Genotype-dependent Burst of Transposable Element Expression in Crowns of Hexaploid Wheat (Triticum aestivum L.) during Cold Acclimation.
ABSTRACT: The expression of 1,613 transposable elements (TEs) represented in the Affymetrix Wheat Genome Chip was examined during cold treatment in crowns of four hexaploid wheat genotypes that vary in tolerance to cold and in flowering time. The TE expression profiles showed a constant level of expression throughout the experiment in three of the genotypes. In winter Norstar, the most cold-hardy of the four genotypes, a subset of the TEs showed a burst of expression after vernalization saturation was achieved. About 47% of the TEs were expressed, and both Class I (retrotransposons) and Class II (DNA transposons) types were well represented. Gypsy and Copia were the most represented among the retrotransposons while CACTA and Mariner were the most represented DNA transposons. The data suggests that the Vrn-A1 region plays a role in the stage-specific induction of TE expression in this genotype.
Project description:We analyzed the distribution of 54 families of transposable elements (TEs; transposons, LTR retrotransposons, and non-LTR retrotransposons) in the chromosomes of Drosophila melanogaster, using data from the sequenced genome. The density of LTR and non-LTR retrotransposons (RNA-based elements) was high in regions with low recombination rates, but there was no clear tendency to parallel the recombination rate. However, the density of transposons (DNA-based elements) was significantly negatively correlated with recombination rate. The accumulation of TEs in regions of reduced recombination rate is compatible with selection acting against TEs, as selection is expected to be weaker in regions with lower recombination. The differences in the relationship between recombination rate and TE density that exist between chromosome arms suggest that TE distribution depends on specific characteristics of the chromosomes (chromatin structure, distribution of other sequences), the TEs themselves (transposition mechanism), and the species (reproductive system, effective population size, etc.), that have differing influences on the effect of natural selection acting against the TE insertions.
Project description:Transposable elements (TEs) constitute >80% of the wheat genome but their dynamics and contribution to size variation and evolution of wheat genomes (Triticum and Aegilops species) remain unexplored. In this study, 10 genomic regions have been sequenced from wheat chromosome 3B and used to constitute, along with all publicly available genomic sequences of wheat, 1.98 Mb of sequence (from 13 BAC clones) of the wheat B genome and 3.63 Mb of sequence (from 19 BAC clones) of the wheat A genome. Analysis of TE sequence proportions (as percentages), ratios of complete to truncated copies, and estimation of insertion dates of class I retrotransposons showed that specific types of TEs have undergone waves of differential proliferation in the B and A genomes of wheat. While both genomes show similar rates and relatively ancient proliferation periods for the Athila retrotransposons, the Copia retrotransposons proliferated more recently in the A genome whereas Gypsy retrotransposon proliferation is more recent in the B genome. It was possible to estimate for the first time the proliferation periods of the abundant CACTA class II DNA transposons, relative to that of the three main retrotransposon superfamilies. Proliferation of these TEs started prior to and overlapped with that of the Athila retrotransposons in both genomes. However, they also proliferated during the same periods as Gypsy and Copia retrotransposons in the A genome, but not in the B genome. As estimated from their insertion dates and confirmed by PCR-based tracing analysis, the majority of differential proliferation of TEs in B and A genomes of wheat (87 and 83%, respectively), leading to rapid sequence divergence, occurred prior to the allotetraploidization event that brought them together in Triticum turgidum and Triticum aestivum, <0.5 million years ago. More importantly, the allotetraploidization event appears to have neither enhanced nor repressed retrotranspositions. We discuss the apparent proliferation of TEs as resulting from their insertion, removal, and/or combinations of both evolutionary forces.
