Plant transposable elements: Their role in evolution

Transposable elements (TE) are natural constituents of plant genomes. However, their presence only becomes apparent if they become dislodged from their resident positions in the genome and transpore into another gene, thereby inducing a mutation. Such TE-induced mutations are somatically unstable because they revert to wild type and hence reconstitute the expression of the mutated gene. The frequent somatic excision of the TE results in a variegated phenotype. Since this instability is inherited in a Mendelian manner the variegated phenotype is nuclear determined. By this criterion TE have been shown to occur in more than 30 species belonging to different families and genera. Many questions arise when dealing with TE: their structure and functions, and the biological significance of the activity of elements in the differentiation of a normal plant or in the evolution of plant genes.     

Bursts of transposable elements as an evolutionary driving force

A burst of transposable elements (TEs) is a massive outbreak that may cause radical genomic rebuilding. This phenomenon has been reported in connection with the formation of taxonomic groups and species and has therefore been associated with major evolutionary events in the past. Over the past few years, several research groups have discovered recent stress‐induced bursts of different TEs. The events for which bursts of TEs have been recorded include domestication, polyploidy, changes in mating systems, interspecific and intergeneric hybridization and abiotic stress. Cases involving abiotic stress, particularly bursts of TEs in natural populations driven by environmental change, are of special interest because this phenomenon may underlie micro‐ and macro‐evolutionary events and ultimately support the maintenance and generation of biological diversity. This study reviews the known cases of bursts of TEs and their possible consequences, with particular emphasis on the speciation process.     

Sectorial mutagenesis by transposable elements

Transposable elements (TEs) generate insertions and cause other mutations in the genomic DNA. It is proposed that during co-evolution between TEs and eukaryotic genomes, an optimal path of the insertion mutagenesis is determined by the surviving TEs. These TEs can become semi-permanently established, chromatin-regulated ‘source’ or ‘mutator genes’, responsible for targeting insertion mutations to specific chromosomal regions. Such mutations can manifest themselves in non-random distribution patterns of interspersed repeats in eukaryotic chromosomes. In this paper we discuss specific models, examples and implications of optimized mutagenesis in eukaryotes. This revised version was published online in July 2006 with corrections to the Cover Date.     

真核生物转座子鉴定和分类计算方法

重复序列是真核生物基因组的重要组成成分,根据其序列特征及在基因组中的存在形式,可以进一步分为串联重复、片段重复和散在重复。其中,散在重复大多起源于转座子。根据转座介质的不同,转座子又可分为DNA和逆转录转座子。转座子的转座和扩增对基因的进化和基因组的稳定具有显著的影响;同时与其他类型的重复序列相比,转座子的结构和分类更为复杂多样,使得对转座子的鉴定和分类更为复杂和困难。鉴于此,文章简要概括了转座子的功能及分类,总结了真核生物转座子鉴定、分类和注释的3个步骤:(1)重复序列库的构建;(2)重复序列的校正和分类;(3)基因组注释。着重介绍了每一步骤所采用的不同计算方法,比较了不同方法的优缺点。只有把多种方法结合起来使用才能实现全基因组转座子的精确鉴定、分类和注释,这将为转座子的全基因组鉴定和分类提供借鉴意义。     

Identifying co‐opted transposable elements using comparative epigenomics

The human genome gives rise to different epigenomic landscapes that define each cell type and can be deregulated in disease. Recent efforts by ENCODE, the NIH Roadmap and the International Human Epigenome Consortium (IHEC) have made significant advances towards assembling reference epigenomic maps of various tissues. Notably, these projects have found that approximately 80% of human DNA was biochemically active in at least one epigenomic assay while only approximately 10% of the sequence displayed signs of purifying selection. Given that transposable elements (TEs) make up at least 50% of the human genome and can be actively transcribed or act as regulatory elements either for their own purposes or be co‐opted for the benefit of their host; we are interested in exploring their overall contribution to the “functional” genome. Traditional methods used to identify functional DNA have relied on comparative genomics, conservation analysis and low throughput validation assays. To discover co‐opted TEs, and distinguish them from noisy genomic elements, we argue that comparative epigenomic methods will also be important.     

