Analysis of a contiguous 211 kb sequence in diploid wheat (Triticum monococcum L.) reveals multiple mechanisms of genome evolution

Are cereal grasses a single genetic system?

In 1993, a passionate and provocative call to arms urged cereal researchers to consider the taxon they study as a single genetic system and collaborate with each other. Since then, that group of scientists has seen their discipline blossom. In an attempt to understand what unity of genetic systems means and how the notion was borne out by later research, we survey the progress and prospects of cereal genomics: sequence assemblies, population-scale sequencing, resistance gene cloning and domestication genetics. Gene order may not be as extraordinarily well conserved in the grasses as once thought. Still, several recurring themes have emerged. The same ancestral molecular pathways defining plant architecture have been co-opted in the evolution of different cereal crops. Such genetic convergence as much as cross-fertilization of ideas between cereal geneticists has led to a rich harvest of genes that, it is hoped, will lead to improved varieties.

Repeat-based phylogenomics shed light on unclear relationships in the monocentric genus Juncus L. (Juncaceae)

The repetitive fraction (repeatome) of eukaryotic genomes is diverse and usually fast evolving, being an important tool for clarify plant systematics. The genus Juncus L. comprises 332 species, karyotypically recognized by having holocentric chromosomes. However, four species were recently described as monocentric, yet our understanding of their genome evolution is largely masked by unclear phylogenetic relationships. Here, we reassess the current Juncus systematics using low-coverage genome skimming data of 33 taxa to construct repeats, nuclear rDNA and plastome-based phylogenetic hypothesis. Furthermore, we characterize the repeatome and chromosomal distribution of Juncus-specific centromeric repeats/CENH3 protein to test the monocentricity reach in the genus. Repeat-base phylogenies revealed topologies congruent with the rDNA tree, but not with the plastome tree. The incongruence between nuclear and plastome chloroplast dataset suggest an ancient hybridization in the divergence of Juncotypus and Tenageia sections 40 Myr ago. The phylogenetic resolution at section level was better fitted with the rDNA/repeat-based approaches, with the recognition of two monophyletic sections (Stygiopsis and Tenageia). We found specific repeatome trends for the main lineages, such as the higher abundances of TEs in the Caespitosi and Iridifolii + Ozophyllum clades. CENH3 immunostaining confirmed the monocentricity of Juncus, which can be a generic synapomorphy for the genus. The heterogeneity of the repeatomes, with high phylogenetic informativeness, identified here may be correlated with their ancient origin (56 Mya) and reveals the potential of comparative genomic analyses for understanding plant systematics and evolution.

Ancient wheats: beneficial effects on insulin resistance

Non-alcoholic fatty liver disease and type 2 diabetes mellitus are two conditions that commonly exist together in the context of the metabolic syndrome. Several scientific advances in understanding this association have identified insulin resistance as the key point in the pathogenesis of both diseases. The first line treatment suggested in the management of these diseases is represented by lifestyle changes and in particular the modification of alimentary regimen, with the transition to a healthy diet. In this context, several studies have focused their attention on the identification of food products with beneficial actions, like ancient wheat (AW). AW are defined as the early cereals that were domesticated in their places of origin in the "Fertile Crescent" of the Middle East, and played a central role as a main source of food for the early civilizations in that region. The present narrative review aims to provide a systematic overview of the state of the art on the effects of AW on insulin resistance.

Transposable Elements: Classification, Identification, and Their Use As a Tool For Comparative Genomics

Most genomes are populated by hundreds of thousands of sequences originated from mobile elements. On the one hand, these sequences present a real challenge in the process of genome analysis and annotation. On the other hand, they are very interesting biological subjects involved in many cellular processes. Here we present an overview of transposable elements biodiversity, and we discuss different approaches to transposable elements detection and analyses.

Independent Domestication of Two Old World Cotton Species

Domesticated cotton species provide raw material for the majority of the world's textile industry. Two independent domestication events have been identified in allopolyploid cotton, one in Upland cotton (Gossypium hirsutum L.) and the other to Egyptian cotton (Gossypium barbadense L.). However, two diploid cotton species, Gossypium arboreum L. and Gossypium herbaceum L., have been cultivated for several millennia, but their status as independent domesticates has long been in question. Using genome resequencing data, we estimated the global abundance of various repetitive DNAs. We demonstrate that, despite negligible divergence in genome size, the two domesticated diploid cotton species contain different, but compensatory, repeat content and have thus experienced cryptic alterations in repeat abundance despite equivalence in genome size. Evidence of independent origin is bolstered by estimates of divergence times based on molecular evolutionary analysis of f7,000 orthologous genes, for which synonymous substitution rates suggest that G. arboreum and G. herbaceum last shared a common ancestor approximately 0.4-2.5 Ma. These data are incompatible with a shared domestication history during the emergence of agriculture and lead to the conclusion that G. arboreum and G. herbaceum were each domesticated independently.

Repetitive DNA and Plant Domestication: Variation in Copy Number and Proximity to Genes of LTR-Retrotransposons among Wild and Cultivated Sunflower (Helianthus annuus) Genotypes

The sunflower (Helianthus annuus) genome contains a very large proportion of transposable elements, especially long terminal repeat retrotransposons. However, knowledge on the retrotransposon-related variability within this species is still limited. We used next-generation sequencing (NGS) technologies to perform a quantitative and qualitative survey of intraspecific variation of the retrotransposon fraction of the genome across 15 genotypes—7 wild accessions and 8 cultivars—of H. annuus. By mapping the Illumina reads of the 15 genotypes onto a library of sunflower long terminal repeat retrotransposons, we observed considerable variability in redundancy among genotypes, at both superfamily and family levels. In another analysis, we mapped Illumina paired reads to two sets of sequences, that is, long terminal repeat retrotransposons and protein-encoding sequences, and evaluated the extent of retrotransposon proximity to genes in the sunflower genome by counting the number of paired reads in which one read mapped to a retrotransposon and the other to a gene. Large variability among genotypes was also ascertained for retrotransposon proximity to genes. Both long terminal repeat retrotransposon redundancy and proximity to genes varied among retrotransposon families and also between cultivated and wild genotypes. Such differences are discussed in relation to the possible role of long terminal repeat retrotransposons in the domestication of sunflower.

Anther and pollen development: A conserved developmental pathway

Pollen development is a critical step in plant development that is needed for successful breeding and seed formation. Manipulation of male fertility has proved a useful trait for hybrid breeding and increased crop yield. However, although there is a good understanding developing of the molecular mechanisms of anther and pollen anther development in model species, such as Arabidopsis and rice, little is known about the equivalent processes in important crops. Nevertheless the onset of increased genomic information and genetic tools is facilitating translation of information from the models to crops, such as barley and wheat; this will enable increased understanding and manipulation of these pathways for agricultural improvement.

