Retrotransposon evolution in diverse plant genomes

Unveiling the distinctive traits of functional rye centromeres: minisatellites, retrotransposons, and R-loop formation

Centromeres play a vital role in cellular division by facilitating kinetochore assembly and spindle attachments. Despite their conserved functionality, centromeric DNA sequences exhibit rapid evolution, presenting diverse sizes and compositions across species. The functional significance of rye centromeric DNA sequences, particularly in centromere identity, remains unclear. In this study, we comprehensively characterized the sequence composition and organization of rye centromeres. Our findings revealed that these centromeres are primarily composed of long terminal repeat retrotransposons (LTR-RTs) and interspersed minisatellites. We systematically classified LTR-RTs into five categories, highlighting the prevalence of younger CRS1, CRS2, and CRS3 of CRSs (centromeric retrotransposons of Secale cereale) were primarily located in the core centromeres and exhibited a higher association with CENH3 nucleosomes. The minisatellites, mainly derived from retrotransposons, along with CRSs, played a pivotal role in establishing functional centromeres in rye. Additionally, we observed the formation of R-loops at specific regions of CRS1, CRS2, and CRS3, with both rye pericentromeres and centromeres exhibiting enrichment in R-loops. Notably, these R-loops selectively formed at binding regions of the CENH3 nucleosome in rye centromeres, suggesting a potential role in mediating the precise loading of CENH3 to centromeres and contributing to centromere specification. Our work provides insights into the DNA sequence composition, distribution, and potential function of R-loops in rye centromeres. This knowledge contributes valuable information to understanding the genetics and epigenetics of rye centromeres, offering implications for the development of synthetic centromeres in future plant modifications and beyond.

Dominance of transposable element-related satDNAs results in great complexity of "satDNA library" and invokes the extension towards "repetitive DNA library"

Research on bivalves is fast-growing, including genome-wide analyses and genome sequencing. Several characteristics qualify oysters as a valuable model to explore repetitive DNA sequences and their genome organization. Here we characterize the satellitomes of five species in the family Ostreidae (Crassostrea angulata, C. virginica, C. hongkongensis, C. ariakensis, Ostrea edulis), revealing a substantial number of satellite DNAs (satDNAs) per genome (ranging between 33 and 61) and peculiarities in the composition of their satellitomes. Numerous satDNAs were either associated to or derived from transposable elements, displaying a scarcity of transposable element-unrelated satDNAs in these genomes. Due to the non-conventional satellitome constitution and dominance of Helitron-associated satDNAs, comparative satellitomics demanded more in-depth analyses than standardly employed. Comparative analyses (including C. gigas, the first bivalve species with a defined satellitome) revealed that 13 satDNAs occur in all six oyster genomes, with Cg170/HindIII satDNA being the most abundant in all of them. Evaluating the "satDNA library model" highlighted the necessity to adjust this term when studying tandem repeat evolution in organisms with such satellitomes. When repetitive sequences with potential variation in the organizational form and repeat-type affiliation are examined across related species, the introduction of the terms "TE library" and "repetitive DNA library" becomes essential.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-024-00218-0.

Characterization of centromeric DNA of Gossypium anomalum reveals sequence-independent enrichment dynamics of centromeric repeats

Centromeres in eukaryotes are composed of highly repetitive DNAs, which evolve rapidly and are thought to achieve a favorable structure in mature centromeres. However, how the centromeric repeat evolves into an adaptive structure is largely unknown. We characterized the centromeric sequences of Gossypium anomalum through chromatin immunoprecipitation against CENH3 antibodies. We revealed that the G. anomalum centromeres contained only retrotransposon-like repeats but were depleted in long arrays of satellites. These retrotransposon-like centromeric repeats were present in the African-Asian and Australian lineage species, suggesting that they might have arisen in the common ancestor of these diploid species. Intriguingly, we observed a substantial increase and decrease in copy numbers among African-Asian and Australian lineages, respectively, for the retrotransposon-derived centromeric repeats without apparent structure or sequence variation in cotton. This result indicates that the sequence content is not a decisive aspect of the adaptive evolution of centromeric repeats or at least retrotransposon-like centromeric repeats. In addition, two active genes with potential roles in gametogenesis or flowering were identified in CENH3 nucleosome-binding regions. Our results provide new insights into the constitution of centromeric repetitive DNA and the adaptive evolution of centromeric repeats in plants.

Satellite DNAs-From Localized to Highly Dispersed Genome Components

According to the established classical view, satellite DNAs are defined as abundant non-coding DNA sequences repeated in tandem that build long arrays located in heterochromatin. Advances in sequencing methodologies and development of specialized bioinformatics tools enabled defining a collection of all repetitive DNAs and satellite DNAs in a genome, the repeatome and the satellitome, respectively, as well as their reliable annotation on sequenced genomes. Supported by various non-model species included in recent studies, the patterns of satellite DNAs and satellitomes as a whole showed much more diversity and complexity than initially thought. Differences are not only in number and abundance of satellite DNAs but also in their distribution across the genome, array length, interspersion patterns, association with transposable elements, localization in heterochromatin and/or in euchromatin. In this review, we compare characteristic organizational features of satellite DNAs and satellitomes across different animal and plant species in order to summarize organizational forms and evolutionary processes that may lead to satellitomes' diversity and revisit some basic notions regarding repetitive DNA landscapes in genomes.

Non-B-form DNA tends to form in centromeric regions and has undergone changes in polyploid oat subgenomes

Centromeres are the specialized regions of the chromosomes that direct faithful chromosome segregation during cell division. Despite their functional conservation, centromeres display features of rapidly evolving DNA and wide evolutionary diversity in size and organization. Previous work found that the noncanonical B-form DNA structures are abundant in the centromeres of several eukaryotic species with a possible implication for centromere specification. Thus far, systematic studies into the organization and function of non-B-form DNA in plants remain scarce. Here, we applied the oat system to investigate the role of non-B-form DNA in centromeres. We conducted chromatin immunoprecipitation sequencing using an antibody to the centromere-specific histone H3 variant (CENH3); this accurately positioned oat centromeres with different ploidy levels and identified a series of centromere-specific sequences including minisatellites and retrotransposons. To define genetic characteristics of oat centromeres, we surveyed the repeat sequences and found that dyad symmetries were abundant in oat centromeres and were predicted to form non-B-DNA structures in vivo. These structures including bent DNA, slipped DNA, Z-DNA, G-quadruplexes, and R-loops were prone to form within CENH3-binding regions. Dynamic conformational changes of predicted non-B-DNA occurred during the evolution from diploid to tetraploid to hexaploid oat. Furthermore, we applied the single-molecule technique of AFM and DNA:RNA immunoprecipitation with deep sequencing to validate R-loop enrichment in oat centromeres. Centromeric retrotransposons exhibited strong associations with R-loop formation. Taken together, our study elucidates the fundamental character of non-B-form DNA in the oat genome and reveals its potential role in centromeres.

