Search

Background Transposable elements (TEs) are common and often present with high copy numbers in cellular genomes. Unlike in cellular organisms, TEs were previously thought to be either rare or absent in viruses. Almost all reported TEs display only one or two copies per viral genome. In addition, the discovery of pandoraviruses with genomes up to 2.5-Mb emphasizes the need for biologists to rethink the fundamental nature of the relationship between viruses and cellular life. Results Herein, we performed the first comprehensive analysis of miniature inverted-repeat transposable elements (MITEs) in the 5170 viral genomes for which sequences are currently available. Four hundred and fifty one copies of ten miniature inverted-repeat transposable elements (MITEs) were found and each MITE had reached relatively large copy numbers (some up to 90) in viruses. Eight MITEs belonging to two DNA superfamilies (hobo/Activator/Tam3 and Chapaev–Mirage–CACTA) were for the first time identified in viruses, further expanding the organismal range of these two superfamilies. TEs may play important roles in shaping the evolution of pandoravirus genomes, which were here found to be very rich in MITEs. We also show that putative autonomous partners of seven MITEs are present in the genomes of viral hosts, suggesting that viruses may borrow the transpositional machinery of their cellular hosts’ autonomous elements to spread MITEs and colonize their own genomes. The presence of seven similar MITEs in viral hosts, suggesting horizontal transfers (HTs) as the major mechanism for MITEs propagation. Conclusions Our discovery highlights that TEs contribute to shape genome evolution of pandoraviruses. We concluded that as for cellular organisms, TEs are part of the pandoraviruses’ diverse mobilome. Electronic supplementary material The online version of this article (10.1186/s13100-018-0125-4) contains supplementary material, which is available to authorized users.

Once perceived as merely selfish, transposable elements (TEs) are now recognized as potent agents of adaptation. One way TEs contribute to evolution is through TE exaptation, a process whereby TEs, which persist by replicating in the genome, transform into novel host genes, which persist by conferring phenotypic benefits. Known exapted TEs (ETEs) contribute diverse and vital functions, and may facilitate punctuated equilibrium, yet little is known about this process. To better understand TE exaptation, we designed an approach to resolve the phylogenetic context and timing of exaptation events and subsequent patterns of ETE diversification. Starting with known ETEs, we search in diverse genomes for basal ETEs and closely related TEs, carefully curate the numerous candidate sequences, and infer detailed phylogenies. To distinguish TEs from ETEs, we also weigh several key genomic characteristics including repetitiveness, terminal repeats, pseudogenic features, and conserved domains. Applying this approach to the well-characterized plant ETEs MUG and FHY3, we show that each group is paraphyletic and we argue that this pattern demonstrates that each originated in not one but multiple exaptation events. These exaptations and subsequent ETE diversification occurred throughout angiosperm evolution including the crown group expansion, the angiosperm radiation, and the primitive evolution of angiosperms. In addition, we detect evidence of several putative novel ETE families. Our findings support the hypothesis that TE exaptation generates novel genes more frequently than is currently thought, often coinciding with key periods of evolution.

Coevolutionary interactions are thought to have spurred the evolution of key innovations and driven the diversification of much of life on Earth. However, the genetic and evolutionary basis of the innovations that facilitate such interactions remains poorly understood. We examined the coevolutionary interactions between plants (Brassicales) and butterflies (Pieridae), and uncovered evidence for an escalating evolutionary arms-race. Although gradual changes in trait complexity appear to have been facilitated by allelic turnover, key innovations are associated with gene and genome duplications. Furthermore, we show that the origins of both chemical defenses and of molecular counter adaptations were associated with shifts in diversification rates during the arms-race. These findings provide an important connection between the origins of biodiversity, coevolution, and the role of gene and genome duplications as a substrate for novel traits.

Plant phenomics offers unique opportunities to accelerate our understanding of gene function and plant response to different environments, and may be particularly useful for studying previously uncharacterized genes. One important type of poorly characterized genes is those derived from transposable elements (TEs), which have departed from a mobility-driven lifestyle to attain new adaptive roles for the host (exapted TEs). We used phenomics approaches, coupled with reverse genetics, to analyze T-DNA insertion mutants of both previously reported and novel protein-coding exapted TEs in the model plant Arabidopsis thaliana. We show that mutations in most of these exapted TEs result in phenotypes, particularly when challenged by abiotic stress. We built statistical multi-dimensional phenotypic profiles and compared them to wild-type and known stress responsive mutant lines for each particular stress condition. We found that these exapted TEs may play roles in responses to phosphate limitation, tolerance to high salt concentration, freezing temperatures, and arsenic toxicity. These results not only experimentally validate a large set of putative functional exapted TEs recently discovered through computational analysis, but also uncover additional novel phenotypes for previously well-characterized exapted TEs in A. thaliana.

Transposable elements (TEs) are mobile genetic elements that were once perceived as merely selfish, but are now recognized as potent agents of adaptation. One way TEs contribute to genome evolution is through TE exaptation, a process whereby TEs, which usually persist by replicating in the genome, transform into novel host genes, which thereafter persist by conferring phenotypic benefits. Exapted TEs are known to contribute diverse and vital functions, and may facilitate punctuated equilibrium, yet we have little understanding about the process of TE exaptation. In order to facilitate our understanding of how TE coding sequences may become exapted, here we incorporate the findings of recent publications into a framework and six-step model.