Project description:To identify the genes involved in the development of low temperature (LT) tolerance in hexaploid wheat, we examined the global changes in expression in response to cold of the 55,052 potentially unique genes represented in the Affymetrix Wheat Genome microarray. We compared the expression of genes in winter-habit (winter Norstar and winter Manitou) and spring-habit (spring Manitou and spring Norstar)) cultivars, wherein the locus for the vernalization gene Vrn-A1 was swapped between the parental winter Norstar and spring Manitou in the derived near-isogenic lines winter Manitou and spring Norstar. Global expression of genes in the crowns of 3-leaf stage plants cold-acclimated at 6°C for 0, 2, 14, 21, 38, 42, 56 and 70 days was examined.Analysis of variance of gene expression separated the samples by genetic background and by the developmental stage before or after vernalization saturation was reached. Using gene-specific ANOVA we identified 12,901 genes (at p < 0.001) that change in expression with respect to both genotype and the duration of cold-treatment. We examined in more detail a subset of these genes (2,771) where expression was highly influenced by the interaction between these two main factors. Functional assignments using GO annotations showed that genes involved in transport, oxidation-reduction, and stress response were highly represented. Clustering based on the pattern of transcript accumulation identified genes that were up or down-regulated by cold-treatment. Our data indicate that the cold-sensitive lines can up-regulate known cold-responsive genes comparable to that of cold-hardy lines. The levels of expression of these genes were highly influenced by the initial rate and the duration of the gene's response to cold. We show that the Vrn-A1 locus controls the duration of gene expression but not its initial rate of response to cold treatment. Furthermore, we provide evidence that Ta.Vrn-A1 and Ta.Vrt1 originally hypothesized to encode for the same gene showed different patterns of expression and therefore are distinct.This study provides novel insight into the underlying mechanisms that regulate the expression of cold-responsive genes in wheat. The results support the developmental model of LT tolerance gene regulation and demonstrate the complex genotype by environment interactions that determine LT adaptation in winter annual cereals.
Project description:The interactions between transposable elements (TEs) and their hosts constitute one of the most profound co-evolutionary processes found in nature. The population dynamics of TEs depends on factors specific to each TE families, such as the rate of transposition and insertional preference, the demographic history of the host and the genomic landscape. How these factors interact has yet to be investigated holistically. Here we are addressing this question in the green anole (Anolis carolinensis) whose genome contains an extraordinary diversity of TEs (including non-LTR retrotransposons, SINEs, LTR-retrotransposons and DNA transposons). We observed a positive correlation between recombination rate and frequency of TEs and densities for LINEs, SINEs and DNA transposons. For these elements, there was a clear impact of demography on TE frequency and abundance, with a loss of polymorphic elements and skewed frequency spectra in recently expanded populations. On the other hand, some LTR-retrotransposons displayed patterns consistent with a very recent phase of intense amplification. To determine how demography, genomic features and intrinsic properties of TEs interact we ran simulations using SLiM3. We determined that i) short TE insertions are not strongly counter-selected, but long ones are, ii) neutral demographic processes, linked selection and preferential insertion may explain positive correlations between average TE frequency and recombination, iii) TE insertions are unlikely to have been massively recruited in recent adaptation. We demonstrate that deterministic and stochastic processes have different effects on categories of TEs and that a combination of empirical analyses and simulations can disentangle these mechanisms.
Project description: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.
Project description:The availability of the sequenced Drosophila melanogaster genome provides an opportunity to study sequence variation between copies within transposable element families. In this study,we analyzed the 624 copies of 22 transposable element (TE) families (14 LTR retrotransposons, five non-LTR retrotransposons, and three transposons). LTR and non-LTR retrotransposons possessed far fewer divergent elements than the transposons,suggesting that the difference depends on the transposition mechanism. However,there was not a continuous range of divergence of the copies in each class,which were either very similar to the canonical elements,or very divergent from them. This sequence homogeneity among TE family copies matches the theoretical models of the dynamics of these repeated sequences. The sequenced Drosophila genome thus appears to be composed of a mixture of TEs that are still active and of ancient relics that have degenerated and the distribution of which along the chromosomes results from natural selection. This clearly demonstrates that the TEs are highly active within the genome,suggesting that the genetic variability of the Drosophila genome is still being renewed by the action of TEs.
Project description:Noncoding RNAs have been extensively described in plant and animal transcriptomes by using high-throughput sequencing technology. Of these noncoding RNAs, a growing number of long intergenic noncoding RNAs (lincRNAs) have been described in multicellular organisms, however the origins and functions of many lincRNAs remain to be explored. In many eukaryotic genomes, transposable elements (TEs) are widely distributed and often account for large fractions of plant and animal genomes yet the contribution of TEs to lincRNAs is largely unknown. By using strand-specific RNA-sequencing, we profiled the expression patterns of lincRNAs in Arabidopsis, rice and maize, and identified 47 611 and 398 TE-associated lincRNAs (TE-lincRNAs), respectively. TE-lincRNAs were more often derived from retrotransposons than DNA transposons and as retrotransposon copy number in both rice and maize genomes so did TE-lincRNAs. We validated the expression of these TE-lincRNAs by strand-specific RT-PCR and also demonstrated tissue-specific transcription and stress-induced TE-lincRNAs either after salt, abscisic acid (ABA) or cold treatments. For Arabidopsis TE-lincRNA11195, mutants had reduced sensitivity to ABA as demonstrated by longer roots and higher shoot biomass when compared to wild-type. Finally, by altering the chromatin state in the Arabidopsis chromatin remodelling mutant ddm1, unique lincRNAs including TE-lincRNAs were generated from the preceding untranscribed regions and interestingly inherited in a wild-type background in subsequent generations. Our findings not only demonstrate that TE-associated lincRNAs play important roles in plant abiotic stress responses but lincRNAs and TE-lincRNAs might act as an adaptive reservoir in eukaryotes.