Asymmetric epigenetic modification and elimination of rDNA sequences by polyploidization in wheat

rRNA genes consist of long tandem repeats clustered on chromosomes, and their products are important functional components of the ribosome. In common wheat (Triticum aestivum), rDNA loci from the A and D genomes were largely lost during the evolutionary process. This biased DNA elimination may be related to asymmetric transcription and epigenetic modifications caused by the polyploid formation. Here, we observed both sets of parental nucleolus organizing regions (NORs) were expressed after hybridization, but asymmetric silencing of one parental NOR was immediately induced by chromosome doubling, and reversing the ploidy status could not reactivate silenced NORs. Furthermore, increased CHG and CHH DNA methylation on promoters was accompanied by asymmetric silencing of NORs. Enrichment of H3K27me3 and H3K9me2 modifications was also observed to be a direct response to increased DNA methylation and transcriptional inactivation of NOR loci. Both A and D genome NOR loci with these modifications started to disappear in the S4 generation and were completely eliminated by the S7 generation in synthetic tetraploid wheat. Our results indicated that asymmetric epigenetic modification and elimination of rDNA sequences between different donor genomes may lead to stable allopolyploid wheat with increased differentiation and diversity.

Major repeat components covering one-third of the ginseng (Panax ginseng C.A. Meyer) genome and evidence for allotetraploidy

Ginseng (Panax ginseng) is a famous medicinal herb, but the composition and structure of its genome are largely unknown. Here we characterized the major repeat components and inspected their distribution in the ginseng genome. By analyzing three repeat-rich bacterial artificial chromosome (BAC) sequences from ginseng, we identified complex insertion patterns of 34 long terminal repeat retrotransposons (LTR-RTs) and 11 LTR-RT derivatives accounting for more than 80% of the BAC sequences. The LTR-RTs were classified into three Ty3/gypsy (PgDel, PgTat and PgAthila) and two Ty1/Copia (PgTork and PgOryco) families. Mapping of 30-Gbp Illumina whole-genome shotgun reads to the BAC sequences revealed that these five LTR-RT families occupy at least 34% of the ginseng genome. The Ty3/Gypsy families were predominant, comprising 74 and 33% of the BAC sequences and the genome, respectively. In particular, the PgDel family accounted for 29% of the genome and presumably played major roles in enlargement of the size of the ginseng genome. Fluorescence in situ hybridization (FISH) revealed that the PgDel1 elements are distributed throughout the chromosomes along dispersed heterochromatic regions except for ribosomal DNA blocks. The intensity of the PgDel2 FISH signals was biased toward 24 out of 48 chromosomes. Unique gene probes showed two pairs of signals with different locations, one pair in subtelomeric regions on PgDel2-rich chromosomes and the other in interstitial regions on PgDel2-poor chromosomes, demonstrating allotetraploidy in ginseng. Our findings promote understanding of the evolution of the ginseng genome and of that of related species in the Araliaceae.

A physical map of the short arm of wheat chromosome 1A

Bread wheat (Triticum aestivum) has a large and highly repetitive genome which poses major technical challenges for its study. To aid map-based cloning and future genome sequencing projects, we constructed a BAC-based physical map of the short arm of wheat chromosome 1A (1AS). From the assembly of 25,918 high information content (HICF) fingerprints from a 1AS-specific BAC library, 715 physical contigs were produced that cover almost 99% of the estimated size of the chromosome arm. The 3,414 BAC clones constituting the minimum tiling path were end-sequenced. Using a gene microarray containing ∼40 K NCBI UniGene EST clusters, PCR marker screening and BAC end sequences, we arranged 160 physical contigs (97 Mb or 35.3% of the chromosome arm) in a virtual order based on synteny with Brachypodium, rice and sorghum. BAC end sequences and information from microarray hybridisation was used to anchor 3.8 Mbp of Illumina sequences from flow-sorted chromosome 1AS to BAC contigs. Comparison of genetic and synteny-based physical maps indicated that ∼50% of all genetic recombination is confined to 14% of the physical length of the chromosome arm in the distal region. The 1AS physical map provides a framework for future genetic mapping projects as well as the basis for complete sequencing of chromosome arm 1AS.

Divergent evolutionary mechanisms of co-located Tak/Lrk and Glu-D3 loci revealed by comparative analysis of grass genomes

Seed storage and disease resistance proteins are major traits of wheat. The study of their gene organization and evolution has some implications in breeding. In this study, we characterized the hexaploid wheat D-genome BAC clone TaBAC703A9 that contains a low molecular weight glutenin locus (Glu-D3) and a resistance gene analogue cluster. With a gene density of one gene per 4.8 kb, the cluster contains four resistance gene analogues, namely Tak703-1, Lrr703, Tak703, and Lrk703. This structural cluster unit was conserved across nine grass genomes, but divergent evolutionary mechanisms have been involved in shaping the Tak/Lrk loci in the different species. Gene duplication was the major force for the Tak/Lrk evolution in oats, maize, barley, wheat, sorghum, and Brachypodium, while tandem duplication drove the expansion of this locus in japonica rice. Despite the close proximity of the Glu-D3 and the Tak/Lrk loci in wheat, the evolutionary mechanisms that drove their amplification differ. The Glu-D3 region had a lower gene density, and its amplification was driven by retroelements.

Genome-wide analysis of Stowaway-like MITEs in wheat reveals high sequence conservation, gene association, and genomic diversification

The diversity and evolution of wheat (Triticum-Aegilops group) genomes is determined, in part, by the activity of transposable elements that constitute a large fraction of the genome (up to 90%). In this study, we retrieved sequences from publicly available wheat databases, including a 454-pyrosequencing database, and analyzed 18,217 insertions of 18 Stowaway-like miniature inverted-repeat transposable element (MITE) families previously characterized in wheat that together account for approximately 1.3 Mb of sequence. All 18 families showed high conservation in length, sequence, and target site preference. Furthermore, approximately 55% of the elements were inserted in transcribed regions, into or near known wheat genes. Notably, we observed significant correlation between the mean length of the MITEs and their copy number. In addition, the genomic composition of nine MITE families was studied by real-time quantitative polymerase chain reaction analysis in 40 accessions of Triticum spp. and Aegilops spp., including diploids, tetraploids, and hexaploids. The quantitative polymerase chain reaction data showed massive and significant intraspecific and interspecific variation as well as genome-specific proliferation and nonadditive quantities in the polyploids. We also observed significant differences in the methylation status of the insertion sites among MITE families. Our data thus suggest a possible role for MITEs in generating genome diversification and in the establishment of nascent polyploid species in wheat.

Mutator System Derivatives Isolated from Sugarcane Genome Sequence

Mutator-like transposase is the most represented transposon transcript in the sugarcane transcriptome. Phylogenetic reconstructions derived from sequenced transcripts provided evidence that at least four distinct classes exist (I-IV) and that diversification among these classes occurred early in Angiosperms, prior to the divergence of Monocots/Eudicots. The four previously described classes served as probes to select and further sequence six BAC clones from a genomic library of cultivar R570. A total of 579,352 sugarcane base pairs were produced from these "Mutator system" BAC containing regions for further characterization. The analyzed genomic regions confirmed that the predicted structure and organization of the Mutator system in sugarcane is composed of two true transposon lineages, each containing a specific terminal inverted repeat and two transposase lineages considered to be domesticated. Each Mutator transposase class displayed a particular molecular structure supporting lineage specific evolution. MUSTANG, previously described domesticated genes, are located in syntenic regions across Sacharineae and, as expected for a host functional gene, posses the same gene structure as in other Poaceae. Two sequenced BACs correspond to hom(eo)logous locus with specific retrotransposon insertions that discriminate sugarcane haplotypes. The comparative studies presented, add information to the Mutator systems previously identified in the maize and rice genomes by describing lineage specific molecular structure and genomic distribution pattern in the sugarcane genome. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s12042-012-9104-y) contains supplementary material, which is available to authorized users.