Transposable elements are mobile genetic elements that have successfully populated eukaryotic genomes and show diversity in their structure and transposition mechanisms. Although first viewed solely as selfish, transposable elements are now known as important vectors to drive the adaptation and evolution of their host genome. Transposable elements can affect host gene structures, gene copy number, gene expression, and even as a source for novel genes. For example, a number of transposable element sequences have been co-opted to contribute to evolutionary innovation, such as the mammalian placenta and the vertebrate immune system. In plants, the need to adapt rapidly to changing environmental conditions is essential and is reflected, as will be discussed, by genome plasticity and an abundance of diverse, active transposon families. This review focuses on transposable elements in plants, particularly those that have beneficial effects on the host. We also emphasize the importance of having proper tools to annotate and classify transposons to better understand their biology.

Transposons are major components of all eukaryotic genomes. Although traditionally regarded as causes of detrimental mutations, recent evidence suggests that transposons may play a role in host gene diversification and evolution. For example, host gene transduction by retroelements has been suggested to be both common and to have the potential to create new chimeric genes by the shuffling of existing sequences. We have previously shown that the maize (Zea mays subsp. mays) retrotransposon Bs1 has transduced sequences from three different host genes. Here, we provide evidence that these transduction events led to the generation of a chimeric new gene that is both transcribed and translated. Expression of Bs1 is tightly controlled and occurs during a narrow developmental window in early ear development. Although all Bs1-associated transduction events took place before Zea speciation, a full uninterrupted open reading frame encoding the BS1 protein may have arisen in domesticated maize or in the diverse populations of its progenitor Z. mays subsp. parviglumis. We discuss potential functions based on domain conservation and evidence for functional constraints between the transduced sequences and their host gene counterparts.

MUG1 is a MULE transposon-related domesticated gene in plants. We assessed the sequence diversity, neutrality, expression, and phylogenetics of the MUG1 gene among Oryza ssp. We found MUG1 expression in all tissues analyzed, with different levels in O. sativa. There were 408 variation sites in the 3886 bp of MUG1 locus. The nucleotide diversity of the MUG1 was higher than functionally known genes in rice. The nucleotide diversity (pi) in the domains was lower than the average nucleotide diversity in whole coding region. The pi values in nonsynonymous sites were lower than those of synonymous sites. Tajima D and Fu and Li D* values were mostly negative values, suggesting purifying selection in MUG1 sequences of Oryza ssp. Genome-specific variation and phylogenetic analyses show a general grouping of MUG1 sequences congruent with Oryza ssp. biogeography; however, our MUG1 phylogenetic results, in combination with separate B and D genome studies, might suggest an early divergence of the Oryza ssp. by continental drift of Gondwanaland. O. longistaminata MUG1 divergence from other AA diploids suggests that it might not be a direct ancestor of the African rice species.

Whole copies of the polygalacturonase (PG) genes from rice ( Oryza sativa subsp. japonica) and a filamentous fungus ( Aspergillus oryzae ) were isolated. The orthologs of the rice PGs were also retrieved from other plant species. The 106 plant PGs analyzed were divided into 5 clades, A, B, C, D, and E. The fungus PGs were classified into 3 clades, of which one formed a loose cluster with clade E of the plant PGs. Four domain motifs (I, II, III, IV) were identified in all PGs. Motifs II and III were split by introns such as G/DDC and CGPGHGIS/IGSLG, respectively. In plant PGs there were 446 introns in total and 3.98 introns per gene. Intron phase distribution was 65.5% for phase 0, 19.7% for phase 1, and 14.8% for phase 2 in plant PGs. In the PGs of A. oryzae there were 37 introns of phase 0 (59.5%), phase 1 (24.3%), and phase 2 (16.2%), with 2.47 introns per gene. The 5 clades of plant PGs were divided into 3 basic gene structure lineages. Intron positions and phases were conserved among the PGs in the first 2 lineages. The third lineage consisted of PGs of clade E, which also carried highly conserved introns at different positions from other PGs. Intron positions were not as highly conserved in fungus PGs as in plant PGs. The introns in the current PGs have been present since before the divergence of monocots from dicots. The results obtained show that differential losses of introns created gene diversity, which was followed by segmental and tandem duplication in plant PGs.

The Rice Annotation Project Database (RAP-DB) was created to provide the genome sequence assembly of the International Rice Genome Sequencing Project (IRGSP), manually curated annotation of the sequence, and other genomics information that could be useful for comprehensive understanding of the rice biology. Since the last publication of the RAP-DB, the IRGSP genome has been revised and reassembled. In addition, a large number of rice-expressed sequence tags have been released, and functional genomics resources have been produced worldwide. Thus, we have thoroughly updated our genome annotation by manual curation of all the functional descriptions of rice genes. The latest version of the RAP-DB contains a variety of annotation data as follows: clone positions, structures and functions of 31 439 genes validated by cDNAs, RNA genes detected by massively parallel signature sequencing (MPSS) technology and sequence similarity, flanking sequences of mutant lines, transposable elements, etc. Other annotation data such as Gnomon can be displayed along with those of RAP for comparison. We have also developed a new keyword search system to allow the user to access useful information. The RAP-DB is available at: http://rapdb.dna.affrc.go.jp/ and http://rapdb.lab.nig.ac.jp/.