Identification of potential new mosquito-associated viruses of adult Aedes aegypti mosquitoes from Tocantins state, Brazil

Medically important arboviruses such as dengue virus (DENV), Zika virus (ZIKV), and chikungunya virus (CHIKV) are primarily transmitted by the globally distributed mosquito Aedes aegypti. Increasing evidence suggests that the transmission of some viruses can be influenced by mosquito-specific and mosquito-borne viruses. Advancements in high-throughput sequencing (HTS) and bioinformatics have expanded our knowledge on the richness of viruses harbored by mosquitoes. HTS was used to characterize the presence of virus sequences in wild-caught adult Ae. aegypti from Tocantins (TO) state, Brazil. Samples of mosquitoes were collected in four cities of Tocantins state and submitted to RNA isolation, followed by sequencing at an Illumina HiSeq platform. Our results showed initially by Krona the presence of 3% of the sequenced reads belonging to the viral database. After further analysis, the virus sequences were found to have homology to two viral families found in insects Phenuiviridae and Metaviridae. Three possible viral strains including putative new viruses were detected and named Phasi Charoen-like phasivirus isolate To-1 (PCLV To-1), Aedes aegypti To virus 1 (AAToV1), and Aedes aegypti To virus 2 (AAToV2). The results presented in this work contribute to the growing knowledge about the diversity of viruses in mosquitoes and might be useful for future studies on the interaction between insect-specific viruses and arboviruses.

The formation and evolution of centromeric satellite repeats in Saccharum species

Centromeres in eukaryotes are composed of tandem DNAs and retrotransposons. However, centromeric repeats exhibit considerable diversity, even among closely related species, and their origin and evolution are largely unknown. We conducted a genome-wide characterization of the centromeric sequences in sugarcane (Saccharum officinarum). Four centromeric tandem repeat sequences, So1, So103, So137 and So119, were isolated. So1 has a monomeric length of 137 bp, typical of a centromeric satellite, and has evolved four variants. However, these So1 variants had distinct centromere distributions and some were unique to an individual centromere. The distributions of the So1 variants were unexpectedly consistent among the Saccharum species that had different basic chromosome numbers or ploidy levels, thus suggesting evolutionary stability for approximately 7 million years in sugarcane. So103, So137 and So119 had unusually longer monomeric lengths that ranged from 327 to 1371 bp and lacked translational phasing on the CENH3 nucleosomes. Moreover, So103, So137 and So119 seemed to be highly similar to retrotransposons, which suggests that they originated from these mobile elements. Notably, all three repeats were flanked by direct repeats, and formed extrachromosomal circular DNAs (eccDNAs). The presence of circular molecules for these retrotransposon-derived centromeric satellites suggests an eccDNA-mediated centromeric satellite formation pathway in sugarcane.

ChIP-cloning analysis uncovers centromere-specific retrotransposons in Brassica nigra and reveals their rapid diversification in Brassica allotetraploids

Centromeres are indispensable functional units of chromosomes. The evolutionary mechanisms underlying the rapid evolution of centromeric repeats, especially those following polyploidy, remain unknown. In this study, we isolated centromeric sequences of Brassica nigra, a model diploid progenitor (B genome) of the allopolyploid species B. juncea (AB genome) and B. carinata (BC genome) by chromatin immunoprecipitation of nucleosomes containing the centromere-specific histone CENH3. Sequence analysis detected no centromeric satellite DNAs, and most B. nigra centromeric repeats were found to originate from Tyl/copia-class retrotransposons. In cytological analyses, six of the seven analyzed repeat clusters had no FISH signals in A or C genomes of the related diploid species B. rapa and B. oleracea. Notably, five repeat clusters had FISH signals in both A and B subgenomes in the tetraploid B. juncea. In the tetraploid B. carinata, only CL23 displayed three pairs of signals in terminal or interstitial regions of the C-derived chromosome, and no evidence of colonization of CLs onto C-subgenome centromeres was found in B. carinata. This observation suggests that centromeric repeats spread and proliferated between genomes after polyploidization. CL3 and CRB are likely ancient centromeric sequences arising prior to the divergence of diploid Brassica which have detected signals across the genus. And in allotetraploids B. juncea and B. carinata, the FISH signal intensity of CL3 and CRB differed among subgenomes. We discussed possible mechanisms for centromeric repeat divergence during Brassica speciation and polyploid evolution, thus providing insights into centromeric repeat establishment and targeting.

Analysis of retrotransposon abundance, diversity and distribution in holocentric Eleocharis (Cyperaceae) genomes

BACKGROUND AND AIMS

Long terminal repeat-retrotransposons (LTR-RTs) comprise a large portion of plant genomes, with massive repeat blocks distributed across the chromosomes. Eleocharis species have holocentric chromosomes, and show a positive correlation between chromosome numbers and the amount of nuclear DNA. To evaluate the role of LTR-RTs in karyotype diversity in members of Eleocharis (subgenus Eleocharis), the occurrence and location of different members of the Copia and Gypsy superfamilies were compared, covering interspecific variations in ploidy levels (considering chromosome numbers), DNA C-values and chromosomal arrangements.

METHODS

The DNA C-value was estimated by flow cytometry. Genomes of Eleocharis elegans and E. geniculata were partially sequenced using Illumina MiSeq assemblies, which were a source for searching for conserved proteins of LTR-RTs. POL domains were used for recognition, comparing families and for probe production, considering different families of Copia and Gypsy superfamilies. Probes were obtained by PCR and used in fluorescence in situ hybridization (FISH) against chromosomes of seven Eleocharis species.

KEY RESULTS

A positive correlation between ploidy levels and the amount of nuclear DNA was observed, but with significant variations between samples with the same ploidy levels, associated with repetitive DNA fractions. LTR-RTs were abundant in E. elegans and E. geniculata genomes, with a predominance of Copia Sirevirus and Gypsy Athila/Tat clades. FISH using LTR-RT probes exhibited scattered and clustered signals, but with differences in the chromosomal locations of Copia and Gypsy. The diversity in LTR-RT locations suggests that there is no typical chromosomal distribution pattern for retrotransposons in holocentric chromosomes, except the CRM family with signals distributed along chromatids.

CONCLUSIONS

These data indicate independent fates for each LTR-RT family, including accumulation between and within chromosomes and genomes. Differential activity and small changes in LTR-RTs suggest a secondary role in nuclear DNA variation, when compared with ploidy changes.

Exploiting repetitive sequences and BAC clones in Festuca pratensis karyotyping

The Festuca genus is thought to be the most numerous genus of the Poaceae family. One of the most agronomically important forage grasses, Festuca pratensis Huds. is treated as a model plant to study the molecular mechanisms associated with tolerance to winter stresses, including frost. However, the precise mapping of the genes governing stress tolerance in this species is difficult as its karyotype remains unrecognized. Only two F. pratensis chromosomes with 35S and 5S rDNA sequences can be easily identified, but its remaining chromosomes have not been distinguished to date. Here, two libraries derived from F. pratensis nuclear DNA with various contents of repetitive DNA sequences were used as sources of molecular probes for fluorescent in situ hybridisation (FISH), a BAC library and a library representing sequences most frequently present in the F. pratensis genome. Using FISH, six groups of DNA sequences were revealed in chromosomes on the basis of their signal position, including dispersed-like sequences, chromosome painting-like sequences, centromeric-like sequences, knob-like sequences, a group without hybridization signals, and single locus-like sequences. The last group was exploited to develop cytogenetic maps of diploid and tetraploid F. pratensis, which are presented here for the first time and provide a remarkable progress in karyotype characterization.

