Transcriptome variations in hybrids of wild emmer wheat (Triticum turgidum ssp. dicoccoides).

BACKGROUND: Wild emmer wheat is a great candidate to revitalize domesticated wheat genetic diversity. Recent years have seen intensive investigation into the evolution and domestication of wild emmer wheat, including whole-genome DNA and transcriptome sequencing. However, the impact of intraspecific hybridization on the transcriptome of wild emmer wheat has been poorly studied. In this study, we assessed changes in methylation patterns and transcriptomic variations in two accessions of wild emmer wheat collected from two marginal populations, Mt. Hermon and Mt. Amasa, and in their stable F4 hybrid. RESULTS: Methylation-Sensitive Amplified Polymorphism (MSAP) detected significant cytosine demethylation in F4 hybrids vs. parental lines, suggesting potential transcriptome variation. After a detailed analysis, we examined nine RNA-Seq samples, which included three biological replicates from the F4 hybrid and its parental lines. RNA-Seq databases contained approximately 200 million reads, with each library consisting of 15 to 25 million reads. There are a total of 62,490 well-annotated genes in these databases, with 6,602 genes showing differential expression between F4 hybrid and parental lines Mt. Hermon and Mt. Amasa. The differentially expressed genes were classified into four main categories based on their expression patterns. Gene ontology (GO) analysis revealed that differentially expressed genes are associated with DNA/RNA metabolism, photosynthesis, stress response, phosphorylation and developmental processes. CONCLUSION: This study highlights the significant transcriptomic changes resulting from intraspecific hybridization within natural plant populations, which might aid the nascent hybrid in adapting to various environmental conditions.

Cytonuclear interactions modulate the plasticity of photosynthetic rhythmicity and growth in wild barley.

In plants, the contribution of the plasmotype (mitochondria and chloroplast) in controlling the circadian clock plasticity and possible consequences on cytonuclear genetic makeup have yet to be fully elucidated. A genome-wide association study in the wild barley (Hordeum vulgare ssp. spontaneum) B1K collection identified overlap with our previously mapped DRIVERS OF CLOCKS (DOCs) loci in wild-cultivated interspecific population. Moreover, we identified non-random segregation and epistatic interactions between nuclear DOCs loci and the chloroplastic RpoC1 gene, indicating an adaptive value for specific cytonuclear gene combinations. Furthermore, we show that DOC1.1, which harbours the candidate SIGMA FACTOR-B (SIG-B) gene, is linked with the differential expression of SIG-B and CCA1 genes and contributes to the circadian gating response to heat. High-resolution temporal growth and photosynthesis measurements of B1K also link the DOCs loci to differential growth, Chl content and quantum yield. To validate the involvement of the Plastid encoded polymerase (PEP) complex, we over-expressed the two barley chloroplastic RpoC1 alleles in Arabidopsis and identified significant differential plasticity under elevated temperatures. Finally, enhanced clock plasticity of de novo ENU (N-Ethyl-N-nitrosourea) -induced barley rpoB1 mutant further implicates the PEP complex as a key player in regulating the circadian clock output. Overall, this study highlights the contribution of specific cytonuclear interaction between rpoC1 (PEP gene) and SIG-B with distinct circadian timing regulation under heat, and their pleiotropic effects on growth implicate an adaptive value.

Exploitation of epigenetic variation of crop wild relatives for crop improvement and agrobiodiversity preservation.

Crop wild relatives (CWRs) are recognized as the best potential source of traits for crop improvement. However, successful crop improvement using CWR relies on identifying variation in genes controlling desired traits in plant germplasms and subsequently incorporating them into cultivars. Epigenetic diversity may provide an additional layer of variation within CWR and can contribute novel epialleles for key traits for crop improvement. There is emerging evidence that epigenetic variants of functional and/or agronomic importance exist in CWR gene pools. This provides a rationale for the conservation of epigenotypes of interest, thus contributing to agrobiodiversity preservation through conservation and (epi)genetic monitoring. Concepts and techniques of classical and modern breeding should consider integrating recent progress in epigenetics, initially by identifying their association with phenotypic variations and then by assessing their heritability and stability in subsequent generations. New tools available for epigenomic analysis offer the opportunity to capture epigenetic variation and integrate it into advanced (epi)breeding programmes. Advances in -omics have provided new insights into the sources and inheritance of epigenetic variation and enabled the efficient introduction of epi-traits from CWR into crops using epigenetic molecular markers, such as epiQTLs.

