) that constitute ~95 % of the cumulative length of the map. Ordering of 80 contigs along the deletion bins of chromosome arm 5DS revealed small-scale breaks in syntenic blocks. Analysis of the gene space of 5DS suggested an increasing gradient of genes organized in islands towards the telomere, with the highest gene density of 5.17 genes/Mb in the 0.67-0.78 deletion bin, 1.4 to 1.6 times that of all other bins.

CONCLUSIONS

Here, we provide a chromosome-specific view into the organization and evolution of the D genome of bread wheat, in comparison to one of its ancestors, revealing recent genome rearrangements. The high-quality physical map constructed in this study paves the way for the assembly of a reference sequence, from which breeding efforts will greatly benefit.