Complex eukaryotes contain millions of transposable elements (TEs), comprising large fractions of their nuclear genomes. TEs consist of structural, regulatory, and coding sequences that are ordinarily associated with transposition, but that occasionally confer on the organism a selective advantage and may thereby become exapted. Exapted transposable element genes (ETEs) are known to play critical roles in diverse systems, from vertebrate adaptive immunity to plant development. Yet despite their evident importance, most ETEs have been identified fortuitously and few systematic searches have been conducted, suggesting that additional ETEs may await discovery. To explore this possibility, we develop a comprehensive systematic approach to searching for ETEs. We use TE-specific conserved domains to identify with high precision genes derived from TEs and screen them for signatures of exaptation based on their similarities to reference sets of known ETEs, conventional (non-TE) genes, and TE genes across diverse genetic attributes including repetitiveness, conservation of genomic location and sequence, and levels of expression and repressive small RNAs. Applying this approach in the model plant Arabidopsis thaliana, we discover a surprisingly large number of novel high confidence ETEs. Intriguingly, unlike known plant ETEs, several of the novel ETE families form tandemly arrayed gene clusters, whereas others are relatively young. Our results not only identify novel TE-derived genes that may have practical applications but also challenge the notion that TE exaptation is merely a relic of ancient life, instead suggesting that it may continue to fundamentally drive evolution.

DNA derived from transposable elements (TEs) constitutes large parts of the genomes of complex eukaryotes, with major impacts not only on genomic research but also on how organisms evolve and function. Although a variety of methods and tools have been developed to detect and annotate TEs, there are as yet no standard benchmarks—that is, no standard way to measure or compare their accuracy. This lack of accuracy assessment calls into question conclusions from a wide range of research that depends explicitly or implicitly on TE annotation. In the absence of standard benchmarks, toolmakers are impeded in improving their tools, annotators cannot properly assess which tools might best suit their needs, and downstream researchers cannot judge how accuracy limitations might impact their studies. We therefore propose that the TE research community create and adopt standard TE annotation benchmarks, and we call for other researchers to join the authors in making this long-overdue effort a success., This work was funded by a Bioinformatics and Computational Biology grant to MB and TB from Genome Canada, Genome Québec, and the Canadian Institutes for Health Research.

Rice, one of the world's most important food plants, has important syntenic relationships with the other cereal species and is a model plant for the grasses. Here we present a map-based, finished quality sequence that covers 95% of the 389 Mb genome, including virtually all of the euchromatin and two complete centromeres. A total of 37,544 non-transposable-element-related protein-coding genes were identified, of which 71% had a putative homologue in Arabidopsis. In a reciprocal analysis, 90% of the Arabidopsis proteins had a putative homologue in the predicted rice proteome. Twenty-nine per cent of the 37,544 predicted genes appear in clustered gene families. The number and classes of transposable elements found in the rice genome are consistent with the expansion of syntenic regions in the maize and sorghum genomes. We find evidence for widespread and recurrent gene transfer from the organelles to the nuclear chromosomes. The map-based sequence has proven useful for the identification of genes underlying agronomic traits. The additional single-nucleotide polymorphisms and simple sequence repeats identified in our study should accelerate improvements in rice production.

The discovery that vertebrate retroviruses could transduce cellular sequences was central to cancer etiology and research. Although not well documented, transduction of cellular sequences by retroelements has been suggested to modify cellular functions. The maize Bs1 transposon was the first non-vertebrate retroelement reported to have transduced a portion of a cellular gene (c-pma). We show that Bs1 has, in addition, transduced portions of at least two more maize cellular genes, namely for 1,3-beta-glucanase (c-bg) and 1,4-beta-xylan endohydrolase (c-xe). We also show that Bs1 has maintained a truncated gag domain with similarity to the magellan gypsy-like long terminal repeat retrotransposon and a region that may correspond to an env-like domain. Our findings suggest that, like oncogenic retroviruses, the three transduced gene fragments and the Bs1 gag domain encode a fusion protein that has the potential to be expressed. We suggest that transduction by retroelements may facilitate the formation of novel hybrid genes in plants.

Members of the Tourist family of miniature inverted-repeat transposable elements (MITEs) are very abundant among a wide variety of plants, are frequently found associated with normal plant genes, and thus are thought to be important players in the organization and evolution of plant genomes. In Arabidopsis, the recent discovery of a Tourist member harboring a putative transposase has shed new light on the mobility and evolution of MITEs. Here, we analyze a family of Tourist transposons endogenous to the genome of the nematode Caenorhabditis elegans (Bristol N2). One member of this large family is 7568 bp in length, harbors an ORF similar to the putative Tourist transposase from Arabidopsis, and is related to the IS5 family of bacterial insertion sequences (IS). Using database searches, we found expressed sequence tags (ESTs) similar to the putative Tourist transposases in plants, insects, and vertebrates. Taken together, our data suggest that Tourist-like and IS5-like transposons form a superfamily of potentially active elements ubiquitous to prokaryotic and eukaryotic genomes.