Genetic loci mediating circadian clock output plasticity and crop productivity under barley domestication.

Circadian clock rhythms are shown to be intertwined with crop adaptation. To realize the adaptive value of changes in these rhythms under crop domestication and improvement, there is a need to compare the genetics of clock and yield traits. We compared circadian clock rhythmicity based on Chl leaf fluorescence and transcriptomics among wild ancestors, landraces, and breeding lines of barley under optimal and high temperatures. We conducted a genome scan to identify pleiotropic loci regulating the clock and field phenotypes. We also compared the allelic diversity in wild and cultivated barley to test for selective sweeps. We found significant loss of thermal plasticity in circadian rhythms under domestication. However, transcriptome analysis indicated that this loss was only for output genes and that temperature compensation in the core clock machinery was maintained. Drivers of the circadian clock (DOC) loci were identified via genome-wide association study. Notably, these loci also modified growth and reproductive outputs in the field. Diversity analysis indicated selective sweep in these pleiotropic DOC loci. These results indicate a selection against thermal clock plasticity under barley domestication and improvement and highlight the importance of identifying genes underlying for understanding the biochemical basis of crop adaptation to changing environments.

A novel miniature transposon-like element discovered in the coding sequence of a gene that encodes for 5-formyltetrahydrofolate in wheat.

BACKGROUND: Transposable elements (TEs) comprise over 80% of the wheat genome and usually possess unique features for specific super-families and families. However, the role of TEs in wheat evolution and reshaping the wheat genome remains largely unclear. RESULTS: In this study, we discovered a miniature (307 bp in length) TE-like sequence in exon 6 of a gene that encodes for 5-formyltetrahydrofolate, in two accessions of wild emmer wheat (T. turgidum ssp. dicoccoides) and has interfered with the gene translation by creating a shorter reading frame as a result of a stop codon. The sequence that was termed Mariam, does not show any structural similarity to known TEs. It does not possess terminal inverted repeats (TIRs) that would allow us to assign this element to one of the TIR DNA super-families, and it does not possess characteristic features of SINE, such as a Pol-III promotor or a poly-A tail. In-silico analysis of five publicly available genome drafts of Triticum and Aegilops species revealed that Mariam element appears in a very low copy number (1-3 insertions) in diploid wheat species and ~ 12 insertions in tetraploid and hexaploidy wheat species. In addition, Mariam element was found to be unique to wheat, as it was not found in other plant genomes. The dynamic nature of Mariam in the wheat genome was assessed by site-specific PCR analysis and revealed that it retained activity in wild emmer populations in a population-specific manner. CONCLUSIONS: This study provides additional insight into the evolutionary impact of TEs in wheat.

Genome-wide analysis of a recently active retrotransposon, Au SINE, in wheat: content, distribution within subgenomes and chromosomes, and gene associations.

KEY MESSAGE: Here, we show that Au SINE elements have strong associations with protein-coding genes in wheat. Most importantly Au SINE insertion within introns causes allelic variation and might induce intron retention. The impact of transposable elements (TEs) on genome structure and function is intensively studied in eukaryotes, especially in plants where TEs can reach up to 90% of the genome in some cases, such as in wheat. Here, we have performed a genome-wide in-silico analysis using the updated publicly available genome draft of bread wheat (T. aestivum), in addition to the updated genome drafts of the diploid donor species, T. urartu and Ae. tauschii, to retrieve and analyze a non-LTR retrotransposon family, termed Au SINE, which was found to be widespread in plant species. Then, we have performed site-specific PCR and realtime RT-PCR analyses to assess the possible impact of Au SINE on gene structure and function. To this end, we retrieved 133, 180 and 1886 intact Au SINE insertions from T. urartu, Ae. tauschii and T. aestivum genome drafts, respectively. The 1886 Au SINE insertions were distributed in the seven homoeologous chromosomes of T. aestivum, while ~ 67% of the insertions were associated with genes. Detailed analysis of 40 genes harboring Au SINE revealed allelic variation of those genes in the Triticum-Aegilops genus. In addition, expression analysis revealed that both regular transcripts and alternative Au SINE-containing transcripts were simultaneously amplified in the same tissue, indicating retention of Au SINE-containing introns. Analysis of the wheat transcriptome revealed that hundreds of protein-coding genes harbor Au SINE in at least one of their mature splice variants. Au SINE might play a prominent role in speciation by creating transcriptome variation.

