Ty1-copia retrotransposon-like elements in chickpea genome: their identification, distribution and use for diversity analysis

Retrotransposons: How the continuous evolutionary front shapes plant genomes for response to heat stress

Long terminal repeat retrotransposons (LTR retrotransposons) are the most abundant group of mobile genetic elements in eukaryotic genomes and are essential in organizing genomic architecture and phenotypic variations. The diverse families of retrotransposons are related to retroviruses. As retrotransposable elements are dispersed and ubiquitous, their "copy-out and paste-in" life cycle of replicative transposition leads to new genome insertions without the excision of the original element. The overall structure of retrotransposons and the domains responsible for the various phases of their replication is highly conserved in all eukaryotes. The two major superfamilies of LTR retrotransposons, Ty1/Copia and Ty3/Gypsy, are distinguished and dispersed across the chromosomes of higher plants. Members of these superfamilies can increase in copy number and are often activated by various biotic and abiotic stresses due to retrotransposition bursts. LTR retrotransposons are important drivers of species diversity and exhibit great variety in structure, size, and mechanisms of transposition, making them important putative actors in genome evolution. Additionally, LTR retrotransposons influence the gene expression patterns of adjacent genes by modulating potential small interfering RNA (siRNA) and RNA-directed DNA methylation (RdDM) pathways. Furthermore, comparative and evolutionary analysis of the most important crop genome sequences and advanced technologies have elucidated the epigenetics and structural and functional modifications driven by LTR retrotransposon during speciation. However, mechanistic insights into LTR retrotransposons remain obscure in plant development due to a lack of advancement in high throughput technologies. In this review, we focus on the key role of LTR retrotransposons response in plants during heat stress, the role of centromeric LTR retrotransposons, and the role of LTR retrotransposon markers in genome expression and evolution.

Characterization of a heat-activated retrotransposon in Vigna angularis

In plants, several transposable elements are conserved across species. We found a homolog of ONSEN, which is a heat-activated retrotransposon originally isolated from Arabidopsis thaliana, in Vigna. The ONSEN-like elements (VaONS) were detected in all the analyzed Japanese accessions of Vigna angularis (adzuki bean) by Southern blot analysis. However, VaONS sequences were observed to be polymorphic in the different accessions. Interestingly, extrachromosomal DNA (ecDNA) was detected in some accessions of adzuki bean, indicating the conserved heat-activation of VaONS. Furthermore, we successfully induced retrotransposition of VaONS in adzuki plant regenerated through callus. Findings of our study should provide a new tool for molecular breeding of adzuki bean.

Isolation and characterization of reverse transcriptase fragments of LTR retrotransposons from the genome of Chenopodium quinoa (Amaranthaceae)

High heterogeneity was observed among conserved domains of reverse transcriptase ( rt ) isolated from quinoa. Only one Ty1- copia rt was highly amplified. Reverse transcriptase sequences were located predominantly in pericentromeric region of quinoa chromosomes. The heterogeneity, genomic abundance, and chromosomal distribution of reverse transcriptase (rt)-coding fragments of Ty1-copia and Ty3-gypsy long terminal repeat retrotransposons were analyzed in the Chenopodium quinoa genome. Conserved domains of the rt gene were amplified and characterized using degenerate oligonucleotide primer pairs. Sequence analyses indicated that half of Ty1-copia rt (51 %) and 39 % of Ty3-gypsy rt fragments contained intact reading frames. High heterogeneity among rt sequences was observed for both Ty1-copia and Ty3-gypsy rt amplicons, with Ty1-copia more heterogeneous than Ty3-gypsy. Most of the isolated rt fragments were present in quinoa genome in low copy numbers, with only one highly amplified Ty1-copia rt sequence family. The gypsy-like RNase H fragments co-amplified with Ty1-copia-degenerate primers were shown to be highly amplified in the quinoa genome indicating either higher abundance of some gypsy families of which rt domains could not be amplified, or independent evolution of this gypsy-region in quinoa. Both Ty1-copia and Ty3-gypsy retrotransposons were preferentially located in pericentromeric heterochromatin of quinoa chromosomes. Phylogenetic analyses of newly amplified rt fragments together with well-characterized retrotransposon families from other organisms allowed identification of major lineages of retroelements in the genome of quinoa and provided preliminary insight into their evolutionary dynamics.

Structural characterization of copia-type retrotransposons leads to insights into the marker development in a biofuel crop, Jatropha curcas L

BACKGROUND

Recently, Jatropha curcas L. has attracted worldwide attention for its potential as a source of biodiesel. However, most DNA markers have demonstrated high levels of genetic similarity among and within jatropha populations around the globe. Despite promising features of copia-type retrotransposons as ideal genetic tools for gene tagging, mutagenesis, and marker-assisted selection, they have not been characterized in the jatropha genome yet. Here, we examined the diversity, evolution, and genome-wide organization of copia-type retrotransposons in the Asian, African, and Mesoamerican accessions of jatropha, then introduced a retrotransposon-based marker for this biofuel crop.