Characterization of SBEIIa homoeologous genes in bread wheat

To elucidate some of the molecular mechanisms involved in genome differentiation and evolution of cultivated wheats, we compared orthologous genes encoding starch branching enzyme IIa (SBEIIa). Bread wheat is an allohexaploid species comprising the three genomes A, B and D, each of which contributes a copy of the SBEIIa gene, involved in starch biosynthesis and known to control important quality traits related to technological and nutritional value of wheat-based food products. Alignment of the nucleotide sequences of these three genes revealed variation, both at the level of single nucleotides and indels. Multiple transposon elements were identified in the intragenic regions, some of which appear to have inserted before the divergence of the wheat diploid genomes. The B genome homoeologue was the most divergent of the three genes. Two MITE transposon insertions were detected within the intronic sequence of SBEIIa-B and two other transposons within SBEIIa-D. The presence/absence of these transposons in a panel of diploid and polyploid Triticum and Aegilops species provided some insights into the phylogeny of wheat.

Rye chromosome-specific polymerase chain reaction products developed by primers designed from the EcoO109I recognition site

From our analysis of repeat sequences in the rye genome, the presence of multiple restriction sites of EcoO109I (5'-PuGGNCCPy-3') across the genome has been predicted. By first using primers designed to contain EcoO109I sites in polymerase chain reaction (PCR), polymorphic DNA markers were effectively obtained. A total of 43 types of 10-mer primers containing EcoO109I sites were applied for PCR by using genomic DNA of Secale cereale self-fertile line IR27 and Triticum aestivum 'Chinese Spring' (CS) as the template. Twenty two primers detected polymorphisms between wheat and rye, and they were applied for PCR using a series of CS wheat--'Imperial' rye chromosome addition lines as templates. Nine chromosome-specific amplification fragments identified on five chromosomes were collected from gels and hybridized with nylon membrane-transferred PCR products from the wheat-rye chromosome addition lines. The gel blot was only observed between the collected fragments; therefore, these fragments were confirmed to be chromosome-specific. These fragments were sequenced and converted to sequence-tagged site (STS) primers. We therefore introduce a new method for building chromosome-specific DNA markers: (i) multiple polymorphic fragments can be obtained from EcoO109I primers and (ii) the addition of three nucleotides to the EcoO109I site restricts the amplification region to generate chromosome-specific fragments.

Marker utility of miniature inverted-repeat transposable elements for wheat biodiversity and evolution

Transposable elements (TEs) account for up to 80% of the wheat genome and are considered one of the main drivers of wheat genome evolution. However, the contribution of TEs to the divergence and evolution of wheat genomes is not fully understood. In this study, we have developed 55 miniature inverted-repeat transposable element (MITE) markers that are based on the presence/absence of an element, with over 60% of these 55 MITE insertions associated with wheat genes. We then applied these markers to assess genetic diversity among Triticum and Aegilops species, including diploid (AA, BB and DD genomes), tetraploid (BBAA genome) and hexaploid (BBAADD genome) species. While 18.2% of the MITE markers showed similar insertions in all species indicating that those are fossil insertions, 81.8% of the markers showed polymorphic insertions among species, subspecies, and accessions. Furthermore, a phylogenetic analysis based on MITE markers revealed that species were clustered based on genus, genome composition, and ploidy level, while 47.13% genetic divergence was observed between the two main clusters, diploids versus polyploids. In addition, we provide evidence for MITE dynamics in wild emmer populations. The use of MITEs as evolutionary markers might shed more light on the origin of the B-genome of polyploid wheat.

Genome change in wheat observed through the structure and expression of α/β-gliadin genes

To better understand genome structure and the expression of α/β-gliadin multigenes in hexaploid wheat, bacterial artificial chromosome (BAC) clones containing α/β-gliadin genes from the three loci, Gli-A2, Gli-B2, and Gli-D2, were screened. Based on their restriction fragment patterns, we selected five BAC clones, namely, two clones for Gli-A2, two clones for Gli-B2, and one clone for Gli-D2, to fully sequence. Approximately 200 kb was sequenced for each locus. In total, twelve α/β-gliadin intact genes and four pseudogenes were found, and retrotransposons or other transposons existed in each BAC clone. Dot-plot analysis revealed the pattern of genome segmental duplication within each BAC. We calculated time since duplication of each set of α/β-gliadin genes and insertion of retrotransposons. Duplication of all adjacent genes within the same BAC clone took place before or after allotetrapolyploidization, but duplication of certain genes occurred before diploid differentiation of wheat species. Retrotransposons were also inserted before and after the segmental duplication events. Furthermore, translocation of α/β-gliadin genes from chromosomes 1 to 6 apparently occurred before the diversification of various wheat genomes. Duplication of genome segments containing α/β-gliadin genes and retrotransposons were brought about through unequal crossing-over or saltatory replication and α/β-gliadin genes per se were duplicated without any recombination events. Out of twelve intact α/β-gliadin genes detected from their sequences, nine were expressed, although their patterns of expression were distinct. Since they have similar cis-elements and promoter structures, the mechanisms underlying their distinct gene expression and possible applications are discussed.

Transposable elements and their identification

Most genomes are populated by thousands of sequences that originated from mobile elements. On the one hand, these sequences present a real challenge in the process of genome analysis and annotation. On the other hand, there are very interesting biological subjects involved in many cellular processes. Here, we present an overview of transposable elements (TEs) biodiversity and their impact on genomic evolution. Finally, we discuss different approaches to the TEs detection and analyses.

Recent insertion of a 52-kb mitochondrial DNA segment in the wheat lineage

The assembly of a 1.3-Mb size region of the wheat genome has provided the opportunity to study a recent nuclear mitochondrial DNA insertion (NUMT). In the present study, we have studied two bacterial artificial chromosomes (BACs) and characterized a 52-kb NUMT segment from the tetraploid and hexaploid wheat BAC libraries. The conserved orthologous NUMT regions from tetraploid and hexaploid wheat Langdon and Chinese Spring shared identical gene haplotypes even though mutations (insertions, deletions, and substitutions) had occurred. The 52-kb NUMT was present in hexaploid variety Chinese Spring, but absent in variety Hope, by sequence comparison of their corresponding region. Amplifying the NUMT junctions using a set of the wheat materials including diploid, tetraploid, and hexaploid lines showed that none of the diploid wheat carried the region and only some tetraploid and hexaploid wheat were positive for the NUMT. Age estimation of the NUMT displayed the mean ages of Langdon NUMT and Chinese Spring NUMT to be 378,000 and 416,000 years ago, respectively. Reverse transcription PCR and sequencing of the nad7 gene showed 28 C → U RNA editing sites and four partial editing sites, as expected for mitochondrial DNA expression. Specific SNPs discriminated between cDNA from the nucleus and the mitochondria and suggested that the nuclear copy was not expressed. The mitochondrial DNA studied was inserted into the genome quite recently within the wheat lineage and gave rise to the non-coding nuclear nad7 gene. The NUMT segment could be lost and acquired frequently during the wheat evolution.