Transposable elements generate population-specific insertional patterns and allelic variation in genes of wild emmer wheat (Triticum turgidum ssp. dicoccoides).

BACKGROUND: Natural populations of the tetraploid wild emmer wheat (genome AABB) were previously shown to demonstrate eco-geographically structured genetic and epigenetic diversity. Transposable elements (TEs) might make up a significant part of the genetic and epigenetic variation between individuals and populations because they comprise over 80% of the wild emmer wheat genome. In this study, we performed detailed analyses to assess the dynamics of transposable elements in 50 accessions of wild emmer wheat collected from 5 geographically isolated sites. The analyses included: the copy number variation of TEs among accessions in the five populations, population-unique insertional patterns, and the impact of population-unique/specific TE insertions on structure and expression of genes. RESULTS: We assessed the copy numbers of 12 TE families using real-time quantitative PCR, and found significant copy number variation (CNV) in the 50 wild emmer wheat accessions, in a population-specific manner. In some cases, the CNV difference reached up to 6-fold. However, the CNV was TE-specific, namely some TE families showed higher copy numbers in one or more populations, and other TE families showed lower copy numbers in the same population(s). Furthermore, we assessed the insertional patterns of 6 TE families using transposon display (TD), and observed significant population-specific insertional patterns. The polymorphism levels of TE-insertional patterns reached 92% among all wild emmer wheat accessions, in some cases. In addition, we observed population-specific/unique TE insertions, some of which were located within or close to protein-coding genes, creating allelic variations in a population-specific manner. We also showed that those genes are differentially expressed in wild emmer wheat. CONCLUSIONS: For the first time, this study shows that TEs proliferate in wild emmer wheat in a population-specific manner, creating new alleles of genes, which contribute to the divergent evolution of homeologous genes from the A and B subgenomes.

Evolution of the BBAA component of bread wheat during its history at the allohexaploid level.

Subgenome integrity in bread wheat (Triticum aestivum; BBAADD) makes possible the extraction of its BBAA component to restitute a novel plant type. The availability of such a ploidy-reversed wheat (extracted tetraploid wheat [ETW]) provides a unique opportunity to address whether and to what extent the BBAA component of bread wheat has been modified in phenotype, karyotype, and gene expression during its evolutionary history at the allohexaploid level. We report here that ETW was anomalous in multiple phenotypic traits but maintained a stable karyotype. Microarray-based transcriptome profiling identified a large number of differentially expressed genes between ETW and natural tetraploid wheat (Triticum turgidum), and the ETW-downregulated genes were enriched for distinct Gene Ontology categories. Quantitative RT-PCR analysis showed that gene expression differences between ETW and a set of diverse durum wheat (T. turgidum subsp durum) cultivars were distinct from those characterizing tetraploid cultivars per se. Pyrosequencing revealed that the expression alterations may occur to either only one or both of the B and A homoeolog transcripts in ETW. A majority of the genes showed additive expression in a resynthesized allohexaploid wheat. Analysis of a synthetic allohexaploid wheat and diverse bread wheat cultivars revealed the rapid occurrence of expression changes to the BBAA subgenomes subsequent to allohexaploidization and their evolutionary persistence.

Structure and extent of DNA methylation-based epigenetic variation in wild emmer wheat (T. turgidum ssp. dicoccoides) populations.