Obtaining retrotransposon sequences, analysis of their genomic distribution and use of retrotransposon-derived genetic markers in lentil (Lens culinaris Medik.)

Retrotransposons with long terminal repeats (LTR-RTs) are widespread mobile elements in eukaryotic genomes. We obtained a total of 81 partial LTR-RT sequences from lentil corresponding to internal retrotransposon components and LTRs. Sequences were obtained by PCR from genomic DNA. Approximately 37% of the LTR-RT internal sequences presented premature stop codons, pointing out that these elements must be non-autonomous. LTR sequences were obtained using the iPBS technique which amplifies sequences between LTR-RTs. A total of 193 retrotransposon-derived genetic markers, mainly iPBS, were used to obtain a genetic linkage map from 94 F₇ inbred recombinant lines derived from the cross between the cultivar Lupa and the wild ancestor L. culinaris subsp. orientalis. The genetic map included 136 markers located in eight linkage groups. Clusters of tightly linked retrotransposon-derived markers were detected in linkage groups LG1, LG2, and LG6, hence denoting a non-random genomic distribution. Phylogenetic analyses identified the LTR-RT families in which internal and LTR sequences are included. Ty3-gypsy elements were more frequent than Ty1-copia, mainly due to the high Ogre element frequency in lentil, as also occurs in other species of the tribe Vicieae. LTR and internal sequences were used to analyze in silico their distribution among the contigs of the lentil draft genome. Up to 8.8% of the lentil contigs evidenced the presence of at least one LTR-RT similar sequence. A statistical analysis suggested a non-random distribution of these elements within of the lentil genome. In most cases (between 97% and 72%, depending on the LTR-RT type) none of the internal sequences flanked by the LTR sequence pair was detected, suggesting that defective and non-autonomous LTR-RTs are very frequent in lentil. Results support that LTR-RTs are abundant and widespread throughout of the lentil genome and that they are a suitable source of genetic markers useful to carry out further genetic analyses.

Isolation and characterization of centromeric repetitive DNA sequences in Saccharum spontaneum

Sugarcane (Saccharum hybrids spp.) is the most important sugar crop that accounts for ~75% of the world’s sugar production. Recently, a whole-genome sequencing project was launched on the wild species S. spontaneum. To obtain information on the DNA composition of the repeat-enriched region of the centromere, we conducted a genome-wide analysis of the DNA sequences associated with CenH3 (a mutant of histone H3 located in eukaryote centromeres) using chromatin immunoprecipitation followed by sequencing (ChIP-seq) method. We demonstrate that the centromeres contain mainly SCEN-like single satellite repeat (Ss1) and several Ty3/gypsy retrotransposon-related repeats (Ss166, Ss51, and Ss68). Ss1 dominates in the centromeric regions and spans up to 500 kb. In contrast, the Ty3/gypsy retrotransposon-related repeats are either clustered spanning over a short range, or dispersed in the centromere regions. Interestingly, Ss1 exhibits a chromosome-specific enrichment in the wild species S. spontaneum and S. robustum, but not in the domesticated species S. officinarum and modern sugarcane cultivars. This finding suggests an autopolyploid genome identity of S. spontaneum with a high level of homology among its eight sub-genomes. We also conducted a genome-wide survey of the repetitive DNAs in S. spontaneum following a similarity-based sequence clustering strategy. These results provide insight into the composition of sugarcane genome as well as the genome assembly of S. spontaneum.

The Diversity of Sequence and Chromosomal Distribution of New Transposable Element-Related Segments in the Rye Genome Revealed by FISH and Lineage Annotation

Transposable elements (TEs) in plant genomes exhibit a great variety of structure, sequence content and copy number, making them important drivers for species diversity and genome evolution. Even though a genome-wide statistic summary of TEs in rye has been obtained using high-throughput DNA sequencing technology, the accurate diversity of TEs in rye, as well as their chromosomal distribution and evolution, remains elusive due to the repetitive sequence assembling problems and the high dynamic and nested nature of TEs. In this study, using genomic plasmid library construction combined with dot-blot hybridization and fluorescence in situ hybridization (FISH) analysis, we successfully isolated 70 unique FISH-positive TE-related sequences including 47 rye genome specific ones: 30 showed homology or partial homology with previously FISH characterized sequences and 40 have not been characterized. Among the 70 sequences, 48 sequences carried Ty3/gypsy-derived segments, 7 sequences carried Ty1/copia-derived segments and 15 sequences carried segments homologous with multiple TE families. 26 TE lineages were found in the 70 sequences, and among these lineages, Wilma was found in sequences dispersed in all chromosome regions except telomeric positions; Abiba was found in sequences predominantly located at pericentromeric and centromeric positions; Wis, Carmilla, and Inga were found in sequences displaying signals dispersed from distal regions toward pericentromeric positions; except DNA transposon lineages, all the other lineages were found in sequences displaying signals dispersed from proximal regions toward distal regions. A high percentage (21.4%) of chimeric sequences were identified in this study and their high abundance in rye genome suggested that new TEs might form through recombination and nested transposition. Our results also gave proofs that diverse TE lineages were arranged at centromeric and pericentromeric positions in rye, and lineages like Abiba might play a role in their structural organization and function. All these results might help in understanding the diversity and evolution of TEs in rye, as well as their driving forces in rye genome organization and evolution.

Rapid proliferation and nucleolar organizer targeting centromeric retrotransposons in cotton

Centromeric chromatin in most eukaryotes is composed of highly repetitive centromeric retrotransposons and satellite repeats that are highly variable even among closely related species. The evolutionary mechanisms that underlie the rapid evolution of centromeric repeats remain unknown. To obtain insight into the evolution of centromeric repeats following polyploidy, we studied a model diploid progenitor (Gossypium raimondii, D-genome) of the allopolyploid (AD-genome) cottons, G. hirsutum and G. barbadense. Sequence analysis of chromatin-immunoprecipitated DNA showed that the G. raimondii centromeric repeats originated from retrotransposon-related sequences. Comparative analysis showed that nine of the 10 analyzed centromeric repeats were absent from the centromeres in the A-genome and related diploid species (B-, F- and G-genomes), indicating that they colonized the centromeres of D-genome lineage after the divergence of the A- and D- ancestral species or that they were ancestrally retained prior to the origin of Gossypium. Notably, six of the nine repeats were present in both the A- and D-subgenomes in tetraploid G. hirsutum, and increased in abundance in both subgenomes. This finding suggests that centromeric repeats may spread and proliferate between genomes subsequent to polyploidization. Two repeats, Gr334 and Gr359 occurred in both the centromeres and nucleolar organizer regions (NORs) in D- and AD-genome species, yet localized to just the NORs in A-, B-, F-, and G-genome species. Contained within is a story of an established centromeric repeat that is eliminated and allopolyploidization provides an opportunity for reinvasion and reestablishment, which broadens our evolutionary understanding behind the cycles of centromeric repeat establishment and targeting.