Revolver and superior: novel transposon-like gene families of the plant kingdom

High-throughput sequencing of eukaryotic genomes has revived interest in the structure and function of repetitive genomic sequences, previously referred to as junk DNA. Repetitive sequences, including transposable elements, are now believed to play a significant role in genomic differentiation and evolution. Some are also expressed as regulatory noncoding RNAs. Vast DNA databases exist for higher eukaryotes; however, with the exception of homologues of known repetitive-sequence-families and transposable elements, most repetitive elements still need to be annotated. Revolver and Superior, both discovered in the Triticeae, are novel classes of transposon-like genes and major components of large cereal genomes. Revolver was isolated from rye via genome subtraction of sequences common to rye and wheat. Superior was isolated from rye by cleavage with EcoO109I, the recognition sites of which consist of a 5'- PuGGNCCPy-3' multi-sequence. Revolver is 2929-3041 bp long with an inverted repeat sequence on each end. The Superior family elements are 1292-1432 bp in length, with divergent 5' regions, indicating the presence of considerable structural diversity. Revolver and Superior are transcriptionally active elements; Revolver harbors a single gene consisting of three exons and two introns, encoding a protein of 139 amino acid residues. Revolver variants range in size from 2665 bp to 4269 bp, with some variants lacking the 5' region, indicating structural diversity around the first exon. Revolver and Superior are dispersed across all seven chromosomes of rye. Revolver has existed since the diploid progenitor of wheat, and has been amplified or lost in several species during the evolution of the Triticeae. This article reviews the recently discovered Revolver and Superior families of plant transposons, which do not share identity with any known autonomous transposable elements or repetitive elements from any living species.

Chromodomains and LTR retrotransposons in plants

A chromodomain is a domain contained in various proteins involved in chromatin remodeling and the regulation of gene expression in eukaryotes during development. Chromodomains perform a wide range of diverse functions including chromatin targeting and interactions between different proteins, RNA and DNA. The chromodomains also have been found as an additional domain at the C-terminal region of Polyproteins (Pol) encoded by transposable elements, which belong to the Gypsy LTR retrotransposons superfamily. Chromoviruses or chromodomain-containing Gypsy LTR retrotransposons form the most widespread clade of Gypsy LTR retrotransposons and can be found in diverse eukaryotes including plants, fungi and vertebrates. The recent finding suggested that chromodomains can be responsible for the targeted integration of LTR retrotransposons and, thus, should be favorable for mobile elements by allowing them to avoid negative selection arising from insertion into coding regions.

Gene space dynamics during the evolution of Aegilops tauschii, Brachypodium distachyon, Oryza sativa, and Sorghum bicolor genomes

Nine different regions totaling 9.7 Mb of the 4.02 Gb Aegilops tauschii genome were sequenced using the Sanger sequencing technology and compared with orthologous Brachypodium distachyon, Oryza sativa (rice), and Sorghum bicolor (sorghum) genomic sequences. The ancestral gene content in these regions was inferred and used to estimate gene deletion and gene duplication rates along each branch of the phylogenetic tree relating the four species. The total gene number in the extant Ae. tauschii genome was estimated to be 36,371. The gene deletion and gene duplication rates and total gene numbers in the four genomes were used to estimate the total gene number in each node of the phylogenetic tree. The common ancestor of the Brachypodieae and Triticeae lineages was estimated to have had 28,558 genes, and the common ancestor of the Panicoideae, Ehrhartoideae, and Pooideae subfamilies was estimated to have had 27,152 or 28,350 genes, depending on the ancestral gene scenario. Relative to the Brachypodieae and Triticeae common ancestor, the gene number was reduced in B. distachyon by 3,026 genes and increased in Ae. tauschii by 7,813 genes. The sum of gene deletion and gene duplication rates, which reflects the rate of gene synteny loss, was correlated with the rate of structural chromosome rearrangements and was highest in the Ae. tauschii lineage and lowest in the rice lineage. The high rate of gene space evolution in the Ae. tauschii lineage accounts for the fact that, contrary to the expectations, the level of synteny between the phylogenetically more related Ae. tauschii and B. distachyon genomes is similar to the level of synteny between the Ae. tauschii genome and the genomes of the less related rice and sorghum. The ratio of gene duplication to gene deletion rates in these four grass species closely parallels both the total number of genes in a species and the overall genome size. Because the overall genome size is to a large extent a function of the repeated sequence content in a genome, we suggest that the amount and activity of repeated sequences are important factors determining the number of genes in a genome.

Assembling a puzzle of dispersed retrotransposable sequences in the genome of chickpea (Cicer arietinum L.)

Several repetitive elements are known to be present in the genome of chickpea (Cicer arietinum L.) including satellite DNA and En/Spm transposons as well as two dispersed, highly repetitive elements, CaRep1 and CaRep2. PCR was used to prove that CaRep1, CaRep2, and previously isolated CaRep3 of C. arietinum represent different segments of a highly repetitive Ty3-gypsy-like retrotransposon (Metaviridae) designated CaRep that makes up large parts of the intercalary heterochromatin. The full sequence of this element including the LTRs and untranslated internal regions was isolated by selective amplification. The restriction pattern of CaRep was different within the annual species of the genus Cicer, suggesting its rearrangement during the evolution of the genus during the last 100 000 years. In addition to CaRep, another LTR and a non-LTR retrotransposon family were isolated, and their restriction patterns and physical localization in the chickpea genome were characterized. The LINE-like element CaLin is only of comparatively low abundance and reveals a considerable heterogeneity. The Ty1-copia-like element (Pseudoviridae) CaTy is located in the distal parts of the intercalary heterochromatin and adjacent euchromatic regions, but it is absent from the centromeric regions. These results together with earlier findings allow to depict the distribution of retroelements on chickpea chromosomes, which extensively resembles the retroelement landscape of the genome of the model legume Medicago truncatula Gaertn.

Diverse retrotransposon families and an AT-rich satellite DNA revealed in giant genomes of Fritillaria lilies

BACKGROUND AND AIMS

The genus Fritillaria (Liliaceae) comprises species with extremely large genomes (1C = 30 000-127 000 Mb) and a bicontinental distribution. Most North American species (subgenus Liliorhiza) differ from Eurasian Fritillaria species by their distinct phylogenetic position and increased amounts of heterochromatin. This study examined the contribution of major repetitive elements to the genome obesity found in Fritillaria and identified repeats contributing to the heterochromatin arrays in Liliorhiza species.

METHODS

Two Fritillaria species of similar genome size were selected for detailed analysis, one from each phylogeographical clade: F. affinis (1C = 45·6 pg, North America) and F. imperialis (1C = 43·0 pg, Eurasia). Fosmid libraries were constructed from their genomic DNAs and used for identification, sequence characterization, quantification and chromosome localization of clones containing highly repeated sequences.