Miniature inverted-repeat transposable elements or MITEs represent a large superfamily of transposons that are moderately to highly repetitive and frequently associated with plant genes. These attributes were exploited in the development of a powerful marker technology called Inter-MITE polymorphism, or IMP, which involves the amplification between two adjacent MITEs. In this report, we describe the utility of the IMP approach in the mapping and fingerprinting of the barley genome. MITEs were systematically mined from barley genomic gene sequences by computer-assisted database searches and structural analysis. Barley MITEs include members of the Stowaway family and a new family we have named Barfly. Using these barley MITEs, a total of 88 IMP markers were mapped onto an existing barley RFLP map that was based on a doubled-haploid segregating population between Hordeum vulgare and Hordeum spontaneum. We demonstrate that the IMP approach can be effectively applied in the fingerprinting of barley cultivars and for genetic similarity analysis. We also provide evidence that barley MITE-based primers can be effectively used in the mapping and fingerprinting of other cereals, suggesting that the IMP approach has broad applicability.

While genome-wide surveys of abundance and diversity of mobile elements have been conducted for some class I transposable element families, little is known about the nature of class II transposable elements on this scale. In this report, we present the results from analysis of the sequence and structural diversity of Mutator-like elements (MULEs) in the genome of Arabidopsis thaliana (Columbia). Sequence similarity searches and subsequent characterization suggest that MULEs exhibit extreme structure, sequence, and size heterogeneity. Multiple alignments at the nucleotide and amino acid levels reveal conserved, potentially transposition-related sequence motifs. While many MULEs share common structural features to Mu elements in maize, some groups lack characteristic long terminal inverted repeats. High sequence similarity and phylogenetic analyses based on nucleotide sequence alignments indicate that many of these elements with diverse structural features may remain transpositionally competent and that multiple MULE lineages may have been evolving independently over long time scales. Finally, there is evidence that MULEs are capable of the acquisition of host DNA segments, which may have implications for adaptive evolution, both at the element and host levels.

Auxin-binding protein 1 subsp. mays (ABP1) has been suggested as a receptor mediating auxin-induced cell expansion and differentiation. In maize (Zea mays), ABP1 is encoded by a single gene, Abp1. The TATA and CAAT promoter elements as well as the transcriptional start site were previously identified and all were found to be located within a transposable element (TE),Tourist-Zm11. In this study we report the cloning and characterization of the Abp1 5′-flanking region in maize and its wild relatives, the teosintes. We provide evidence for insertion polymorphism corresponding to Tourist-Zm11 and two novel TEs, Batuta and Pilgrim. Despite this polymorphic structure, the Abp1 core promoter in maize and the teosintes is conserved, is located downstream of the TE insertions in the 5′-flanking region, and is TATA-less. We discuss the potential evolutionary impact of these TEs on the regulation of Abp1 gene expression.

Recent availability of extensive genome sequence information offers new opportunities to analyze genome organization, including transposon diversity and accumulation, at a level of resolution that was previously unattainable. In this report, we used sequence similarity search and analysis protocols to perform a fine-scale analysis of a large sample (≈17.2 Mb) of the Arabidopsis thaliana (Columbia) genome for transposons. Consistent with previous studies, we report that the A. thaliana genome harbors diverse representatives of most known superfamilies of transposons. However, our survey reveals a higher density of transposons of which over one-fourth could be classified into a single novel transposon family designated as Basho , which appears unrelated to any previously known superfamily. We have also identified putative transposase-coding ORFs for miniature inverted-repeat transposable elements (MITEs), providing clues into the mechanism of mobility and origins of the most abundant transposons associated with plant genes. In addition, we provide evidence that most mined transposons have a clear distribution preference for A + T-rich sequences and show that structural variation for many mined transposons is partly due to interelement recombination. Taken together, these findings further underscore the complexity of transposons within the compact genome of A. thaliana .

The rice disease resistance gene Xa21, which encodes a receptor-like kinase, is a member of a multigene family. Based on comparisons of genomic sequences of seven family members, seventeen transposon-like elements were identified in the 5' and 3' flanking regions and introns of these genes. Sequence characterization revealed that these elements are diverse, showing similarity to maize Ds, CACTA and miniature inverted repeat-like elements, as well as novel elements. Only two elements were located in presumed coding regions, indicating that integration of transposable elements at the Xa21 disease resistance locus occurred preferentially in noncoding regions.

Despite the central importance of noncoding DNA to gene regulation and evolution, understanding of the extent of selection on plant noncoding DNA remains limited compared to that of other organisms. Here we report sequencing of genomes from three Brassicaceae species (Leavenworthia alabamica, Sisymbrium irio and Aethionema arabicum) and their joint analysis with six previously sequenced crucifer genomes. Conservation across orthologous bases suggests that at least 17% of the Arabidopsis thaliana genome is under selection, with nearly one-quarter of the sequence under selection lying outside of coding regions. Much of this sequence can be localized to approximately 90,000 conserved noncoding sequences (CNSs) that show evidence of transcriptional and post-transcriptional regulation. Population genomics analyses of two crucifer species, A. thaliana and Capsella grandiflora, confirm that most of the identified CNSs are evolving under medium to strong purifying selection. Overall, these CNSs highlight both similarities and several key differences between the regulatory DNA of plants and other species.