BACKGROUND: The genetic structure and differentiation of wild emmer wheat suggests that genetic diversity is eco-geographically structured. However, very little is known about the structure and extent of the heritable epigenetic variation and its influence on local adaptation in natural populations. RESULTS: The structure and extent of the heritable methylation-based epigenetic variation were assessed within and among natural populations of Triticum turgidum ssp. dicoccoides. We used methylation sensitive amplified polymorphism (MSAP) and transposon methylation display (TMD) techniques, to assess the methylation status of random genomic CCGG sites and CCGG sites flanking transposable elements (TEs), respectively. Both techniques were applied to the DNA of 50 emmer accessions which were collected from five different geographically isolated regions. In order to ensure the assessment of heritable epigenetic variation, all accessions were grown under common garden conditions for two generations. In all accessions, the difference in methylation levels of CCGG sites, including CCGG sites that flanked TEs, were not statistically significant and relatively high, ranging between 46 and 76 %. The pattern of methylation was significantly different among accessions, such that clear and statistically significant population-specific methylation patterns were observed. CONCLUSION: In this study, we have observed population-unique heritable methylation patterns in emmer wheat accessions originating from five geographically isolated regions. Our data indicate that methylation-based epigenetic diversity might be eco-geographically structured and might be partly determined by climatic and edaphic factors.

Genome-wide analysis of short interspersed nuclear elements SINES revealed high sequence conservation, gene association and retrotranspositional activity in wheat.

Short interspersed nuclear elements (SINEs) are non-autonomous non-LTR retroelements that are present in most eukaryotic species. While SINEs have been intensively investigated in humans and other animal systems, they are poorly studied in plants, especially in wheat (Triticum aestivum). We used quantitative PCR of various wheat species to determine the copy number of a wheat SINE family, termed Au SINE, combined with computer-assisted analyses of the publicly available 454 pyrosequencing database of T. aestivum. In addition, we utilized site-specific PCR on 57 Au SINE insertions, transposon methylation display and transposon display on newly formed wheat polyploids to assess retrotranspositional activity, epigenetic status and genetic rearrangements in Au SINE, respectively. We retrieved 3706 different insertions of Au SINE from the 454 pyrosequencing database of T. aestivum, and found that most of the elements are inserted in A/T-rich regions, while approximately 38% of the insertions are associated with transcribed regions, including known wheat genes. We observed typical retrotransposition of Au SINE in the second generation of a newly formed wheat allohexaploid, and massive hypermethylation in CCGG sites surrounding Au SINE in the third generation. Finally, we observed huge differences in the copy numbers in diploid Triticum and Aegilops species, and a significant increase in the copy numbers in natural wheat polyploids, but no significant increase in the copy number of Au SINE in the first four generations for two of three newly formed allopolyploid species used in this study. Our data indicate that SINEs may play a prominent role in the genomic evolution of wheat through stress-induced activation.

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.

Copy number variation of transposable elements in Triticum-Aegilops genus suggests evolutionary and revolutionary dynamics following allopolyploidization.

KEY MESSAGE: Here, we report on copy number variation of transposable elements and on the genome-specific proliferation in wheat. In addition, we report on revolutionary and evolutionary dynamics of transposons. Wheat is a valuable model for understanding the involvement of transposable elements (TEs) in speciation as wheat species (Triticum-Aegilops group) have diverged from a common ancestor, have undergone two events of speciation through allopolyploidy, and contain a very high fraction of TEs. However, an unbiased genome-wide examination of TE variation among these species has not been conducted. Our research utilized quantitative real time PCR to assess the relative copy numbers of 16 TE families in various Triticum and Aegilops species. We found (1) high variation and genome-specificity of TEs in wheat species, suggesting they were active throughout the evolution of wheat, (2) neither Ae. searsii nor Ae. speltoides by themselves can be the only contributors of the B genome to wheat, and (3) nonadditive changes in TE quantities in polyploid wheat. This study indicates the apparent involvement of large TEs in creating genetic variation in revolutionary and evolutionary scales following allopolyploidization events, presumably assisting in the diploidization of homeologous chromosomes.

Transcriptional activation of retrotransposons alters the expression of adjacent genes in wheat.