KEY RESULTS AND CONCLUSIONS

Repeats corresponding to 6·7 and 4·7 % of the F. affinis and F. imperialis genome, respectively, were identified. Chromoviruses and the Tat lineage of Ty3/gypsy group long terminal repeat retrotransposons were identified as the predominant components of the highly repeated fractions in the F. affinis and F. imperialis genomes, respectively. In addition, a heterogeneous, extremely AT-rich satellite repeat was isolated from F. affinis. The FriSAT1 repeat localized in heterochromatic bands makes up approx. 26 % of the F. affinis genome and substantial genomic fractions in several other Liliorhiza species. However, no evidence of a relationship between heterochromatin content and genome size variation was observed. Also, this study was unable to reveal any predominant repeats which tracked the increasing/decreasing trends of genome size evolution in Fritillaria. Instead, the giant Fritillaria genomes seem to be composed of many diversified families of transposable elements. We hypothesize that the genome obesity may be partly determined by the failure of removal mechanisms to counterbalance effectively the retrotransposon amplification.

Comparative sequence analysis of the potato cyst nematode resistance locus H1 reveals a major lack of co-linearity between three haplotypes in potato (Solanum tuberosum ssp.)

The H1 locus confers resistance to the potato cyst nematode Globodera rostochiensis pathotypes 1 and 4. It is positioned at the distal end of chromosome V of the diploid Solanum tuberosum genotype SH83-92-488 (SH) on an introgression segment derived from S. tuberosum ssp. andigena. Markers from a high-resolution genetic map of the H1 locus (Bakker et al. in Theor Appl Genet 109:146-152, 2004) were used to screen a BAC library to construct a physical map covering a 341-kb region of the resistant haplotype coming from SH. For comparison, physical maps were also generated of the two haplotypes from the diploid susceptible genotype RH89-039-16 (S. tuberosum ssp. tuberosum/S. phureja), spanning syntenic regions of 700 and 319 kb. Gene predictions on the genomic segments resulted in the identification of a large cluster consisting of variable numbers of the CC-NB-LRR type of R genes for each haplotype. Furthermore, the regions were interspersed with numerous transposable elements and genes coding for an extensin-like protein and an amino acid transporter. Comparative analysis revealed a major lack of gene order conservation in the sequences of the three closely related haplotypes. Our data provide insight in the evolutionary mechanisms shaping the H1 locus and will facilitate the map-based cloning of the H1 resistance gene.

Mining disease-resistance genes in roses: functional and molecular characterization of the rdr1 locus

The interaction of roses with the leaf spot pathogen Diplocarpon rosae (the cause of black spot on roses) is an interesting pathosystem because it involves a long-lived woody perennial, with life history traits very different from most model plants, and a hemibiotrophic pathogen with moderate levels of gene flow. Here we present data on the molecular structure of the first monogenic dominant resistance gene from roses, Rdr1, directed against one isolate of D. rosae. Complete sequencing of the locus carrying the Rdr1 gene resulted in a sequence of 265,477 bp with a cluster of nine highly related TIR-NBS-LRR (TNL) candidate genes. After sequencing revealed candidate genes for Rdr1, we implemented a gene expression analysis and selected five genes out of the nine TNLs. We then silenced the whole TNL gene family using RNAi (Rdr1-RNAi) constructed from the most conserved sequence region and demonstrated a loss of resistance in the normally resistant genotype. To identify the functional TNL gene, we further screened the five TNL candidate genes with a transient leaf infiltration assay. The transient expression assay indicated a single TNL gene (muRdr1H), partially restoring resistance in the susceptible genotype. Rdr1 was found to localize within the muRdr1 gene family; the genes within this locus contain characteristic motifs of active TNL genes and belong to a young cluster of R genes. The transient leaf assay can be used to further analyze the rose black spot interaction and its evolution, extending the analyses to additional R genes and to additional pathogenic types of the pathogen.

Normalization of genomic DNA using duplex-specific nuclease

An application of duplex-specific nuclease (DSN) normalization technology to whole-genome shotgun sequencing of genomes with a large proportion of repetitive DNA is described. The method uses a thermostable DSN from the Kamchatka crab that specifically hydrolyzes dsDNA. In model experiments on human genomic DNA, we demonstrated that DSN normalization of double-stranded DNA formed during C0t analysis is effective against abundant repetitive sequences with high sequence identity, while retaining highly divergent repeats and coding regions at base-line levels. Thus, DSN normalization applied to C0t analysis can be used to eliminate evolutionarily young repetitive elements from genomic DNA before sequencing, and should prove invaluable in studies of large eukaryotic genomes, such as those of higher plants.

Rice structural variation: a comparative analysis of structural variation between rice and three of its closest relatives in the genus Oryza

Rapid progress in comparative genomics among the grasses has revealed similar gene content and order despite exceptional differences in chromosome size and number. Large- and small-scale genomic variations are of particular interest, especially among cultivated and wild species, as they encode rapidly evolving features that may be important in adaptation to particular environments. We present a genome-wide study of intermediate-sized structural variation (SV) among rice (Oryza sativa) and three of its closest relatives in the genus Oryza (Oryza nivara, Oryza rufipogon and Oryza glaberrima). We computationally identified regional expansions, contractions and inversions in the Oryza species genomes relative to O. sativa by combining data from paired-end clone alignments to the O. sativa reference genome and physical maps. A subset of the computational predictions was validated using a new approach for BAC size determination. The result was a confirmed catalog of 674 expansions (25-38 Mb) and 611 (4-19 Mb) contractions, and 140 putative inversions (14-19 Mb) between the three Oryza species and O. sativa. In the expanded regions unique to O. sativa we found enrichment in transposable elements (TEs): long terminal repeats (LTRs) were randomly located across the chromosomes, and their insertion times corresponded to the date of the A genome radiation. Also, rice-expanded regions contained an over-representation of single-copy genes related to defense factors in the environment. This catalog of confirmed SV in reference to O. sativa provides an entry point for future research in genome evolution, speciation, domestication and novel gene discovery.

Effective isolation of retrotransposons and repetitive DNA families from the wheat genome

New classes of repetitive DNA elements were effectively identified by isolating small fragments of the elements from the wheat genome. A wheat A genome library was constructed from Triticum monococcum by degenerate cleavage with EcoO109I, the recognition sites of which consisted of 5'-PuGGNCCPy-3' multi-sequences. Three novel repetitive sequences pTm6, pTm69 and pTm58 derived from the A genome were screened and tested for high copy number using a blotting approach. pTm6 showed identity with integrase domains of the barley Ty1-Copia-retrotransposon BARE-1 and pTm58 showed similarity to the barley Ty3-gypsy-like retrotransposon Romani. pTm69, however, constituted a tandem array with useful genomic specificities, but did not share any identity with known repetitive elements. This study also sought to isolate wheat D-genome-specific repetitive elements regardless of the level of methylation, by genomic subtraction. Total genomic DNA of Aegilops tauschii was cleaved into short fragments with a methylation-insensitive 4 bp cutter, MboI, and then common DNA sequences between Ae. tauschii and Triticum turgidum were subtracted by annealing with excess T. turgidum genomic DNA. The D genome repetitive sequence pAt1 was isolated and used to identify an additional novel repetitive sequence family from wheat bacterial artificial chromosomes with a size range of 1 395-1 850 bp. The methods successfully led pathfinding of two unique repetitive families.