Retrotransposons are an abundant and ancient component of plant genomes, yet recent evidence indicates that element activity in many modern plants is restricted to times of stress. Stress activation of plant retrotransposons may be a significant factor in somaclonal variation, in addition to providing an important means to isolate new active elements. Long terminal repeat retrotransposons and a second class of elements we have called miniature inverted-repeat transposable elements (MITEs) have recently been found to be associated with the genes of diverse plants where some contribute regulatory sequences. Because of their sequence diversity and small size, MITEs may be a valuable evolutionary tool for altering patterns of gene expression.

While evolution is often understood exclusively in terms of adaptation, innovation often begins when a feature adapted for one function is co-opted for a different purpose, such as when feathers originally adapted for insulation became used for flight. Co-opted features are called exaptations. Transposable elements are often viewed as molecular parasites, yet they are frequently the source of evolutionary innovation. One way in which transposable elements contribute to evolution is that their sequences can be co-opted to perform phenotypically beneficial functions. Transposable element gene exaptations have contributed to major innovations such as the vertebrate adaptive immune system and the mammalian placenta. They also often become transcription factors, and transposable element-derived transcription factor binding sites can form new regulatory networks. In this chapter, we review transposable element coding sequence exaptations in plants.

Tourist was originally described as a 128-bp insertion mutation in the maize wx-B2 allele. Subsequent analysis revealed that Tourist elements are in the introns or flanking sequences of 11 maize genes and a single barley gene. In this study we report that Tourist elements are frequently associated with the wild-type genes of two other grasses, rice and sorghum. Six of 35 rice and 5 of 8 sorghum complete gene sequences reported to date contain Tourist elements. Furthermore, 11 additional maize genes have been found to contain Tourist elements, bringing the current total of elements associated with maize genes to 23. Sequence comparison of Tourist elements has led to the identification of four subfamilies, designated A-D. Evidence is presented for the recent mobility of elements in three of these subfamilies and in three of the four grass species. These data suggest that Tourist elements are highly repetitive in the genomes of some and perhaps all members of the grasses.

Auxin plays crucial roles in plant development. Auxin-binding protein 1 (ABP1) is an auxin receptor required to coordinate cell division and expansion during postembryonic shoot development, and differential auxin responses during root growth. While ABP1 is encoded by a single gene in maize, multiple gene copies exist in teosinte, the wild relatives of maize. We have previously shown that some of the differences between these genes are caused by multiple transposon insertions in the promoter. Here we show that the different ABP1 promoter types confer differential gene expression levels on a firefly luciferase reporter gene. We also discovered a negative regulatory sequence upstream of the conserved transcriptional start site. When this sequence is insulated by an Ac-like transposon or deleted in natural ABP1 gene variants, expression levels are enhanced. Promoters combining both a MITE and a solo-LTR showed small but significant increase in expression compared to those containing only one insertion. This increase seems to be additive, suggesting that it may be due to enhancer sequences present within these transposons. Our results point to a potential role of the ABP1-associated transposons in the modulation of ABP1 gene expression.

The wx-B2 mutation results from a 128-bp transposable element-like insertion in exon 11 of the maize Waxy gene. Surprisingly, 11 maize genes and one barley gene in the GenBank and EMBL data bases were found to contain similar elements in flanking or intron sequences. Members of this previously undescribed family of elements, designated Tourist, are short (133 bp on average), have conserved terminal inverted repeats, are flanked by a 3-bp direct repeat, and display target site specificity. Based on estimates of repetitiveness of three Tourist elements in maize genomic DNA, the copy number of the Tourist element family may exceed that of all previously reported eukaryotic inverted repeat elements. Taken together, our data suggest that Tourist may be the maize equivalent of the human Alu family of elements with respect to copy number, genomic dispersion, and the high frequency of association with genes.

Rice, the primary source of dietary calories for half of humanity, is the first crop plant for which a high-quality reference genome sequence from a single variety was produced. We used resequencing microarrays to interrogate 100 Mb of the unique fraction of the reference genome for 20 diverse varieties and landraces that capture the impressive genotypic and phenotypic diversity of domesticated rice. Here, we report the distribution of 160,000 nonredundant SNPs. Introgression patterns of shared SNPs revealed the breeding history and relationships among the 20 varieties; some introgressed regions are associated with agronomic traits that mark major milestones in rice improvement. These comprehensive SNP data provide a foundation for deep exploration of rice diversity and gene–trait relationships and their use for future rice improvement.

We present here the annotation of the complete genome of rice Oryza sativa L. ssp. japonica cultivar Nipponbare. All functional annotations for proteins and non-protein-coding RNA (npRNA) candidates were manually curated. Functions were identified or inferred in 19,969 (70%) of the proteins, and 131 possible npRNAs (including 58 antisense transcripts) were found. Almost 5000 annotated protein-coding genes were found to be disrupted in insertional mutant lines, which will accelerate future experimental validation of the annotations. The rice loci were determined by using cDNA sequences obtained from rice and other representative cereals. Our conservative estimate based on these loci and an extrapolation suggested that the gene number of rice is ∼32,000, which is smaller than previous estimates. We conducted comparative analyses between rice and Arabidopsis thaliana and found that both genomes possessed several lineage-specific genes, which might account for the observed differences between these species, while they had similar sets of predicted functional domains among the protein sequences. A system to control translational efficiency seems to be conserved across large evolutionary distances. Moreover, the evolutionary process of protein-coding genes was examined. Our results suggest that natural selection may have played a role for duplicated genes in both species, so that duplication was suppressed or favored in a manner that depended on the function of a gene.