Use of Repetitive Sequences for Molecular and Cytogenetic Characterization of Avena Species from Portugal

Genomic diversity of Portuguese accessions of Avena species--diploid A. strigosa and hexaploids A. sativa and A. sterilis--was evaluated through molecular and cytological analysis of 45S rDNA, and other repetitive sequences previously studied in cereal species--rye subtelomeric sequence (pSc200) and cereal centromeric sequence (CCS1). Additionally, retrotransposons and microsatellites targeting methodologies--IRAP (inter-retrotransposon amplified polymorphism) and REMAP (retrotransposon-microsatellite amplified polymorphism)--were performed. A very high homology was detected for ribosomal internal transcribed sequences (ITS1 and ITS2) between the species analyzed, although nucleolar organizing regions (NOR) fluorescent in situ hybridization (FISH) analysis revealed distinct number of Nor loci between diploid and hexaploid species. Moreover, morphological diversity, evidenced by FISH signals with different sizes, was observed between distinct accessions within each species. pSc200 sequences were for the first time isolated from Avena species but proven to be highly similar in all genotypes analyzed. The use of primers designed for CCS1 unraveled a sequence homologous to the Ty3/gypsy retrotransposon Cereba, that was mapped to centromeric regions of diploid and hexaploid species, being however restricted to the more related A and D haplomes. Retrotransposon-based methodologies disclosed species- and accessions-specific bands essential for the accurate discrimination of all genotypes studied. Centromeric, IRAP and REMAP profiles therefore allowed accurate assessment of inter and intraspecific variability, demonstrating the potential of these molecular markers on future oat breeding programs.

Sequencing of 15 622 gene-bearing BACs clarifies the gene-dense regions of the barley genome

Barley (Hordeum vulgare L.) possesses a large and highly repetitive genome of 5.1 Gb that has hindered the development of a complete sequence. In 2012, the International Barley Sequencing Consortium released a resource integrating whole-genome shotgun sequences with a physical and genetic framework. However, because only 6278 bacterial artificial chromosome (BACs) in the physical map were sequenced, fine structure was limited. To gain access to the gene-containing portion of the barley genome at high resolution, we identified and sequenced 15 622 BACs representing the minimal tiling path of 72 052 physical-mapped gene-bearing BACs. This generated ~1.7 Gb of genomic sequence containing an estimated 2/3 of all Morex barley genes. Exploration of these sequenced BACs revealed that although distal ends of chromosomes contain most of the gene-enriched BACs and are characterized by high recombination rates, there are also gene-dense regions with suppressed recombination. We made use of published map-anchored sequence data from Aegilops tauschii to develop a synteny viewer between barley and the ancestor of the wheat D-genome. Except for some notable inversions, there is a high level of collinearity between the two species. The software HarvEST:Barley provides facile access to BAC sequences and their annotations, along with the barley-Ae. tauschii synteny viewer. These BAC sequences constitute a resource to improve the efficiency of marker development, map-based cloning, and comparative genomics in barley and related crops. Additional knowledge about regions of the barley genome that are gene-dense but low recombination is particularly relevant.

Genome-dependent chromosome dynamics in three successive generations of the allotetraploid Festuca pratensis × Lolium perenne hybrid

We focus on the identification of complete and recombined ribosomal DNA-bearing chromosomes, and the dynamics of chromosomal number and position of ribosomal DNA (rDNA) loci in the F2-F4 generations derived from the F1 hybrid of Festuca pratensis Huds. (2n = 4x = 28) × Lolium perenne L. (2n = 4x = 28). Lolium genomic DNA and rRNA genes were mapped by means of genomic and fluorescence in situ hybridization (GISH and FISH). The results revealed that plants of the three generations share various rDNA loci profiles with chromosome structural changes, possibly as a result of chromosomal inter- and intra-rearrangements. We observed an asymmetrical variation in the number of recombinant arms with and without rDNA loci between parental genomes. The Lolium genome was more affected by rearrangements in arms with rDNA loci, while Festuca was more affected in arms without them. Statistically significant differences between L. perenne and F. pratensis genomes concerned the number of recombined chromosomes without rDNA, and the number of recombined rDNA-bearing chromosomal arms of marked chromosomes, showing a tendency of F. pratensis genome-like chromosomes to be less stable, compared with L. perenne. We postulate a novel genome-dependent range and type of chromosome variation in plants of the F2-F4 generations derived from F. pratensis × L. perenne hybrid.

Comparative molecular cytogenetic analyses of a major tandemly repeated DNA family and retrotransposon sequences in cultivated jute Corchorus species (Malvaceae)

BACKGROUND AND AIMS

The cultivated jute species Corchorus olitorius and Corchorus capsularis are important fibre crops. The analysis of repetitive DNA sequences, comprising a major part of plant genomes, has not been carried out in jute but is useful to investigate the long-range organization of chromosomes. The aim of this study was the identification of repetitive DNA sequences to facilitate comparative molecular and cytogenetic studies of two jute cultivars and to develop a fluorescent in situ hybridization (FISH) karyotype for chromosome identification.

METHODS

A plasmid library was generated from C. olitorius and C. capsularis with genomic restriction fragments of 100-500 bp, which was complemented by targeted cloning of satellite DNA by PCR. The diversity of the repetitive DNA families was analysed comparatively. The genomic abundance and chromosomal localization of different repeat classes were investigated by Southern analysis and FISH, respectively. The cytosine methylation of satellite arrays was studied by immunolabelling.

KEY RESULTS

Major satellite repeats and retrotransposons have been identified from C. olitorius and C. capsularis. The satellite family CoSat I forms two undermethylated species-specific subfamilies, while the long terminal repeat (LTR) retrotransposons CoRetro I and CoRetro II show similarity to the Metaviridea of plant retroelements. FISH karyotypes were developed by multicolour FISH using these repetitive DNA sequences in combination with 5S and 18S-5·8S-25S rRNA genes which enable the unequivocal chromosome discrimination in both jute species.

CONCLUSIONS

The analysis of the structure and diversity of the repeated DNA is crucial for genome sequence annotation. The reference karyotypes will be useful for breeding of jute and provide the basis for karyotyping homeologous chromosomes of wild jute species to reveal the genetic and evolutionary relationship between cultivated and wild Corchorus species.

Improving pharmaceutical protein production in Oryza sativa

Application of plant expression systems in the production of recombinant proteins has several advantages, such as low maintenance cost, absence of human pathogens, and possession of complex post-translational glycosylation capabilities. Plants have been successfully used to produce recombinant cytokines, vaccines, antibodies, and other proteins, and rice (Oryza sativa) is a potential plant used as recombinant protein expression system. After successful transformation, transgenic rice cells can be either regenerated into whole plants or grown as cell cultures that can be upscaled into bioreactors. This review summarizes recent advances in the production of different recombinant protein produced in rice and describes their production methods as well as methods to improve protein yield and quality. Glycosylation and its impact in plant development and protein production are discussed, and several methods of improving yield and quality that have not been incorporated in rice expression systems are also proposed. Finally, different bioreactor options are explored and their advantages are analyzed.

Highly diverse chromoviruses of Beta vulgaris are classified by chromodomains and chromosomal integration

Background

Chromoviruses are one of the three genera of Ty3-gypsy long terminal repeat (LTR) retrotransposons, and are present in high copy numbers in plant genomes. They are widely distributed within the plant kingdom, with representatives even in lower plants such as green and red algae. Their hallmark is the presence of a chromodomain at the C-terminus of the integrase. The chromodomain exhibits structural characteristics similar to proteins of the heterochromatin protein 1 (HP1) family, which mediate the binding of each chromovirus type to specific histone variants. A specific integration via the chromodomain has been shown for only a few chromoviruses. However, a detailed study of different chromoviral clades populating a single plant genome has not yet been carried out.