Small RNAs, DNA methylation and transposable elements in wheat

Background

More than 80% of the wheat genome is composed of transposable elements (TEs). Since active TEs can move to different locations and potentially impose a significant mutational load, their expression is suppressed in the genome via small non-coding RNAs (sRNAs). sRNAs guide silencing of TEs at the transcriptional (mainly 24-nt sRNAs) and post-transcriptional (mainly 21-nt sRNAs) levels. In this study, we report the distribution of these two types of sRNAs among the different classes of wheat TEs, the regions targeted within the TEs, and their impact on the methylation patterns of the targeted regions.

Results

We constructed an sRNA library from hexaploid wheat and developed a database that included our library and three other publicly available sRNA libraries from wheat. For five completely-sequenced wheat BAC contigs, most perfectly matching sRNAs represented TE sequences, suggesting that a large fraction of the wheat sRNAs originated from TEs. An analysis of all wheat TEs present in the Triticeae Repeat Sequence database showed that sRNA abundance was correlated with the estimated number of TEs within each class. Most of the sRNAs perfectly matching miniature inverted repeat transposable elements (MITEs) belonged to the 21-nt class and were mainly targeted to the terminal inverted repeats (TIRs). In contrast, most of the sRNAs matching class I and class II TEs belonged to the 24-nt class and were mainly targeted to the long terminal repeats (LTRs) in the class I TEs and to the terminal repeats in CACTA transposons. An analysis of the mutation frequency in potentially methylated sites revealed a three-fold increase in TE mutation frequency relative to intron and untranslated genic regions. This increase is consistent with wheat TEs being preferentially methylated, likely by sRNA targeting.

Conclusions

Our study examines the wheat epigenome in relation to known TEs. sRNA-directed transcriptional and post-transcriptional silencing plays important roles in the short-term suppression of TEs in the wheat genome, whereas DNA methylation and increased mutation rates may provide a long-term mechanism to inactivate TEs.

Recent Insights into Mechanisms of Genome Size Change in Plants

Genome sizes vary considerably across all eukaryotes and even among closely related species. The genesis and evolutionary dynamics of that variation have generated considerable interest, as have the patterns of variation themselves. Here we review recent developments in our understanding of genome size evolution in plants, drawing attention to the higher order processes that can influence the mechanisms generating changing genome size.

A highly conserved gene island of three genes on chromosome 3B of hexaploid wheat: diverse gene function and genomic structure maintained in a tightly linked block

BACKGROUND

The complexity of the wheat genome has resulted from waves of retrotransposable element insertions. Gene deletions and disruptions generated by the fast replacement of repetitive elements in wheat have resulted in disruption of colinearity at a micro (sub-megabase) level among the cereals. In view of genomic changes that are possible within a given time span, conservation of genes between species tends to imply an important functional or regional constraint that does not permit a change in genomic structure. The ctg1034 contig completed in this paper was initially studied because it was assigned to the Sr2 resistance locus region, but detailed mapping studies subsequently assigned it to the long arm of 3B and revealed its unusual features.

RESULTS

BAC shotgun sequencing of the hexaploid wheat (Triticum aestivum cv. Chinese Spring) genome has been used to assemble a group of 15 wheat BACs from the chromosome 3B physical map FPC contig ctg1034 into a 783,553 bp genomic sequence. This ctg1034 sequence was annotated for biological features such as genes and transposable elements. A three-gene island was identified among >80% repetitive DNA sequence. Using bioinformatics analysis there were no observable similarity in their gene functions. The ctg1034 gene island also displayed complete conservation of gene order and orientation with syntenic gene islands found in publicly available genome sequences of Brachypodium distachyon, Oryza sativa, Sorghum bicolor and Zea mays, even though the intergenic space and introns were divergent.

CONCLUSION

We propose that ctg1034 is located within the heterochromatic C-band region of deletion bin 3BL7 based on the identification of heterochromatic tandem repeats and presence of significant matches to chromodomain-containing gypsy LTR retrotransposable elements. We also speculate that this location, among other highly repetitive sequences, may account for the relative stability in gene order and orientation within the gene island.Sequence data from this article have been deposited with the GenBank Data Libraries under accession no. GQ422824.

Absence of close-facing retrotransposons: a comparison of molecular data and theory

Retrotransposons occur in extremely large numbers in many eukaryotic genomes. However, little is known of the factors which affect the distribution of close proximity elements. In this work we investigate the frequency of close facing retrotransposons in a plant species with extremely high numbers of retrotransposons. Molecular observations are compared with predictions of a mathematical model that assumes a uniform probability of retrotransposon insertion into the genome. The mathematical model plays the role of a null hypothesis. We find that compared with the predictions of the model, there is a statistically significant deficit of identical copies of facing retroelements that are close to one another. This suggests that an efficient mechanism exists that removes or limits close facing retroelements.

Analysis of transposons and repeat composition of the sunflower (Helianthus annuus L.) genome

A sample-sequencing strategy combined with slot-blot hybridization and FISH was used to study the composition of the repetitive component of the sunflower genome. One thousand six hundred thirty-eight sequences for a total of 954,517 bp were analyzed. The fraction of sequences that can be classified as repetitive using computational and hybridization approaches amounts to 62% in total. Almost two thirds remain as yet uncharacterized in nature. Of those characterized, most belong to the gypsy superfamily of LTR-retrotransposons. Unlike in other species, where single families can account for large fractions of the genome, it appears that no transposon family has been amplified to very high levels in sunflower. All other known classes of transposable elements were also found. One family of unknown nature (contig 61) was the most repeated in the sunflower genome. The evolution of the repetitive component in the Helianthus genus and in other Asteraceae was studied by comparative analysis of the hybridization of total genomic DNAs from these species to the sunflower small-insert library and compared to gene-based phylogeny. Very little similarity is observed between Helianthus species and two related Asteraceae species outside of the genus. Most repetitive elements are similar in annual and perennial Helianthus species indicating that sequence amplification largely predates such divergence. Gypsy-like elements are more represented in the annuals than in the perennials, while copia-like elements are similarly represented, attesting a different amplification history of the two superfamilies of LTR-retrotransposons in the Helianthus genus.

Bifurcation and enhancement of autonomous-nonautonomous retrotransposon partnership through LTR Swapping in soybean

Long terminal repeat (LTR) retrotransposons, the most abundant genomic components in flowering plants, are classifiable into autonomous and nonautonomous elements based on their structural completeness and transposition capacity. It has been proposed that selection is the major force for maintaining sequence (e.g., LTR) conservation between nonautonomous elements and their autonomous counterparts. Here, we report the structural, evolutionary, and expression characterization of a giant retrovirus-like soybean (Glycine max) LTR retrotransposon family, SNARE. This family contains two autonomous subfamilies, SARE(A) and SARE(B), that appear to have evolved independently since the soybean genome tetraploidization event approximately 13 million years ago, and a nonautonomous subfamily, SNRE, that originated from SARE(A). Unexpectedly, a subset of the SNRE elements, which amplified from a single founding SNRE element within the last approximately 3 million years, have been dramatically homogenized with either SARE(A) or SARE(B) primarily in the LTR regions and bifurcated into distinct subgroups corresponding to the two autonomous subfamilies. We uncovered evidence of region-specific swapping of nonautonomous elements with autonomous elements that primarily generated various nonautonomous recombinants with LTR sequences from autonomous elements of different evolutionary lineages, thus revealing a molecular mechanism for the enhancement of preexisting partnership and the establishment of new partnership between autonomous and nonautonomous elements.