Retrotransposons are a principal component of most eukaryotic genomes, representing roughly 40% of the human genome and 50-80% of some grass genomes. They are usually transcriptionally silent but can be activated under certain stresses. Despite their considerable contribution to genome structure, their impact on the expression of adjacent genes is not well understood. The steady-state transcript levels originating from Wis 2-1A retrotransposons are much higher in newly synthesized wheat amphiploids (two or more diverged genomes in the same nucleus). On activation, both Wis 2-1A long terminal repeats drive the readout synthesis of new transcripts from adjacent sequences including the antisense or sense strands of known genes. Here we report that activation of these antisense or sense transcripts is associated with silencing or activation of the corresponding genes, respectively. These data, together with the abundance of retrotransposons in genomes and their ability to be activated by various signals, support the view of transposons as potential controlling elements.

Mobilization of Stowaway-like MITEs in newly formed allohexaploid wheat species.

Transposable elements (TEs) dominate the genetic capacity of most eukaryotes, especially plants, where they can account for up to 90 % of the genome, such as in wheat. The relationship between TEs and their hosts and the role of TEs in organismal biology are poorly understood. In this study, we have applied next generation sequencing, together with a transposon display technique in order to test whether a Stowaway-like MITE, termed Minos, transposes following allopolyploidization events in wheat. We have generated a 454-pyrosequencing database of Minos-specific amplicons (transposon display products) from a newly formed wheat allohexaploid and its parental lines and retrieved hundreds of novel MITE insertions in the allohexaploid. Clear mobilization of Minos was also seen by site-specific PCR analysis and sequence validation. In addition, using real-time qPCR analysis we observed an insignificant change in the relative quantity of Minos from the expected value of merging the two parental genomes, indicating that, despite its activation, no significant burst in Minos copy number can be seen in the newly formed allohexaploid. Interestingly, we found that CCGG sites surrounding Minos underwent massive hypermethylation following the allohexaploidization process. Our data suggest that MITEs have maintained their capacity for activity throughout the evolution of wheat and might be epigenetically deregulated in the first generations following allopolyploidization.

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.

Analysis of copy-number variation, insertional polymorphism, and methylation status of the tiniest class I (TRIM) and class II (MITE) transposable element families in various rice strains.

Transposable elements (TEs) dominate the genetic capacity of most eukaryotes, especially plants, where they may compose up to 90% of the genome. Many studies, both in plants and animals reported that in fact non-autonomous elements that have lost their protein-coding sequences and became miniature elements were highly associated with genes, and showed a high level of transpositional activity such as mPing family in rice. In this study, we have investigated in detail the copy number, insertional polymorphism and the methylation status of the tiniest LTR retrotransposon family, termed TRIM, in nine rice strains, in comparison with mPing. While TRIM showed similar copy numbers (average of 79 insertions) in all the nine rice strains, the copy number of mPing varied dramatically (ranging from 6 to 203 insertions) in the same strains. Site-specific PCR analysis revealed that ~58% of the TRIM elements have identical insertion sites among the nine rice strains, while none of the mPing elements (100% polymorphism) have identical insertion sites in the same strains. Finally, over 65% of the TRIM insertion sites were cytosine methylated in all nine rice strains, while the level of the methylated mPing insertion sites ranged between 43 and 81.5%. The findings of this study indicate that unlike mPing, TRIM is most probably a fossil TE family in rice. In addition, the data shows that there might be a strong correlation between TE methylation and copy number.

Transposable elements are associated with genome-specific gene expression in bread wheat.

Introduction: Recent studies in wheat emphasized the importance of TEs, which occupy ~85% of the wheat genome, as a major source of intraspecific genetic variation due to their recent activity and involvement in genomic rearrangements. The contribution of TEs to structural and functional variations in bread wheat genes is not fully understood. Methods: Here, publicly available RNA-Seq databases of bread wheat were integrated to identify TE insertions within gene bodies (exons\ introns) and assess the impact of TE insertions on gene expression variations of homoeologs gene groups. Overall, 70,818 homoeologs genes were analyzed: 55,170 genes appeared in each one of the three subgenomes (termed ABD), named triads; 12,640 genes appeared in two of the three subgenomes (in A and B only, termed AB; or in A and D only, termed AD; or in B and D only, termed BD);, named dyads; and 3,008 genes underwent duplication in one of the three subgenomes (two copies in: subgenome A, termed AABD; subgenome B, termed ABBD; or subgenome D, termed ABDD), named tetrads. Results: To this end, we found that ~36% of the 70,818 genes contained at least one TE insertion within the gene body, mostly in triads. Analysis of 14,258 triads revealed that the presence of TE insertion in at least one of the triad genes (7,439 triads) was associated with balanced expression (similar expression levels) between the homoeolog genes. TE insertions within the exon or in the untranslated regions (UTRs) of one or more of the homoeologs in a triad were significantly associated with homoeolog expression bias. Furthermore, we found a statistically significant correlation between the presence\absence of TEs insertions belonging to six TE superfamilies and 17 TE subfamilies and the suppression of a single homoeolog gene. A significant association was observed between the presence of TE insertions from specific superfamilies and the expression of genes that are associated with biotic and abiotic stress responses. Conclusion: Our data strongly indicate that TEs might play a prominent role in controlling gene expression in a genome-specific manner in bread wheat.