Results

We conducted a comprehensive survey of chromoviruses within the Beta vulgaris (sugar beet) genome, and found a highly diverse chromovirus population, with significant differences in element size, primarily caused by their flanking LTRs. In total, we identified and annotated full-length members of 16 families belonging to the four plant chromoviral clades: CRM, Tekay, Reina, and Galadriel. The families within each clade are structurally highly conserved; in particular, the position of the chromodomain coding region relative to the polypurine tract is clade-specific. Two distinct groups of chromodomains were identified. The group II chromodomain was present in three chromoviral clades, whereas families of the CRM clade contained a more divergent motif. Physical mapping using representatives of all four clades identified a clade-specific integration pattern. For some chromoviral families, we detected the presence of expressed sequence tags, indicating transcriptional activity.

Conclusions

We present a detailed study of chromoviruses, belonging to the four major clades, which populate a single plant genome. Our results illustrate the diversity and family structure of B. vulgaris chromoviruses, and emphasize the role of chromodomains in the targeted integration of these viruses. We suggest that the diverse sets of plant chromoviruses with their different localization patterns might help to facilitate plant-genome organization in a structural and functional manner.

FISH in analysis of gamma ray-induced micronuclei formation in barley

A micronucleus test in combination with fluorescent in situ hybridization (FISH) using telomere-, centromere-specific probes and 5S and 25S rDNA was used for a detailed analysis of the effects of gamma ray irradiation on the root tip meristem cells of barley, Hordeum vulgare (2n = 14). FISH with four DNA probes was used to examine the involvement of specific chromosomes or chromosome fragments in gamma ray-induced micronuclei formation and then to explain their origin. Additionally, a comparison of the possible origin of the micronuclei induced by physical and chemical treatment: maleic hydrazide (MH) and N-nitroso-N-methylurea (MNU) was done. The micronuclei induced by gamma ray could originate from acentric fragments after chromosome breakage or from whole lagging chromosomes as a result of a dysfunction of the mitotic apparatus. No micronuclei containing only centromeric signals were found. An application of rDNA as probes allowed it to be stated that 5S rDNA–bearing chromosomes are involved in micronuclei formation more often than NOR chromosomes. This work allowed the origin of physically- and chemically-induced micronuclei in barley cells to be compared: the origin of micronuclei was most often from terminal fragments. FISH confirmed its usefulness in the characterization of micronuclei content, as well as in understanding and comparing the mechanisms of the actions of mutagens applied in plant genotoxicity.

Divergence in centromere structure distinguishes related genomes in Coix lacryma-jobi and its wild relative

Knowledge about the composition and structure of centromeres is critical for understanding how centromeres perform their functional roles. Here, we report the sequences of one centromere-associated bacterial artificial chromosome clone from a Coix lacryma-jobi library. Two Ty3/gypsy-class retrotransposons, centromeric retrotransposon of C. lacryma-jobi (CRC) and peri-centromeric retrotransposon of C. lacryma-jobi, and a (peri)centromere-specific tandem repeat with a unit length of 153 bp were identified. The CRC is highly homologous to centromere-specific retrotransposons reported in grass species. An 80-bp DNA region in the 153-bp satellite repeat was found to be conserved to centromeric satellite repeats from maize, rice, and pearl millet. Fluorescence in situ hybridization showed that the three repetitive sequences were located in (peri-)centromeric regions of both C. lacryma-jobi and Coix aquatica. However, the 153-bp satellite repeat was only detected on 20 out of the 30 chromosomes in C. aquatica. Immunostaining with an antibody against rice CENH3 indicates that the 153-bp satellite repeat and CRC might be both the major components for functional centromeres, but not all the 153-bp satellite repeats or CRC sequences are associated with CENH3. The evolution of centromeric repeats of C. lacryma-jobi during the polyploidization was discussed.

Nested Ty3-gypsy retrotransposons of a single Beta procumbens centromere contain a putative chromodomain

LTR retrotransposons belong to a major group of DNA sequences that are often localized in plant centromeres. Using BAC inserts originating from the centromere of a monosomic wild beet (Beta procumbens) chromosome fragment in Beta vulgaris, two complete LTR retrotransposons were identified. Both elements, designated Beetle1 and Beetle2, possess a coding region with genes in the order characteristic for Ty3-gypsy retrotransposons. Beetle1 and Beetle2 have a chromodomain in the C-terminus of the integrase gene and are highly similar to the centromeric retrotransposons (CRs) of rice, maize, and barley. Both retroelements were localized in the centromeric region of B. procumbens chromosomes by fluorescence in-situ hybridization. They can therefore be classified as centromere-specific chromoviruses. PCR analysis using RNA as template indicated that Beetle1 and Beetle2 are transcriptionally active. On the basis of the sequence diversity between the LTR sequences, it was estimated that Beetle1 and Beetle2 transposed within the last 60,000 years and 130,000 years, respectively. The centromeric localization of Beetle1 and Beetle2 and their transcriptional activity combined with high sequence conservation within each family play an important structural role in the centromeres of B. procumbens chromosomes.

Novel clades of chromodomain-containing Gypsy LTR retrotransposons from mosses (Bryophyta)

Retrotransposons are the major component of plant genomes. Chromodomain-containing Gypsy long terminal repeat (LTR) retrotransposons are widely distributed in eukaryotes. Four distinct clades of chromodomain-containing Gypsy retroelements are known from the vascular plants: Reina, CRM, Galadriel and Tekay. At the same time, almost nothing is known about the repertoire of LTR retrotransposons in bryophyte genomes. We have combined a search of chromodomain-containing Gypsy retroelements in Physcomitrella genomic sequences and an experimental investigation of diverse moss species. The computer-based mining of the chromodomain-containing LTR retrotransposons allowed us to describe four different elements from Physcomitrella. Four novel clades were identified that are evolutionarily distinct from the chromodomain-containing Gypsy LTR retrotransposons of other plants.

Structure and dynamics of retrotransposons at wheat centromeres and pericentromeres

Little is known of the dynamics of centromeric DNA in polyploid plants. We report the sequences of two centromere-associated bacterial artificial chromosome clones from a Triticum boeoticum library. Both autonomous and non-autonomous wheat centromeric retrotransposons (CRWs) were identified, both being closely associated with the centromeres of wheat. Fiber-fluorescence in situ hybridization and chromatin immunoprecipitation analysis showed that wheat centromeric retrotransposons (CRWs) represent a dominant component of the wheat centromere and are associated with centromere function. CRW copy number showed variation among different genomes: the D genome chromosomes contained fewer copies than either the A or B genome chromosomes. The frequency of lengthy continuous CRW arrays was higher than that in either rice or maize. The dynamics of CRWs and other retrotransposons at centromeric and pericentromeric regions during diploid speciation and polyploidization of wheat and its related species are discussed.