Two different CC-NBS-LRR genes are required for Lr10-mediated leaf rust resistance in tetraploid and hexaploid wheat

Comparative study of disease resistance genes in crop plants and their relatives provides insight on resistance gene function, evolution and diversity. Here, we studied the allelic diversity of the Lr10 leaf rust resistance gene, a CC-NBS-LRR coding gene originally isolated from hexaploid wheat, in 20 diploid and tetraploid wheat lines. Besides a gene in the tetraploid wheat variety 'Altar' that is identical to the hexaploid wheat Lr10, two additional, functional resistance alleles showing sequence diversity were identified by virus-induced gene silencing in tetraploid wheat lines. In contrast to most described NBS-LRR proteins, the N-terminal CC domain of LR10 was found to be under strong diversifying selection. A second NBS-LRR gene at the Lr10 locus, RGA2, was shown through silencing to be essential for Lr10 function. Interestingly, RGA2 showed much less sequence diversity than Lr10. These data demonstrate allelic diversity of functional genes at the Lr10 locus in tetraploid wheat, and these new genes can now be analyzed for agronomic relevance. Lr10-based resistance is highly unusual both in its dependence on two, only distantly, related CC-NBS-LRR proteins, as well as in the pattern of diversifying selection in the N-terminal domain. This indicates a new and complex molecular mechanism of pathogen detection and signal transduction.

Detailed analysis of a contiguous 22-Mb region of the maize genome

Most of our understanding of plant genome structure and evolution has come from the careful annotation of small (e.g., 100 kb) sequenced genomic regions or from automated annotation of complete genome sequences. Here, we sequenced and carefully annotated a contiguous 22 Mb region of maize chromosome 4 using an improved pseudomolecule for annotation. The sequence segment was comprehensively ordered, oriented, and confirmed using the maize optical map. Nearly 84% of the sequence is composed of transposable elements (TEs) that are mostly nested within each other, of which most families are low-copy. We identified 544 gene models using multiple levels of evidence, as well as five miRNA genes. Gene fragments, many captured by TEs, are prevalent within this region. Elimination of gene redundancy from a tetraploid maize ancestor that originated a few million years ago is responsible in this region for most disruptions of synteny with sorghum and rice. Consistent with other sub-genomic analyses in maize, small RNA mapping showed that many small RNAs match TEs and that most TEs match small RNAs. These results, performed on approximately 1% of the maize genome, demonstrate the feasibility of refining the B73 RefGen_v1 genome assembly by incorporating optical map, high-resolution genetic map, and comparative genomic data sets. Such improvements, along with those of gene and repeat annotation, will serve to promote future functional genomic and phylogenomic research in maize and other grasses.

Rapid DNA loss as a counterbalance to genome expansion through retrotransposon proliferation in plants

Transposable elements, particularly LTR-retrotransposons, comprise the primary vehicle for genome size expansion in plants, while DNA removal through illegitimate recombination and intrastrand homologous recombination serve as the most important counteracting forces to plant genomic obesity. Despite extensive research, the relative impact of these opposing forces and hence the directionality of genome size change remains unknown. In Gossypium (cotton), the 3-fold genome size variation among diploids is due largely to copy number variation of the gypsy-like retrotransposon Gorge3. Here we combine comparative sequence analysis with a modeling approach to study the directionality of genome size change in Gossypium. We demonstrate that the rate of DNA removal in the smaller genomes is sufficient to reverse genome expansion through Gorge3 proliferation. These data indicate that rates of DNA loss can be highly variable even within a single plant genus, and that the known mechanisms of DNA loss can indeed reverse the march toward genomic obesity.

Do genetic recombination and gene density shape the pattern of DNA elimination in rice long terminal repeat retrotransposons?

In flowering plants, the accumulation of small deletions through unequal homologous recombination (UR) and illegitimate recombination (IR) is proposed to be the major process counteracting genome expansion, which is caused primarily by the periodic amplification of long terminal repeat retrotransposons (LTR-RTs). However, the full suite of evolutionary forces that govern the gain or loss of transposable elements (TEs) and their distribution within a genome remains unclear. Here, we investigated the distribution and structural variation of LTR-RTs in relation to the rates of local genetic recombination (GR) and gene densities in the rice (Oryza sativa) genome. Our data revealed a positive correlation between GR rates and gene densities and negative correlations between LTR-RT densities and both GR and gene densities. The data also indicate a tendency for LTR-RT elements and fragments to be shorter in regions with higher GR rates; the size reduction of LTR-RTs appears to be achieved primarily through solo LTR formation by UR. Comparison of indica and japonica rice revealed patterns and frequencies of LTR-RT gain and loss within different evolutionary timeframes. Different LTR-RT families exhibited variable distribution patterns and structural changes, but overall LTR-RT compositions and genes were organized according to the GR gradients of the genome. Further investigation of non-LTR-RTs and DNA transposons revealed a negative correlation between gene densities and the abundance of DNA transposons and a weak correlation between GR rates and the abundance of long interspersed nuclear elements (LINEs)/short interspersed nuclear elements (SINEs). Together, these observations suggest that GR and gene density play important roles in shaping the dynamic structure of the rice genome.

Rapid development of PCR-based genome-specific repetitive DNA junction markers in wheat

In hexaploid wheat (Triticum aestivum L.) (AABBDD, C=17 000 Mb), repeat DNA accounts for approximately 90% of the genome, of which transposable elements (TEs) constitute 60%-80%. Despite the dynamic evolution of TEs, our previous study indicated that the majority of TEs are conserved and collinear between the homologous wheat genomes, based on identical insertion patterns. In this study, we exploited the unique and abundant TE insertion junction regions identified from diploid Aegilops tauschii to develop genome-specific repeat DNA junction markers (RJM) for use in hexaploid wheat. In this study, both BAC end and random shotgun sequences were used to search for RJM. Of the 300 RJM primer pairs tested, 269 (90%) amplified single bands from diploid Ae. tauschii. Of these 269 primer pairs, 260 (97%) amplified hexaploid wheat and 9 (3%) amplified Ae. tauschii only. Among the RJM primers that amplified hexaploid wheat, 88% were successfully assigned to individual chromosomes of the hexaploid D genome. Among the 38 RJM primers mapped on chromosome 6D, 31 (82%) were unambiguously mapped to delineated bins of the chromosome using various wheat deletion lines. Our results suggest that the unique RJM derived from the diploid D genome could facilitate genetic, physical, and radiation mapping of the hexaploid wheat D genome.