Project description:Teleosts are unique among vertebrates, with a wide range of haploid genome sizes in very close lineages, varying from less than 400 mega base pairs (Mb) for pufferfish to over 3000 Mb for salmon. The cause of the difference in genome size remains largely unexplained.In this study, we reveal that the differential success of transposable elements (TEs) correlates with the variation of genome size across four representative teleost species (zebrafish, medaka, stickleback, and tetraodon). The larger genomes represent a higher diversity within each clade (superfamily) and family and a greater abundance of TEs compared with the smaller genomes; zebrafish, representing the largest genome, shows the highest diversity and abundance of TEs in its genome, followed by medaka and stickleback; while the tetraodon, representing the most compact genome, displays the lowest diversity and density of TEs in its genome. Both of Class I (retrotransposons) and Class II TEs (DNA transposons) contribute to the difference of TE accumulation of teleost genomes, however, Class II TEs are the major component of the larger teleost genomes analyzed and the most important contributors to genome size variation across teleost lineages. The hAT and Tc1/Mariner superfamilies are the major DNA transposons of all four investigated teleosts. Divergence distribution revealed contrasting proliferation dynamics both between clades of retrotransposons and between species. The TEs within the larger genomes of the zebrafish and medaka represent relatively stronger activity with an extended time period during the evolution history, in contrast with the very young activity in the smaller stickleback genome, or the very low level of activity in the tetraodon genome.Overall, our data shows that teleosts represent contrasting profiles of mobilomes with a differential density, diversity and activity of TEs. The differences in TE accumulation, dominated by DNA transposons, explain the main size variations of genomes across the investigated teleost species, and the species differences in both diversity and activity of TEs contributed to the variations of TE accumulations across the four teleost species. TEs play major roles in teleost genome evolution.
Project description:Identification of polymorphic transposable elements (TEs) is important because TE polymorphism creates genetic diversity and influences the function of genes in the host genome. However, de novo scanning of polymorphic TEs remains a challenge. Here, we report a novel computational method, called PTEMD (polymorphic TEs and their movement detection), for de novo discovery of genome-wide polymorphic TEs. PTEMD searches highly identical sequences using reads supported breakpoint evidences. Using PTEMD, we identified 14 polymorphic TE families (905 sequences) in rice blast fungus Magnaporthe oryzae, and 68 (10,618 sequences) in maize. We validated one polymorphic TE family experimentally, MoTE-1; all MoTE-1 family members are located in different genomic loci in the three tested isolates. We found that 57.1% (8 of 14) of the PTEMD-detected polymorphic TE families in M. oryzae are active. Furthermore, our data indicate that there are more polymorphic DNA transposons in maize than their counterparts of retrotransposons despite the fact that retrotransposons occupy largest fraction of genomic mass. We demonstrated that PTEMD is an effective tool for identifying polymorphic TEs in M. oryzae and maize genomes. PTEMD and the genome-wide polymorphic TEs in M. oryzae and maize are publically available at http://www.kanglab.cn/blast/PTEMD_V1.02.htm.
Project description:Eukaryotic genomes contain transposable elements (TE) that can move into new locations upon activation. Since uncontrolled transposition of TEs, including the retrotransposons and DNA transposons, can lead to DNA breaks and genomic instability, multiple mechanisms, including heterochromatin-mediated repression, have evolved to repress TE activation. Studies in model organisms have shown that TEs become activated upon aging as a result of age-associated deregulation of heterochromatin. Considering that different organisms or cell types may undergo distinct heterochromatin changes upon aging, it is important to identify pathways that lead to TE activation in specific tissues and cell types. Through deep sequencing of isolated RNAs, we report an increased expression of many retrotransposons in the old Drosophila fat body, an organ equivalent to the mammalian liver and adipose tissue. This de-repression correlates with an increased number of DNA damage foci and decreased level of Drosophila lamin-B in the old fat body cells. Depletion of the Drosophila lamin-B in the young or larval fat body results in a reduction of heterochromatin and a corresponding increase in retrotransposon expression and DNA damage. Further manipulations of lamin-B and retrotransposon expression suggest a role of the nuclear lamina in maintaining the genome integrity of the Drosophila fat body by repressing retrotransposons.