RESULTS

In total, 157 PCR fragments that were amplified using the degenerate primers for the reverse transcriptase (RT) domain of copia-type retroelements were sequenced and aligned to construct the neighbor-joining tree. Phylogenetic analysis demonstrated that isolated copia RT sequences were classified into ten families, which were then grouped into three lineages. An in-depth study of the jatropha genome for the RT sequences of each family led to the characterization of full consensus sequences of the jatropha copia-type families. Estimated copy numbers of target sequences were largely different among families, as was presence of genes within 5 kb flanking regions for each family. Five copia-type families were as appealing candidates for the development of DNA marker systems. A candidate marker from family Jc7 was particularly capable of detecting genetic variation among different jatropha accessions. Fluorescence in situ hybridization (FISH) to metaphase chromosomes reveals that copia-type retrotransposons are scattered across chromosomes mainly located in the distal part regions.

CONCLUSION

This is the first report on genome-wide analysis and the cytogenetic mapping of copia-type retrotransposons of jatropha, leading to the discovery of families bearing high potential as DNA markers. Distinct dynamics of individual copia-type families, feasibility of a retrotransposon-based insertion polymorphism marker system in examining genetic variability, and approaches for the development of breeding strategies in jatropha using copia-type retrotransposons are discussed.

Identification and characterization of jute LTR retrotransposons:: Their abundance, heterogeneity and transcriptional activity

Long Terminal Repeat (LTR) retrotransposons constitute a significant part of eukaryotic genomes and play an important role in genome evolution especially in plants. Jute is an important fiber crop with a large genome of 1,250 Mbps. This genome is still mostly unexplored. In this study we aimed at identifying and characterizing the LTR retrotransposons of jute with a view to understanding the jute genome better. In this study, the Reverse Transcriptase domain of Ty1-copia and Ty3-gypsy LTR retrotransposons of jute were amplified by degenerate primers and their expressions were examined by reverse transcription PCR. Copy numbers of reverse transcriptase (RT) genes of Ty1-copia and Ty3-gypsy elements were determined by dot blot analysis. Sequence analysis revealed higher heterogeneity among Ty1-copia retrotransposons than Ty3-gypsy and clustered each of them in three groups. Copy number of RT genes in Ty1-copia was found to be higher than that of Ty3-gypsy elements from dot blot hybridization. Cumulatively Ty1-copia and Ty3-gypsy may constitute around 19% of the jute genome where two groups of Ty1-copia were found to be transcriptionally active. Since the LTR retrotransposons constitute a large portion of jute genome, these findings imply the importance of these elements in the evolution of jute genome.

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

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

Genetic mapping of the apospory-specific genomic region in Pennisetum squamulatum using retrotransposon-based molecular markers

Pennisetum squamulatum reproduces by apomixis, a type of asexual reproduction through seeds. Apomixis in P. squamulatum is transmitted as a dominant Mendelian trait, and a genomic region, the apospory-specific genomic region (ASGR), is sufficient for inheritance of the trait. The ASGR is physically large (>50 Mb), highly heterochromatic, hemizygous, and recombinationally suppressed. These characteristics have hindered high-resolution genetic mapping and map-based cloning of apomixis genes. In this study, the long terminal repeat (LTR) regions of ASGR-abundant retrotransposons in the genome of P. squamulatum and ASGR-linked bacterial artificial chromosome clones were identified and sequenced for designing LTR-specific primers. Two hundred and ninety single-dose sequence specific amplified polymorphism (SSAP) markers were generated from 38 primer combinations. The SSAP markers combined with two previous ASGR-mapped markers were used for genetic linkage analysis and construction of a genetic map resulting in the formation of 27 linkage groups at LOD 10, one of which contained >60% of the SSAP markers. After removing identical markers (identical band scoring) on the largest linkage group, 46 markers were finally used for genetic mapping at LOD 10. The markers distributed across 10 different loci covering 19 cM; however, 45 markers were distributed within 9 cM. Six markers were recovered and sequenced. Five markers were successfully converted into sequence characterized amplified regions (SCARs). Segregation of SCAR markers was not always consistent with the SSAP markers of origin suggesting a greater level of error in the SSAP map resulting in an inflated map distance for the ASGR. One SCAR marker (Pst 56-1205-400) detected expression of an ASGR retrotransposon in root, anther, leaf and ovary of P. squamulatum, although sequencing of the RT-PCR product failed to find a functional open reading frame for the transcript.