Divergence in centromere structure distinguishes related genomes in Coix lacryma-jobi and its wild relative

Knowledge about the composition and structure of centromeres is critical for understanding how centromeres perform their functional roles. Here, we report the sequences of one centromere-associated bacterial artificial chromosome clone from a Coix lacryma-jobi library. Two Ty3/gypsy-class retrotransposons, centromeric retrotransposon of C. lacryma-jobi (CRC) and peri-centromeric retrotransposon of C. lacryma-jobi, and a (peri)centromere-specific tandem repeat with a unit length of 153 bp were identified. The CRC is highly homologous to centromere-specific retrotransposons reported in grass species. An 80-bp DNA region in the 153-bp satellite repeat was found to be conserved to centromeric satellite repeats from maize, rice, and pearl millet. Fluorescence in situ hybridization showed that the three repetitive sequences were located in (peri-)centromeric regions of both C. lacryma-jobi and Coix aquatica. However, the 153-bp satellite repeat was only detected on 20 out of the 30 chromosomes in C. aquatica. Immunostaining with an antibody against rice CENH3 indicates that the 153-bp satellite repeat and CRC might be both the major components for functional centromeres, but not all the 153-bp satellite repeats or CRC sequences are associated with CENH3. The evolution of centromeric repeats of C. lacryma-jobi during the polyploidization was discussed.

[Recent progress in plant genome size evolution]

It has been known that eukaryotic genomes span a wide range of sizes regardless of organism complexity. The observed differences in genome size are primarily due to polyploidy level and abundance of non-coding DNA, especially the contribution of transposable elements (TEs). Here we reviewed the current progress in genome size variation of plant species and the underlying evolutionary forces that contribute to genome expansion or contraction. Polyploidization and the accumulation of transposable element are the primary contributors to genome expansion. As to the mechanisms of DNA loss, unequal homologous recombination and illegitimate recombination are thought to be the counterbalances to the unlimited expansion of a genome. The evolutionary direction of plant genome size is also discussed, which tends to favor larger genomes with deletion mechanisms acting to only attenuate genome expansion but not reverse.

Genetic maps and the use of synteny

Genetic linkage maps represent the order of known molecular genetic markers along a given chromosome for a given species. This provides an insight into the organisation of a plant genome. In comparative genomics, synteny is the preserved order of genes on chromosomes of related species which results from descent from a common ancestor. Comparative mapping is a valuable technique to identify similarities and differences between species and enables the transfer of information from one map to another and assists in the reconstruction of ancestral genomes. This chapter demonstrates the application of online resources to identify candidate genes underlying a QTL, conduct genome comparisons, identify syntenic regions and view comparative genetic maps in grass and Brassica species.

Analysis of intraspecies diversity in wheat and barley genomes identifies breakpoints of ancient haplotypes and provides insight into the structure of diploid and hexaploid triticeae gene pools

A large number of wheat (Triticum aestivum) and barley (Hordeum vulgare) varieties have evolved in agricultural ecosystems since domestication. Because of the large, repetitive genomes of these Triticeae crops, sequence information is limited and molecular differences between modern varieties are poorly understood. To study intraspecies genomic diversity, we compared large genomic sequences at the Lr34 locus of the wheat varieties Chinese Spring, Renan, and Glenlea, and diploid wheat Aegilops tauschii. Additionally, we compared the barley loci Vrs1 and Rym4 of the varieties Morex, Cebada Capa, and Haruna Nijo. Molecular dating showed that the wheat D genome haplotypes diverged only a few thousand years ago, while some barley and Ae. tauschii haplotypes diverged more than 500,000 years ago. This suggests gene flow from wild barley relatives after domestication, whereas this was rare or absent in the D genome of hexaploid wheat. In some segments, the compared haplotypes were very similar to each other, but for two varieties each at the Rym4 and Lr34 loci, sequence conservation showed a breakpoint that separates a highly conserved from a less conserved segment. We interpret this as recombination breakpoints of two ancient haplotypes, indicating that the Triticeae genomes are a heterogeneous and variable mosaic of haplotype fragments. Analysis of insertions and deletions showed that large events caused by transposable element insertions, illegitimate recombination, or unequal crossing over were relatively rare. Most insertions and deletions were small and caused by template slippage in short homopolymers of only a few base pairs in size. Such frequent polymorphisms could be exploited for future molecular marker development.

Automated paleontology of repetitive DNA with REANNOTATE

BACKGROUND

Dispersed repeats are a major component of eukaryotic genomes and drivers of genome evolution. Annotation of DNA sequences homologous to known repetitive elements has been mainly performed with the program REPEATMASKER. Sequences annotated by REPEATMASKER often correspond to fragments of repetitive elements resulting from the insertion of younger elements or other rearrangements. Although REPEATMASKER annotation is indispensable for studying genome biology, this annotation does not contain much information on the common origin of fossil fragments that share an insertion event, especially where clusters of nested insertions of repetitive elements have occurred.

RESULTS

Here I present REANNOTATE, a computational tool to process REPEATMASKER annotation for automated i) defragmentation of dispersed repetitive elements, ii) resolution of the temporal order of insertions in clusters of nested elements, and iii) estimating the age of the elements, if they have long terminal repeats. I have re-annotated the repetitive content of human chromosomes, providing evidence for a recent expansion of satellite repeats on the Y chromosome and, from the retroviral age distribution, for a higher rate of evolution on the Y relative to autosomes.

CONCLUSION

REANNOTATE is ready to process existing annotation for automated evolutionary analysis of all types of complex repeats in any genome. The tool is freely available under the GPL at http://www.bioinformatics.org/reannotate.

LTR retrotransposons reveal recent extensive inter-subspecies nonreciprocal recombination in Asian cultivated rice

Background

Long Terminal Repeats retrotransposons (LTR elements) are ubiquitous Eukaryotic transposable elements (TEs). They are considered to be one of the major forces underlying plant genome evolution. Because of relatively high evolutionary speed, active transposition of LTR elements in the host genomes provides rich information on their short-term history. As more and more genomes, especially those of closely related organisms, have been sequenced, it is possible to perform global comparative study of their LTR retrotransposons to reveal events in the history.

Results

The present research is designed to investigate important evolutionary events in the origin of Asian cultivated rice through the comparison of LTR elements. We have developed LTR_INSERT, a new method for LTR elements discovery in two closely related genomes. Our method has a distinctive feature that it is capable of judging whether an insertion occurs prior or posterior to the divergence of genomes. LTR_INSERT identifies 993 full-length LTR elements, annotates 15916 copies related with them, and discovers at least 16 novel LTR families in the whole-genome comparative map of two cultivated rice subspecies. From the full-length LTR elements, we estimate that a significant proportion of the rice genome has experienced inter-subspecies nonreciprocal recombination (ISNR) in as recent as 53,000 years. Large-scale samplings further support that more than 15% of the rice genome has been involved in such recombination. In addition, LTR elements confirm that the genome of O. sativa ssp. indica and that of japonica diverged about 600,000 years ago.

Conclusion

A new LTR retrotransposon identification method integrating both comparative genomics and ab initio algorithm is introduced and applied to Asian cultivated rice genomes. At whole-genome level, this work confirms that recent ISNR is an important factor that molds modern cultivated rice genome.