Exploiting the miniature inverted-repeat transposable elements insertion polymorphisms as an efficient DNA marker system for genome analysis and evolutionary studies in wheat and related species.

Transposable elements (TEs) constitute ~80% of the complex bread wheat genome and contribute significantly to wheat evolution and environmental adaptation. We studied 52 TE insertion polymorphism markers to ascertain their efficiency as a robust DNA marker system for genetic studies in wheat and related species. Significant variation was found in miniature inverted-repeat transposable element (MITE) insertions in relation to ploidy with the highest number of "full site" insertions occurring in the hexaploids (32.6 ± 3.8), while the tetraploid and diploid progenitors had 22.3 ± 0.6 and 15.0 ± 3.5 "full sites," respectively, which suggested a recent rapid activation of these transposons after the formation of wheat. Constructed phylogenetic trees were consistent with the evolutionary history of these species which clustered mainly according to ploidy and genome types (SS, AA, DD, AABB, and AABBDD). The synthetic hexaploids sub-clustered near the tetraploid species from which they were re-synthesized. Preliminary genotyping in 104 recombinant inbred lines (RILs) showed predominantly 1:1 segregation for simplex markers, with four of these markers already integrated into our current DArT-and SNP-based linkage map. The MITE insertions also showed stability with no single excision observed. The MITE insertion site polymorphisms uncovered in this study are very promising as high-potential evolutionary markers for genomic studies in wheat.

Massive alterations of the methylation patterns around DNA transposons in the first four generations of a newly formed wheat allohexaploid.

Rapid and reproducible genomic changes can be induced during the early stages of the life of nascent allopolyploid species. In a previous study, it was shown that following allopolyploidization, cytosine methylation changes can affect up to 11% of the wheat genome. However, the methylation patterns around transposable elements (TEs) were never studied in detail. We used transposon methylation display (TMD) to assess the methylation patterns of CCGG sites flanking three TE families (Balduin, Apollo, and Thalos) in the first four generations of a newly formed wheat allohexaploid. In addition, transposon display (TD), using a methylation-insensitive restriction enzyme, was applied to search for genomic rearrangements at the TE insertion sites. We observed that up to 54% of CCGG sites flanking the three TE families showed changes in methylation patterns in the first four generations of a newly formed wheat allohexaploid, where hypermethylation was predominant. Over 70% of the changes in TMD patterns occurred in the first two generations of the newly formed allohexaploid. Furthermore, analysis of 555 TE insertion sites by TD and 18 cases by site-specific PCR revealed a full additive pattern in the allohexaploid, an indication for lack of massive rearrangements. These data indicate that following allopolyplodization, DNA-TE insertion sites can undergo a significantly high level of methylation changes compared with methylation changes of other genomic sequences.

Where the Wild Things Are: Transposable Elements as Drivers of Structural and Functional Variations in the Wheat Genome.

Transposable elements (TEs) are major contributors to genome plasticity and thus are likely to have a dramatic impact on genetic diversity and speciation. Recent technological developments facilitated the sequencing and assembly of the wheat genome, opening the gate for whole genome analysis of TEs in wheat, which occupy over 80% of the genome. Questions that have been long unanswered regarding TE dynamics throughout the evolution of wheat, are now being addressed more easily, while new questions are rising. In this review, we discuss recent advances in the field of TE dynamics in wheat and possible future directions.