The sunflower (Helianthus annuus L.) genome reflects a recent history of biased accumulation of transposable elements

Aside from polyploidy, transposable elements are the major drivers of genome size increases in plants. Thus, understanding the diversity and evolutionary dynamics of transposable elements in sunflower (Helianthus annuus L.), especially given its large genome size (～3.5 Gb) and the well‐documented cases of amplification of certain transposons within the genus, is of considerable importance for understanding the evolutionary history of this emerging model species. By analyzing approximately 25% of the sunflower genome from random sequence reads and assembled bacterial artificial chromosome (BAC) clones, we show that it is composed of over 81% transposable elements, 77% of which are long terminal repeat (LTR) retrotransposons. Moreover, the LTR retrotransposon fraction in BAC clones harboring genes is disproportionately composed of chromodomain‐containing Gypsy LTR retrotransposons (‘chromoviruses’), and the majority of the intact chromoviruses contain tandem chromodomain duplications. We show that there is a bias in the efficacy of homologous recombination in removing LTR retrotransposon DNA, thereby providing insight into the mechanisms associated with transposable element (TE) composition in the sunflower genome. We also show that the vast majority of observed LTR retrotransposon insertions have likely occurred since the origin of this species, providing further evidence that biased LTR retrotransposon activity has played a major role in shaping the chromatin and DNA landscape of the sunflower genome. Although our findings on LTR retrotransposon age and structure could be influenced by the selection of the BAC clones analyzed, a global analysis of random sequence reads indicates that the evolutionary patterns described herein apply to the sunflower genome as a whole.     

Constitutive heterochromatin and transposable elements in Drosophila melanogaster

Several families of transposable elements (TEs), most of them belonging to the retrotransposon catagory, are particularly enriched in Drosophila melanogaster constitutive heterochromatin. The enrichment of TE-homologous sequences into heterochromatin is not a peculiar feature of the Drosophila genome, but appears to be widespread among higher eukaryotes. The constitutive heterochromatin of D. melanogaster contains several genetically active domains; this raises the possibility that TE-homologous sequences inserted into functional heterochromatin compartments may be expressed. In this review, I present available data on the genetic and molecular organization of D. melanogaster constitutive heterochromatin and its relationship with transposable elements. The implications of these findings on the possible impact of heterochromatic TEs on the function and evolution of the host genome are also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Colonization of heterochromatic genes by transposable elements in Drosophila

As a further step toward understanding transposable element-host genome interactions, we investigated the molecular anatomy of introns from five heterochromatic and 22 euchromatic protein-coding genes of Drosophila melanogaster. A total of 79 kb of intronic sequences from heterochromatic genes and 355 kb of intronic sequences from euchromatic genes have been used in Blast searches against Drosophila transposable elements (TEs). The results show that TE-homologous sequences belonging to 19 different families represent about 50% of intronic DNA from heterochromatic genes. In contrast, only 0.1% of the euchromatic intron DNA exhibits homology to known TEs. Intraspecific and interspecific size polymorphisms of introns were found, which are likely to be associated with changes in TE-related sequences. Together, the enrichment in TEs and the apparent dynamic state of heterochromatic introns suggest that TEs contribute significantly to the evolution of genes located in heterochromatin.     

A novel class of <Emphasis Type="Italic">Helitron</Emphasis>- related transposable elements in maize contain portions of multiple pseudogenes

We recently described a maize mutant caused by an insertion of a Helitron type transposable element (Lal, S.K., Giroux, M.J., Brendel, V., Vallejos, E. and Hannah, L.C., 2003, Plant Cell, 15: 381–391). Here we describe another Helitron insertion in the barren stalk1 gene of maize. The termini of a 6525 bp insertion in the proximal promoter region of the mutant reference allele of maize barren stalk1 gene (ba1-ref) shares striking similarity to the Helitron insertion we reported in the Shrunken-2 gene. This insertion is embedded with pseudogenes that differ from the pseudogenes discovered in the mutant Shrunken-2 insertion. Using the common terminal ends of the mutant insertions as a query, we discovered other Helitron insertions in maize BAC clones. Based on the comparison of the insertion site and PCR amplified genomic sequences, these elements inserted between AT dinucleotides. These putative non-autonomous Helitroninsertions completely lacked sequences similar to RPA (replication protein A) and DNA Helicases reported in other species. A blastn analysis indicated that both the 5 and 3 termini of Helitrons are repeated in the maize genome. These data provide strong evidence that Helitron type transposable elements are active and may have played an essential role in the evolution and expansion of the maize genome.     