Centromeric retrotransposon lineages predate the maize/rice divergence and differ in abundance and activity

Centromeric retrotransposons (CR) are located almost exclusively at the centromeres of plant chromosomes. Analysis of the emerging Zea mays inbred B73 genome sequence revealed two novel subfamilies of CR elements of maize (CRM), bringing the total number of known CRM subfamilies to four. Orthologous subfamilies of each of these CRM subfamilies were discovered in the rice lineage, and the orthologous relationships were demonstrated with extensive phylogenetic analyses. The much higher number of CRs in maize versus Oryza sativa is due primarily to the recent expansion of the CRM1 subfamily in maize. At least one incomplete copy of a CRM1 homolog was found in O. sativa ssp. indica and O. officinalis, but no member of this subfamily could be detected in the finished O. sativa ssp. japonica genome, implying loss of this prolific subfamily in that subspecies. CRM2 and CRM3, as well as the corresponding rice subfamilies, have been recently active but are present in low numbers. CRM3 is a full-length element related to the non-autonomous CentA, which is the first described CRM. The oldest subfamily (CRM4), as well as its rice counterpart, appears to contain only inactive members that are not located in currently active centromeres. The abundance of active CR elements is correlated with chromosome size in the three plant genomes for which high quality genomic sequence is available, and the emerging picture of CR elements is one in which different subfamilies are active at different evolutionary times. We propose a model by which CR elements might influence chromosome and genome size.

Repetitive DNA is associated with centromeric domains in Trypanosoma brucei but not Trypanosoma cruzi

Centromeres in Trypanosoma cruzi and Trypanosoma brucei can be localised to regions between directional gene clusters that contain degenerate retroelements, and in the case of T. brucei, repetitive DNA.

Background

Trypanosomes are parasitic protozoa that diverged early from the main eukaryotic lineage. Their genomes display several unusual characteristics and, despite completion of the trypanosome genome projects, the location of centromeric DNA has not been identified.

Results

We report evidence on the location and nature of centromeric DNA in Trypanosoma cruzi and Trypanosoma brucei. In T. cruzi, we used telomere-associated chromosome fragmentation and found that GC-rich transcriptional 'strand-switch' domains composed predominantly of degenerate retrotranposons are a shared feature of regions that confer mitotic stability. Consistent with this, etoposide-mediated topoisomerase-II cleavage, a biochemical marker for active centromeres, is concentrated at these domains. In the 'megabase-sized' chromosomes of T. brucei, topoisomerase-II activity is also focused at single loci that encompass regions between directional gene clusters that contain transposable elements. Unlike T. cruzi, however, these loci also contain arrays of AT-rich repeats stretching over several kilobases. The sites of topoisomerase-II activity on T. brucei chromosome 1 and T. cruzi chromosome 3 are syntenic, suggesting that centromere location has been conserved for more than 200 million years. The T. brucei intermediate and minichromosomes, which lack housekeeping genes, do not exhibit site-specific accumulation of topoisomerase-II, suggesting that segregation of these atypical chromosomes might involve a centromere-independent mechanism.

Conclusion

The localization of centromeric DNA in trypanosomes fills a major gap in our understanding of genome organization in these important human pathogens. These data are a significant step towards identifying and functionally characterizing other determinants of centromere function and provide a framework for dissecting the mechanisms of chromosome segregation.

CENH3 interacts with the centromeric retrotransposon cereba and GC-rich satellites and locates to centromeric substructures in barley

The chromosomal location of centromere-specific histone H3 (CENH3) is the assembly site for the kinetochore complex of active centromeres. Chromatin immunoprecipitation data indicated that CENH3 interacts in barley with cereba, a centromeric retroelement (CR)-like element conserved among cereal centromeres and barley-specific GC-rich centromeric satellite sequences. Anti-CENH3 signals on extended chromatin fibers always colocalized with the centromeric sequences but did not encompass the entire area covered by such centromeric repeats. This indicates that the CENH3 protein is bound only to a fraction of the centromeric repeats. At mitotic metaphase, CENH3, histone H3, and serine 10 phosphorylated histone H3 predominated within distinct structural subdomains of the centromere, as demonstrated by immunogold labeling for high resolution scanning electron microscopy.

Molecular assembly of meiotic proteins Asy1 and Zyp1 and pairing promiscuity in rye (Secale cereale L.) and its synaptic mutant sy10

Assembly of two orthologous proteins associated with meiotic chromosome axes in Arabidopsis thaliana (Asy1 and Zyp1) was studied immunologically at meiotic prophase of meiosis of wild-type rye (Secale cereale) and its synaptic mutant sy10, using antibodies derived from A. thaliana. The temporal and spatial expression of the two proteins were similar in wild-type rye, but with one notable difference. Unlike A. thaliana, in which foci of the transverse filament protein Zyp1 appear to linearize commensurately with synapsis, linear tracts of Asy1 and Zyp1 protein form independently at leptotene and early zygotene of rye and coalign into triple structures resembling synaptonemal complexes (SCs) only at later stages of synapsis. The sy10 mutant used in this study also forms spatially separate linear tracts of Asy1 and Zyp1 proteins at leptotene and early zygotene, and these coalign but do not form regular triple structures at midprophase. Electron microscopy of spread axial elements reveals extensive asynapsis with some exchanges of pairing partners. Indiscriminate SCs support nonhomologous chiasma formation at metaphase I, as revealed by multi-color fluorescence in situ hybridization enabling reliable identification of all the chromosomes of the complement. Scrutiny of chiasmate associations of chromosomes at this stage revealed some specificity in the associations of homologous and nonhomologous chromosomes. Inferences about the nature of synapsis in this mutant were drawn from such observations.

Tuareg, a novel miniature-inverted repeat family of pearl millet (Pennisetum glaucum) related to the PIF superfamily of maize

Miniature-inverted repeat transposable elements (MITEs) are abundantly repeated in plant genomes and are especially found in genic regions where they could contribute regulatory elements for gene expression. We describe with molecular and cytological tools the first MITE family reported in pearl millet: Tuareg. It was initially detected in the pearl millet ortholog of Teosinte-branched1, an important developmental gene involved in the domestication of maize. The Tuareg family was amplified recently in the pearl millet genome and elements were found more abundant in wild than in domesticated plants. We found that they shared similarity in their terminal repeats with the previously described mPIF MITEs and that they are also present in other Pennisetum species, in maize and more distantly related grasses. The Tuareg family may be part of MITEs activated by PIF-like transposases and it could have been mobile since pearl millet domestication.

Multi-substrate chromosome preparations for high throughput comparative FISH

BACKGROUND

A modification of a standard method of fluorescence in situ hybridisation (FISH) is described, by which a combination of several substrates and probes on single microscope slides enables more accurate comparisons of the distribution and abundance of chromosomal sequences and improves the relatively low throughput of standard FISH methods.

RESULTS

The utility and application of multi-colour, multi-substrate FISH is illustrated by the simultaneous physical mapping of retrotransposon sequences to three species of Avena, and single locus BAC (bacterial artificial chromosome) clones and rDNA probes to three species of Brachypodium, demonstrating how this would enable better understanding of complex phylogenetic relationships among some of the species belonging to these two genera.

CONCLUSION

The results show that use of multi-substrate chromosome preparations significantly increases the utility of FISH in comparative analyses of the distribution and abundance of chromosomal sequences in closely related plant species.