Transposons comprise a major component of eukaryotic genomes, yet it remains controversial whether they are merely genetic parasites or instead significant contributors to organismal function and evolution. In plants, thousands of DNA transposons were recently shown to contain duplicated cellular gene fragments, a process termed transduplication. Although transduplication is a potentially rich source of novel coding sequences, virtually all appear to be pseudogenes in rice. Here we report the results of a genome-wide survey of transduplication in Mutator-like elements (MULEs) in Arabidopsis thaliana, which shows that the phenomenon is generally similar to rice transduplication, with one important exception: KAONASHI (KI). A family of more than 97 potentially functional genes and apparent pseudogenes, evidently derived at least 15 MYA from a cellular small ubiquitin-like modifier-specific protease gene, KI is predominantly located in potentially autonomous non-terminal inverted repeat MULEs and has evolved under purifying selection to maintain a conserved peptidase domain. Similar to the associated transposase gene but unlike cellular genes, KI is targeted by small RNAs and silenced in most tissues but has elevated expression in pollen. In an Arabidopsis double mutant deficient in histone and DNA methylation with elevated KI expression compared to wild type, at least one KI-MULE is mobile. The existence of KI demonstrates that transduplicated genes can retain protein-coding capacity and evolve novel functions. However, in this case, our evidence suggests that the function of KI may be selfish rather than cellular.

DNA transposons are known to frequently capture duplicated fragments of host genes. The evolutionary impact of this phenomenon depends on how frequently the fragments retain protein-coding function as opposed to becoming pseudogenes. Gene fragment duplication by Mutator-like elements (MULEs) has previously been documented in maize, Arabidopsis, and rice. Here we present a rigorous genome-wide analysis of MULEs in the model plant Oryza sativa (domesticated rice). We identify 8274 MULEs with intact termini and target-site duplications (TSDs) and show that 1337 of them contain duplicated host gene fragments. Through a detailed examination of the 5% of duplicated gene fragments that are transcribed, we demonstrate that virtually all cases contain pseudogenic features such as fragmented conserved protein domains, frameshifts, and premature stop codons. In addition, we show that the distribution of the ratio of nonsynonymous to synonymous amino acid substitution rates for the duplications agrees with the expected distribution for pseudogenes. We conclude that MULE-mediated host gene duplication results in the formation of pseudogenes, not novel functional protein-coding genes; however, the transcribed duplications possess characteristics consistent with a potential role in the regulation of host gene expression.

Studies that have attempted to estimate the rate of deleterious mutation have typically been conducted under low levels of ultraviolet-B (UV-B) radiation, a naturally occurring mutagen. We conducted experiments to test whether the inclusion of natural levels of UV-B radiation in mutation-accumulation (MA) experiments influences the rate and effects of mildly deleterious mutation in the plant Arabidopsis thaliana. Ten generations of MA proved insufficient to observe significant changes in means or among-line variances in experimental lines maintained either with or without supplemental UV-B radiation. Maximum-likelihood estimates of mutation rate for total flower number revealed a small but significant rate of mutation for MA lines propagated under supplemental UV-B exposure, but not for those in which supplemental UV-B was omitted. A fraction of the flower number mutations under UV-B (∼25–30%) are estimated to increase flower number. Results from the application of transposon display to plant materials obtained after MA, in both the presence and absence of supplemental UV-B, suggest that the average rate of transposition for the class I and II transposable elements (TEs) surveyed was no more than 10−4. Overall, the estimates of mutation parameters are qualitatively similar to what has been observed in other MA experiments with this species in which supplemental UV-B levels have not been used. As well, it appears that naturally occurring levels of UV-B do not lead to detectable increases in levels of transposable element activity.

While transposons have traditionally been viewed as genomic parasites or "junk DNA," the discovery of transposon-derived host genes has fueled an ongoing debate over the evolutionary role of transposons. In particular, while mobility-related open reading frames have been known to acquire host functions, the contribution of these types of events to the evolution of genes is not well understood. Here we report that genome-wide searches for Mutator transposase-derived host genes in Arabidopsis thaliana (Columbia-0) and Oryza sativa ssp. japonica (cv. Nipponbare) (domesticated rice) identified 121 sequences, including the taxonomically conserved MUSTANG1. Syntenic MUSTANG1 orthologs in such varied plant species as rice, poplar, Arabidopsis, and Medicago truncatula appear to be under purifying selection. However, despite the evidence of this pathway of gene evolution, MUSTANG1 belongs to one of only two Mutator-like gene families with members in both monocotyledonous and dicotyledonous plants, suggesting that Mutator-like elements seldom evolve into taxonomically widespread host genes.