The marker SCK13(603) associated with resistance to ascochyta blight in chickpea is located in a region of a putative retrotransposon

The sequence characterized amplified region (SCAR) marker SCK13(603), associated with ascochyta blight resistance in a chickpea recombinant inbred line (RIL) population, was used as anchored sequence for genome walking. The PCRs performed in the walking steps to walk in the same direction produced eight bands in 5' direction and five bands in 3' direction with a length ranking from 530 to 2,871 bp. The assembly of the bands sequences along with the sequence of SCK13(603) resulted in 7,815 bp contig. Blastn analyses showed stretches of DNA sequence mainly distributed from the nucleotides 1,500 to 4,500 significantly similar to Medicago truncatula genomic DNA. Three open reading frames (ORFs) were identified and blastp analysis of predicted amino acids sequences revealed that ORF1, ORF2 and ORF3 had significant similarity to a CCHC zinc finger protein, to an integrase, and to a precursor of the glucoamylase s1/s2, respectively, from M. truncatula. The high homology of the putative proteins derived from ORF1 and ORF2 with retrotransposon proteins and the prediction of the existence of conserved domains usually present in retrotransposon proteins indicate that the marker SCK13(603) is located in a region of a putative retrotransposon. The information generated in this study has contributed to increase the knowledge of this important region for blight resistance in chickpea.

Isolation and characterization of genomic retrotransposon sequences from octoploid strawberry (Fragaria x ananassa Duch.)

Strawberry (Fragaria spp.) is a kind of herbaceous perennial plant that propagates vegetatively. The conserved domains of reverse transcriptase (RT) genes of Ty1-copia and Ty3-gypsy groups of LTR retrotransposons were amplified from the cultivated strawberry (Fragaria x ananassa Duch.). Sequence analysis of clones demonstrated that 5 of 19 Ty1-copia group unique sequences and 2 of 10 Ty3-gypsy unique sequences in F. x ananassa genome possessed either stop codon or frameshift. Ty1-copia group sequences are highly heterogeneous (divergence ranged from 1 to 69.8%), but the Ty3-gypsy group sequences are less (divergence ranged from 1 to 10%). Southern dot blot hybridization result suggested that both of the LTR retrotransposons are present in the genome of cultivated strawberry with high copy number (Ty1-copia group 2,875 Ty3-gypsy group 348). RT-PCR amplification from total RNA, which was extracted from leaves of micropropagated strawberry plants, did not yield either of the RT fragments. This is the first report on the presence of RT sequences of Ty1-copia and Ty3-gypsy group retrotransposons in F. x ananassa genome.

Reverse transcriptase domain sequences from Mungbean (Vigna radiata) LTR retrotransposons: sequence characterization and phylogenetic analysis

The conserved domains of reverse transcriptase (RT) genes of Ty1-copia and Ty3-gypsy groups of long terminal repeat (LTR) retrotransposons were amplified from mungbean (Vigna radiata) genome using degenerate primers, cloned and sequenced. Among these 34% and 65% of respective clones of copia and gypsy RT sequences possessed stop codons or frame-shifts or both. The RT sequences corresponding to both the groups exhibit significant levels of heterogeneity. Presence of mungbean copia and gypsy RT sequences in other papilionoid legumes of the same (Phaseoleae) and different lineages (Loteae, Trifoleae, Cicereae) indicates existence of these elements prior to the radiation of papilionoid legumes and also supports the recent interpretations of close relationship between Phaseoleae and Loteae tribes of Papilionoideae subfamily. On the other hand significant homologies of some mungbean copia as well as gypsy RT sequences with those of unrelated plant species suggest their origin from different plant lineages and also that heterogeneous population of related elements were already existed throughout (even before the divergence of monocot and dicot) the evolution of these genera from their common ancestor.

Retrotransposon populations of Vicia species with varying genome size

The (non-LTR) LINE and Ty3-gypsy-type LTR retrotransposon populations of three Vicia species that differ in genome size (Vicia faba, Vicia melanops and Vicia sativa) have been characterised. In each species the LINE retrotransposons comprise a complex, very heterogeneous set of sequences, while the Ty3-gypsy elements are much more homogeneous. Copy numbers of all three retrotransposon groups (Ty1-copia, Ty3-gypsy and LINE) in these species have been estimated by random genomic sequencing and Southern hybridisation analysis. The Ty3-gypsy elements are extremely numerous in all species, accounting for 18-35% of their genomes. The Ty1-copia group elements are somewhat less abundant and LINE elements are present in still lower amounts. Collectively, 20-45% of the genomes of these three Vicia species are comprised of retrotransposons. These data show that the three retrotransposon groups have proliferated to different extents in members of the Vicia genus and high proliferation has been associated with homogenisation of the retrotransposon population.