Recombination, rearrangement, reshuffling, and divergence in a centromeric region of rice

Centromeres have many unusual biological properties, including kinetochore attachment and severe repression of local meiotic recombination. These properties are partly an outcome, partly a cause, of unusual DNA structure in the centromeric region. Although several plant and animal genomes have been sequenced, most centromere sequences have not been completed or analyzed in depth. To shed light on the unique organization, variability, and evolution of centromeric DNA, detailed analysis of a 1.97-Mb sequence that includes centromere 8 (CEN8) of japonica rice was undertaken. Thirty-three long-terminal repeat (LTR)-retrotransposon families (including 11 previously unknown) were identified in the CEN8 region, totaling 245 elements and fragments that account for 67% of the region. The ratio of solo LTRs to intact elements in the CEN8 region is approximately 0.9:1, compared with approximately 2.2:1 in noncentromeric regions of rice. However, the ratio of solo LTRs to intact elements in the core of the CEN8 region ( approximately 2.5:1) is higher than in any other region investigated in rice, suggesting a hotspot for unequal recombination. Comparison of the CEN8 region of japonica and its orthologous segments from indica rice indicated that approximately 15% of the intact retrotransposons and solo LTRs were inserted into CEN8 after the divergence of japonica and indica from a common ancestor, compared with approximately 50% for previously studied euchromatic regions. Frequent DNA rearrangements were observed in the CEN8 region, including a 212-kb subregion that was found to be composed of three rearranged tandem repeats. Phylogenetic analysis also revealed recent segmental duplication and extensive rearrangement and reshuffling of the CentO satellite repeats.

The transcribed 165-bp CentO satellite is the major functional centromeric element in the wild rice species Oryza punctata

Centromeres are required for faithful segregation of chromosomes in cell division. It is not clear what kind of sequences act as functional centromeres and how centromere sequences are organized in Oryza punctata, a BB genome species. In this study, we found that the CentO centromeric satellites in O. punctata share high homology with the CentO satellites in O. sativa. The O. punctata centromeres are characterized by megabase tandem arrays that are flanked by centromere-specific retrotransposons. Immunostaining with an antibody specific to CENH3 indicates that the 165-bp CentO satellites are the major component for functional centromeres. Moreover, both strands of CentO satellites are highly methylated and transcribed and produce small interfering RNA, which may be important for the maintenance of centromeric heterochromatin and centromere function.

Chromatin immunoprecipitation cloning reveals rapid evolutionary patterns of centromeric DNA in Oryza species

The functional centromeres of rice (Oryza sativa, AA genome) chromosomes contain two key DNA components: the CRR centromeric retrotransposons and a 155-bp satellite repeat, CentO. However, several wild Oryza species lack the CentO repeat. We developed a chromatin immunoprecipitation-based technique to clone DNA fragments derived from chromatin containing the centromeric histone H3 variant CenH3. Chromatin immunoprecipitation cloning was carried out in the CentO-less species Oryza rhizomatis (CC genome) and Oryza brachyantha (FF genome). Three previously uncharacterized genome-specific satellite repeats, CentO-C1, CentO-C2, and CentO-F, were discovered in the centromeres of these two species. An 80-bp DNA region was found to be conserved in CentO-C1, CentO, and centromeric satellite repeats from maize and pearl millet, species which diverged from rice many millions of years ago. In contrast, the CentO-F repeat shows no sequence similarity to other centromeric repeats but has almost completely replaced other centromeric sequences in O. brachyantha, including the CRR-related sequences that normally constitute a significant fraction of the centromeric DNA in grass species.

Phylogenomic analysis of chromoviruses

Genome sequences of model organisms provide a unique opportunity to obtain insight into the complete diversity of any transposable element (TE) group. A limited number of chromoviruses, the chromodomain containing genus of Metaviridae, is known from plant, fungal and vertebrate genomes. By searching diverse eukaryotic genome databases, we have found a surprisingly large number of new, structurally intact and highly conserved chromoviral elements, greatly exceeding the number of previously known chromoviruses. In this study, we examined the diversity, origin and evolution of chromoviruses in Eukaryota. Chromoviral diversity in plants, fungi and vertebrates, as shown by phylogenetic analyses, was found to be much greater than previously expected. A novel centromere-specific chromoviral lineage was found to be widespread and highly conserved in all seed plants. The age of chromoviruses has been significantly extended by finding their representatives in the most basal plant lineages (green and red algae), in Heterokonta (oomycetes) and in Cercozoa (plasmodiophorids). The evolutionary origin of chromoviruses has been found to be no earlier than in Cercozoa, since none can be found in the basal eukaryotic lineages, despite the extensive genome data. The evolutionary dynamics of chromoviruses can be explained by a strict vertical transmission in plants and fungi, while in Metazoa it is more complex. The currently available genome data clearly show that chromoviruses are the most widespread and one of the oldest Metaviridae clade.

Stable barley chromosomes without centromeric repeats

The satellite sequences (AGGGAG)(n) and Ty3/gypsy-like retrotransposons are known to localize at the barley centromeres. Using a gametocidal system, which induces chromosomal mutations in barley chromosomes added to common wheat, we obtained an isochromosome for the short arm of barley chromosome 7H (7HS) that lacked the barley-specific satellite sequence (AGGGAG)(n). Two telocentric derivatives of the isochromosome arose in the progeny: 7HS* with and 7HS** without the pericentromeric C-band. FISH analysis demonstrated that both telosomes lacked not only the barley-specific centromeric (AGGGAG)(n) repeats and retroelements but also any of the known wheat centromeric tandem repeats, including the 192-bp, 250-bp, and TaiI sequences. Although they lacked these centromeric repeats, 7HS* and 7HS** both showed normal mitotic and meiotic transmission. Translocation of barley centromeric repeats to a wheat chromosome 4A did not generate a dicentric chromosome. Indirect immunostaining revealed that all tested centromere-specific proteins (rice CENH3, maize CENP-C, and putative barley homologues of the yeast kinetochore proteins CBF5 and SKP1) and histone H3 phosphorylated at serines 10 and 28 localized at the centromeric region of 7HS*. We conclude that the barley centromeric repeats are neither sufficient nor obligatory to assemble kinetochores, and we discuss the possible formation of a novel centromere in a barley chromosome.

A genomic perspective on the chromodomain-containing retrotransposons: Chromoviruses

Chromoviruses, chromodomain-containing retrotransposons, are the only Metaviridae (Ty3/gypsy group of retrotransposons) clade with a Eukaryota-wide distribution. They have a common evolutionary origin and are the most prolific and diverse Metaviridae clade. The fusion of a retrotransposon and a chromodomain, was most probably responsible for their extreme evolutionary success in Eukaryota. Analysis of the massive amount of genome sequence data for different eukaryotic lineages has provided an in depth insight into the diversity, evolution, neofunctionalization, high rate of genomic turnover and origin of chromoviruses in Eukaryota. This review attempts to summarise the unique aspects of chromoviruses from a genomic perspective.