Transposable elements (TEs) comprise a major component of eukaryotic genomes, and exhibit striking deviations from random distribution across the genomes studied, including humans, flies, nematodes, and plants. Although considerable progress has been made in documenting these patterns, the causes are subject to debate. Here, we use the genome sequence of Arabidopsis thaliana to test for the importance of competing models of natural selection against TE insertions. We show that, despite TE accumulation near the centromeres, recombination does not generally correlate with TE abundance, suggesting that selection against ectopic recombination does not influence TE distribution in A. thaliana. In contrast, a consistent negative correlation between gene density and TE abundance, and a strong under-representation of TE insertions in introns suggest that selection against TE disruption of gene expression is playing a more important role in A. thaliana. High rates of self-fertilization may reduce the importance of recombination rate in genome structuring in inbreeding organisms such as A. thaliana and Caenorhabditis elegans.

The genomes of plants, like virtually all other eukaryotic organisms, harbor a diverse array of mobile elements, or transposons. In terms of numbers, the predominant type of transposons in many plants is the miniature inverted-repeat transposable element (MITE). There are three archetypal MITEs, known as Tourist, Stowaway, and Emigrant, each of which can be defined by a specific terminal inverted-repeat (TIR) sequence signature. Although their presence was known for over a decade, only recently have open reading frames (ORFs) been identified that correspond to putative transposases for each of the archetypes. We have identified two Stowaway elements that encode a putative transposase and are similar to members of the previously characterized IS630–Tc1-mariner superfamily. In this report, we provide a high-resolution phylogenetic analysis of the evolutionary relationship between Stowaway, Emigrant, and members of the IS630–Tc1-mariner superfamily. We show that although Emigrant is closely related to the pogo-like family of elements, Stowaway may represent a novel family. Integration of our results with previously published data leads to the conclusion that the three main types of MITEs have different evolutionary histories despite similarity in structure.Key words: Stowaway, Emigrant, MITE, mariner, transposon.

A new group of long terminal repeats (LTR) retrotransposons, termed t erminal- r epeat retrotransposons i n m iniature (TRIM), are described that are present in both monocotyledonous and dicotyledonous plant. TRIM elements have terminal direct repeat sequences between ≈100 and 250 bp in length that encompass an internal domain of ≈100–300 bp. The internal domain contains primer binding site and polypurine tract motifs but lacks the coding domains required for mobility. Thus TRIM elements are not capable of autonomous transposition and probably require the help of mobility-related proteins encoded by other retrotransposons. The structural organization of TRIM elements suggests an evolutionary relationship to either LTR retrotransposons or retroviruses. The past mobility of TRIM elements is indicated by the presence of flanking 5-bp direct repeats found typically at LTR retrotransposon insertion sites, the high degree of sequence conservation between elements from different genomic locations, and the identification of r elated to e mpty sites (RESites). TRIM elements seem to be involved actively in the restructuring of plant genomes, affecting the promoter, coding region and intron-exon structure of genes. In solanaceous species and maize, TRIM elements provided target sites for further retrotransposon insertions. In Arabidopsis , evidence is provided that the TRIM element also can be involved in the transduction of host genes.

Theoretical models predict that the mating system should be an important factor driving the dynamics of transposable elements in natural populations due to differences in selective pressure on both element and host. We used a PCR-based approach to examine the abundance and levels of insertion polymorphism of Ac-III, a recently identified Ac-like transposon family, in natural populations of the selfing plant Arabidopsis thaliana and its close outcrossing relative, Arabidopsis lyrata. Although several insertions appeared to be ancient and shared between species, there is strong evidence for recent activity of this element family in both species. Sequences of the regions flanking insertions indicate that all Ac-III transposons segregating in natural populations are in noncoding regions and provide no evidence for local transposition events. Transposon display analysis suggests the presence of slightly higher numbers of insertion sites per individual but fewer total polymorphic insertions in the self-pollinating A. thaliana than A. lyrata. Element insertions appear to be segregating at significantly lower frequencies in A. lyrata than A. thaliana, which is consistent with a reduction in transposition rate, reduction in effective population size, or reduced efficacy of natural selection against element insertions in selfing populations.

The comprehensive analysis of the genome sequence of the plant Arabidopsis thaliana has been completed recently. The genome sequence and associated analyses provide the foundations for rapid progress in many fields of plant research, such as the exploitation of genetic variation in Arabidopsis ecotypes, the assessment of the transcriptome and proteome, and the association of genome changes at the sequence level with evolutionary processes. Nevertheless, genome sequencing and analysis are only the first steps towards a new plant biology. Much remains to be done to refine the analysis of encoded genes, to define the functions of encoded proteins systematically, and to establish new generations of databases to capture and relate diverse data sets generated in widely distributed laboratories.

Oryza sativa L. (domesticated rice) is a monocotyledonous plant, and its 430 Mb genome has been targeted for complete sequencing. We performed a high-resolution computer-based survey for transposable elements on 910 Kb of rice genomic DNA sequences. Both class I and II transposable elements were present, contributing 19.9% of the sequences surveyed. Class II elements greatly outnumbered class I elements (166 versus 22), although class I elements made up a greater percentage (12.2% versus 6.6%) of nucleotides surveyed. Several Mutator-like elements (MULEs) were identified, including rice elements that harbor truncated host cellular genes. MITEs (miniature inverted-repeat transposable elements) account for 71.6% of the mined transposable elements and are clearly the predominant type of transposable element in the sequences examined. Moreover, a putative Stowaway transposase has been identified based on shared sequence similarity with the mined MITEs and previously identified plant mariner-like elements (MLEs). Members of a group of novel rice elements resembling the structurally unusual members of the Basho family in Arabidopsis suggest a wide distribution of these transposons among plants. Our survey provides a preview of transposable element diversity and abundance in rice, and allows for comparison with genomes of other plant species.