Distinguishing ecological from evolutionary approaches to transposable elements

Considerable variation exists not only in the kinds of transposable elements (TEs) occurring within the genomes of different species, but also in their abundance and distribution. Noting a similarity to the assortment of organisms among ecosystems, some researchers have called for an ecological approach to the study of transposon dynamics. However, there are several ways to adopt such an approach, and it is sometimes unclear what an ecological perspective will add to the existing co‐evolutionary framework for explaining transposon‐host interactions. This review aims to clarify the conceptual foundations of transposon ecology in order to evaluate its explanatory prospects. We begin by identifying three unanswered questions regarding the abundance and distribution of TEs that potentially call for an ecological explanation. We then offer an operational distinction between evolutionary and ecological approaches to these questions. By determining the amount of variance in transposon abundance and distribution that is explained by ecological and evolutionary factors, respectively, it is possible empirically to assess the prospects for each of these explanatory frameworks. To illustrate how this methodology applies to a concrete example, we analyzed whole‐genome data for one set of distantly related mammals and another more closely related group of arthropods. Our expectation was that ecological factors are most informative for explaining differences among individual TE lineages, rather than TE families, and for explaining their distribution among closely related as opposed to distantly related host genomes. We found that, in these data sets, ecological factors do in fact explain most of the variation in TE abundance and distribution among TE lineages across less distantly related host organisms. Evolutionary factors were not significant at these levels. However, the explanatory roles of evolution and ecology become inverted at the level of TE families or among more distantly related genomes. Not only does this example demonstrate the utility of our distinction between ecological and evolutionary perspectives, it further suggests an appropriate explanatory domain for the burgeoning discipline of transposon ecology. The fact that ecological processes appear to be impacting TE lineages over relatively short time scales further raises the possibility that transposons might serve as useful model systems for testing more general hypotheses in ecology.     

植物中活性MITEs转座子研究进展

MITEs(Miniature inverted-repeat transposable elements)转座子是一种特殊的转座子,其既有DNA转座子的转座特性——"剪切-粘贴"转座方式,又有RNA转座子的高拷贝特性。目前已被报道的MITEs种类和数量虽然很多,但是关于有转座活性的MITEs的报道却甚少。本文总结了近几年来有关活性MITEs的相关报道,发现具有转座活性的MITEs种类大都分布在Tourist家族,分别是m Ping、m Ging、Ph Tourist1、Tmi1和Ph Tst-3,另外还有Stowaway-like家族的d Tstu1和MITE-39以及Mutator家族的Ah MITE1。文中还分析了这些活性MITEs的结构(TIR和TSD)、拷贝数、进化模式以及转座特性等,为鉴定其他活性MITEs以及MITEs转座和扩增机制的研究奠定了基础。     

转座因子和宿主基因组的进化

转座因子主要是一些“自在”或“无功能”的DNA,其对宿主进化无关紧要的观点受到了质疑。新近的报道指出,它们有增强宿主基因组自身进化,对环境变化作出反应的潜在能力,很可能是遗传多样性的主要源泉。     

Discovering and detecting transposable elements in genome sequences

The contribution of transposable elements (TEs) to genome structure and evolution as well as their impact on genome sequencing, assembly, annotation and alignment has generated increasing interest in developing new methods for their computational analysis. Here we review the diversity of innovative approaches to identify and annotate TEs in the post-genomic era, covering both the discovery of new TE families and the detection of individual TE copies in genome sequences. These approaches span a broad spectrum in computational biology including de novo, homology-based, structure-based and comparative genomic methods. We conclude that the integration and visualization of multiple approaches and the development of new conceptual representations for TE annotation will further advance the computational analysis of this dynamic component of the genome.     