A direct repeat sequence associated with the centromeric retrotransposons in wheat

A high-density BAC filter of Triticum monococcum was screened for the presence of a centromeric retrotransposon using the integrase region as a probe. Southern hybridization to the BAC digests using total genomic DNA probes of Triticum monococcum, Triticum aestivum, and Hordeum vulgare detected differentially hybridizing restriction fragments between wheat and barley. The fragments that hybridized to genomic DNA of wheat but not to that of barley were subcloned. Fluorescence in situ hybridization (FISH) analysis indicated that the clone pHind258 hybridized strongly to centromeric regions in wheat and rye and weakly to those in barley. The sequence of pHind258 was homologous to integrase and long terminal repeats of centromeric Ty3-gypsy retrotransposons of cereal species. Additionally, pHind258 has a pair of 192-bp direct repeats. FISH analysis indicated that the 192-bp repeat probe hybridized to centromeres of wheat and rye but not to those of barley. We found differential FISH signal intensities among wheat chromosomes using the 192-bp probe. In general, the A-genome chromosomes possess strong FISH signals, the B-genome chromosomes possess moderate signals, and the D-genome chromosomes possess weak signals. This was consistent with the estimated copy numbers of the 192-bp repeats in the ancestral species of hexaploid wheat.

Genome conflict in the gramineae

The genomes of grasses and cereals include a diverse and large collection of selfish genetic elements, many of which are fossil relics of ancient origin. Some of these elements are active and, because of their selfish nature and the way in which they exist to perpetuate themselves, they cause a conflict for genomes both within and between species in hybrids and allopolyploids. The conflict arises from how the various elements may undergo 'drive', through transposition, centromere and neocentromere drive, and in mitotic and meiotic drive processes in supernumerary B chromosomes. Experimental and newly formed hybrids and polyploids, where new combinations of genomes are brought together for the first time, find themselves sharing a common nuclear and cytoplasmic environment, and they can respond with varying degrees of instability to adjust to their new partnerships. B chromosomes are harmful to fertility and to the physiology of the cells and plants that carry them. In this review we take a broad view of genome conflict, drawing together aspects arising from a range of genetic elements that have not hitherto been considered in their entirety, and we find some common themes linking these various elements in their activities.

Evolutionary dynamics of transposable elements at the centromere

Transposable elements are the single most abundant class of genetic material in higher eukaryotes. These elements show a genome-wide distribution but are found in disproportionate abundance at the centromeric and/or pericentric regions in a wide range of phylogenetic species. We propose at least three possible ways in which these elements could have directly contributed to the evolution of the architecture and function of the centromere in various organisms. An "extradition" mechanism also appears to have evolved, which enables the developing or established centromere to deal with the potentially disruptive effects of any subsequently arising transposable elements.

Variable and hierarchical size distribution of L1-retroelement-enriched CENP-A clusters within a functional human neocentromere

Human neocentromeres are fully functional centromeres that arise epigenetically from non-centromeric precursor sequences that are devoid of alpha-satellite DNA. Using chromatin immunoprecipitation (ChIP) and BAC-array analysis, we have previously described a 330 kb binding domain for CENP-A (a histone H3 variant that confers centromere-specific nucleosomal property) at the 10q25 neocentromere found on a chromosome 10-derived marker chromosome mardel(10). For the further detailed analysis of the CENP-A-associated chromatin, we have generated a high-resolution genomic array consisting of PCR fragments with an average size of 8 kb, providing an approximately 20-fold increment in analytical resolution. ChIP and PCR-array analysis reveals seven distinct CENP-A-binding clusters within the 330 kb domain, demonstrating the interspersion of CENP-A-associated nucleosomal blocks within the neocentromeric chromatin. Independent ChIP-PCR analysis verified this distribution profile and indicated that histone H3-containing nucleosomes directly intervene the CENP-A-binding clusters. The CENP-A-binding clusters are uneven in size, with the central cluster (>50 kb) being significantly larger than the flanking ones (10-30 kb), and the flanking clusters arranged in an interesting hierarchical and symmetrical configuration of alternating larger and smaller sizes around the central cluster. In silico sequence analysis indicates an approximately 2.5-fold increase in the prevalence of L1 retroelements within the CENP-A-binding clusters when compared with the non-CENP-A-binding regions. These results provide insight into the possible role of retroelements in determining the positioning of CENP-A binding at human neocentromeres, and that a hierarchical and symmetrical arrangement of CENP-A-binding clusters of varying sizes may be an important structural requirement for mammalian kinetochore assembly and/or to provide stability to withstand polar microtubule forces.

Genomic neighborhoods for Arabidopsis retrotransposons: a role for targeted integration in the distribution of the Metaviridae

BACKGROUND

Retrotransposons are an abundant component of eukaryotic genomes. The high quality of the Arabidopsis thaliana genome sequence makes it possible to comprehensively characterize retroelement populations and explore factors that contribute to their genomic distribution.

RESULTS

We identified the full complement of A. thaliana long terminal repeat (LTR) retroelements using RetroMap, a software tool that iteratively searches genome sequences for reverse transcriptases and then defines retroelement insertions. Relative ages of full-length elements were estimated by assessing sequence divergence between LTRs: the Pseudoviridae were significantly younger than the Metaviridae. All retroelement insertions were mapped onto the genome sequence and their distribution was distinctly non-uniform. Although both Pseudoviridae and Metaviridae tend to cluster within pericentromeric heterochromatin, this association is significantly more pronounced for all three Metaviridae sublineages (Metavirus, Tat and Athila). Among these, Tat and Athila are strictly associated with pericentromeric heterochromatin.

CONCLUSIONS

The non-uniform genomic distribution of the Pseudoviridae and the Metaviridae can be explained by a variety of factors including target-site bias, selection against integration into euchromatin and pericentromeric accumulation of elements as a result of suppression of recombination. However, comparisons based on the age of elements and their chromosomal location indicate that integration-site specificity is likely to be the primary factor determining distribution of the Athila and Tat sublineages of the Metaviridae. We predict that, like retroelements in yeast, the Athila and Tat elements target integration to pericentromeric regions by recognizing a specific feature of pericentromeric heterochromatin.

Insertion bias and purifying selection of retrotransposons in the Arabidopsis thaliana genome

An analysis of the distribution and age of LTR retrotransposons in the Arabidopsis genome revealed that Pseudoviridae insert randomly along the chromosome and have been recently active whereas Metaviridae were more active in the past and preferentially target heterochromatin.

Background

Genome evolution and size variation in multicellular organisms are profoundly influenced by the activity of retrotransposons. In higher eukaryotes with compact genomes retrotransposons are found in lower copy numbers than in larger genomes, which could be due to either suppression of transposition or to elimination of insertions, and are non-randomly distributed along the chromosomes. The evolutionary mechanisms constraining retrotransposon copy number and chromosomal distribution are still poorly understood.

Results

I investigated the evolutionary dynamics of long terminal repeat (LTR)-retrotransposons in the compact Arabidopsis thaliana genome, using an automated method for obtaining genome-wide, age and physical distribution profiles for different groups of elements, and then comparing the distributions of young and old insertions. Elements of the Pseudoviridae family insert randomly along the chromosomes and have been recently active, but insertions tend to be lost from euchromatic regions where they are less likely to fix, with a half-life estimated at approximately 470,000 years. In contrast, members of the Metaviridae (particularly Athila) preferentially target heterochromatin, and were more active in the past.

Conclusion

Diverse evolutionary mechanisms have constrained both the copy number and chromosomal distribution of retrotransposons within a single genome. In A. thaliana, their non-random genomic distribution is due to both selection against insertions in euchromatin and preferential targeting of heterochromatin. Constant turnover of euchromatic insertions and a decline in activity for the elements that target heterochromatin have both limited the contribution of retrotransposon DNA to genome size expansion in A. thaliana.