Several recent reports indicate that mobile elements are frequently found in and flanking many wild-type plant genes. To determine the extent of this association, we performed computer-based systematic searches to identify mobile elements in the genes of two "model" plants, Oryza sativa (domesticated rice) and Arabidopsis thaliana. Whereas 32 common sequences belonging to nine putative mobile element families were found in the noncoding regions of rice genes, none were found in Arabidopsis genes. Five of the nine families (Gaijin, Castaway, Ditto, Wanderer, and Explorer) are first described in this report, while the other four were described previously (Tourist, Stowaway, p-SINE1, and Amy/LTP). Sequence similarity, structural similarity, and documentation of past mobility strongly suggests that many of the rice common sequences are bona fide mobile elements. Members of four of the new rice mobile element families are similar in some respects to members of the previously identified inverted-repeat element families, Tourist and Stowaway. Together these elements are the most prevalent type of transposons found in the rice genes surveyed and form a unique collection of inverted-repeat transposons we refer to as miniature inverted-repeat transposable elements or MITEs. The sequence and structure of MITEs are clearly distinct from short or long interspersed nuclear elements (SINEs or LINEs), the most common transposable elements associated with mammalian nuclear genes. Mobile elements, therefore, are associated with both animal and plant genes, but the identity of these elements is strikingly different.

The benefits of ever-growing numbers of sequenced eukaryotic genomes will not be fully realized until we learn to decipher vast stretches of noncoding DNA, largely composed of transposable elements. Transposable elements persist through self-replication, but some genes once encoded by transposable elements have, through a process called molecular domestication, evolved new functions that increase fitness. Although they have conferred numerous adaptations, the number of such domesticated transposable element genes remains unknown, so their evolutionary and functional impact cannot be fully assessed. Systematic searches that exploit genomic signatures of natural selection have been employed to identify potential domesticated genes, but their predictions have yet to be experimentally verified. To this end, we investigated a family of domesticated genes called MUSTANG (MUG), identified in a previous bioinformatic search of plant genomes. We show that MUG genes are functional. Mutants of Arabidopsis thaliana MUG genes yield phenotypes with severely reduced plant fitness through decreased plant size, delayed flowering, abnormal development of floral organs, and markedly reduced fertility. MUG genes are present in all flowering plants, but not in any non-flowering plant lineages, such as gymnosperms, suggesting that the molecular domestication of MUG may have been an integral part of early angiosperm evolution. This study shows that systematic searches can be successful at identifying functional genetic elements in noncoding regions and demonstrates how to combine systematic searches with reverse genetics in a fruitful way to decipher eukaryotic genomes., Author Summary The genomes of complex organisms are mostly made up not of ordinary genes but of transposable elements. Transposable elements have been called “selfish DNA” because they normally persist by copying themselves, not by helping the organism to survive or reproduce. Yet transposable elements can help organisms to evolve; for instance, transposable element genes sometimes acquire new functions that do benefit the organism. Because they are difficult to distinguish from transposable elements, little is known about these “domesticated genes.” Although studies have attempted to identify them computationally, the predictions have not been verified experimentally. Here, we examine some of the first domesticated genes to be predicted computationally, the MUSTANG family of plant genes. We show that the predictions were correct: MUSTANGs are, like ordinary genes, functional. MUSTANG mutations result in serious defects in how plants grow, flower, and reproduce. Since they are present only in flowering plants, MUSTANG probably originated when flowers first evolved, perhaps taking on a key role. This study is important both because it shows that MUSTANG is critical to plant fitness and because, in the future, a similar approach can be used to find additional domesticated genes and to better understand how transposable elements contribute to evolution.

In most respects, preparation of maize genomic libraries in lambda phage is similar to preparing any other genomic library. We follow the procedures for cloning with lambda phage vectors detailed by Sambrook et al. (1989) with the variations described below. We focus here on those aspects that may require special attention when using maize DNA. Please note the “Other Considerations” before beginning to clone.

Members of a new inverted repeat element family, named Stowaway, have been found in close association with more than 40 monocotyledonous and dicotyledonous plant genes listed in the GenBank and EMBL nucleic acid data bases. Stowaway elements are characterized by a conserved terminal inverted repeat, small size, target site specificity (TA), and potential to form stable DNA secondary structures. Some elements are located at the extreme 3' ends of sequenced cDNAs and supply polyadenylation signals to their host genes. Other elements are in the 5' upstream regions of severa1 genes and appear to contain previously identlfied cis-acting regulatory domains. Although the Stowaway elements share many structural features with the recently discovered Tourist elements, the two families share no significant sequence similarity. Together, the Stowaway and Tourist families serve to define an important new class of short inverted repeat elements found in possibly all flowering plant genomes.