CRISPR相关转座元件的研究及应用进展

CRISPR-Cas的基因编辑能力引发了人们对该系统的研究热潮。除了实现基因的敲除和插入,CRISPR-Cas系统还可以被应用于基因簇重组、单碱基编辑和基因转录调控,推动了生物工程领域的发展。然而,有限的同源重组效率使CRISPR-Cas系统的应用受到了一定的限制。与CRISPR-Cas系统相比,移动遗传元件(mobile genetic elements,MGE)在转座酶的调控下,不需要依赖同源重组即可将指定DNA片段定向插入到细胞染色体中。近几年,人们发现了具有转座机制的CRISPR相关的转座元件,它可以介导DNA靶向整合,同时其出色的重编程能力为该领域的研究带来了新的发展。本文主要介绍近年来CRISPR-Cas系统相关转座元件的研究方向和应用进展,以及人工融合的dCas9-transposase系统的应用策略。文中还提出了CRISPR相关转座元件未来的应用前景和潜在挑战,为基因编辑工具的发展方向提供了参考意见。     

Amplification dynamics of miniature inverted-repeat transposable elements and their impact on rice trait variability

Transposable elements (TEs) are a rich source of genetic variability. Among TEs, miniature inverted-repeat TEs (MITEs) are of particular interest as they are present in high copy numbers in plant genomes and are closely associated with genes. MITEs are deletion derivatives of class II transposons, and can be mobilized by the transposases encoded by the latter through a typical cut-and-paste mechanism. However, MITEs are typically present at much higher copy numbers than class II transposons. We present here an analysis of 103 109 transposon insertion polymorphisms (TIPs) in 738 Oryza sativa genomes representing the main rice population groups. We show that an important fraction of MITE insertions has been fixed in rice concomitantly with its domestication. However, another fraction of MITE insertions is present at low frequencies. We performed MITE TIP-genome-wide association studies (TIP-GWAS) to study the impact of these elements on agronomically important traits and found that these elements uncover more trait associations than single nucleotide polymorphisms (SNPs) on important phenotypes such as grain width. Finally, using SNP-GWAS and TIP-GWAS we provide evidence of the replicative amplification of MITEs.     

The rice R gene family: two distinct subfamilies containing several miniature inverted-repeat transposable elements

The R and B genes of maize regulate the anthocyanin biosynthetic pathway and constitute a small gene family whose evolution has been shaped by polyploidization and transposable element activity. To compare the evolution of regulatory genes in the distinct but related genomes of rice and maize, we previously isolated two R homologues from rice (Oryza sativa). The Ra1 gene on chromosome 4 can activate the anthocyanin pathway, whereas the Rb gene, of undetermined function, maps to chromosome 1. In this study, rice R genes have been further characterized. First, we found that an Rb cDNA can induce pigmentation in maize suspension cells. Second, another rice R homologue (Ra2) was identified that is more closely related to Ra1 than to Rb. Domesticated rice and its wild relatives harbor multiple Ra-like and Rb-like genes despite the fact that rice is a true diploid with the smallest genome of all the grass species analyzed to date. Finally, several miniature inverted-repeat transposable elements (MITEs) were found in R family members. Their possible role in hastening the divergence of R genes is discussed.     

Population genetics models of transposable elements

The control of transposable element copy number is of considerable theoretical and empirical interest. Under simple models, copy numbers may increase without limit. Mechanisms that can prevent such an increase include those in which the effect of selection increases with copy number, those in which the rate of transposition decreases with copy number, and those where unlimited increase in copy number is prevented by the consequences of functional heterogeneity in the transposable element family. Finite population sizes may attenuate the power of natural selection to act on transposable element copy number in a number of ways that may be of particular importance in laboratory populations. First, a small host population size will create occasional periods in which the variance between individuals in copy number is diminished, and with it the power of natural selection, even when the expected variance is Poisson. Second, small population sizes will produce high-frequency transposable element sites, systematically reducing the variance in copy number. The consequences will be particularly profound when the selective damage of transposable elements follows from their heterozygosity, as when ectopic exchange limits copy number. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genome and stresses: Reactions against aggressions,behavior of transposable elements

The action of stresses on the genome can be considered as responses of cells or organisms to external aggressions. Stress factors are of environmental origin (climatic or trophic) or of genomic nature (introduction of foreign genetic material, for example). In both cases, important perturbations can occur and modify hereditary potentialities, creating new combinations compatible with survival; such a situation may increase the variability of the genome, and allow evolutive processes to take place. The behavior of transposable elements under stress conditions is thus of particular interest, since these sequences are sources of mutations and therefore of genetic variability; they may play an important role in population adaptation. The survey of the available experimental results suggests that, although some examples of mutations and transposable elements movements induced by external factors are clearly described, environmental injuries or introduction of foreign material into a genome are not systematically followed by drastic genomic changes.