A Ty3/gypsy retrotransposon-like sequence localizes to the centromeric regions of cereal chromosomes

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.

Comprehensive analysis of dynamics of histone H4 acetylation in mitotic barley cells

Nucleosomal histones are covalently modified at specific amino acid residues. In the case of histone H4, four lysines (K5, K8, K12, and K16) are acetylated. In the current studies, we examined the dynamics of histone H4 acetylation at K8 and K12 in mitotic barley cells using a three-dimensional immunofluorescent method. Based on the results and previous studies on the dynamics of K5 and K16 acetylation, we provide a comprehensive view of the dynamics of H4 acetylation. Interphase nuclei exhibit strong acetylation in the centromeric region at K5, K8 and K12. In the case of K12, strong acetylation at nucleolar organizing regions was observed from prophase to anaphase. The dynamics of K12 were closely related to those of K5. On the other hand, K8 exhibited a pattern of almost uniform acetylation from prophase to telophase and strong acetylation in distal regions of chromosomes at both metaphase and anaphase, which is very similar to the dynamics of K16 acetylation. Thus, it appears that there is pair-wise acetylation of K12 and K5 in the nucleolar organizing regions and of K8 and K16 in the gene-rich regions. Together, these results suggest that pair-wise dynamics of H4 acetylation regulate chromosomal structure and function during the cell cycle.

Structure and genomic organization of centromeric repeats in Arabidopsis species

Centromeric repetitive sequences were isolated from Arabidopsis halleri ssp. gemmifera and A. lyrata ssp. kawasakiana. Two novel repeat families isolated from A. gemmifera were designated pAge1 and pAge2. These repeats are 180 bp in length and are organized in a head-to-tail manner. They are similar to the pAL1 repeats of A. thaliana and the pAa units of A. arenosa. Both A. gemmifera and A. kawasakiana possess the pAa, pAge1 and pAge2 repeat families. Sequence comparisons of different centromeric repeats revealed that these families share a highly conserved region of approximately 50 bp. Within each of the four repeat families, two or three regions showed low levels of sequence variation. The average difference in nucleotide sequence was approximately 10% within families and 30% between families, which resulted in clear distinctions between families upon phylogenetic analysis. FISH analysis revealed that the localization patterns for the pAa, pAge1 and pAge2 families were chromosome specific in A. gemmifera and A. kawasakiana. In one pair of chromosomes in A. gemmifera, and three pairs of chromosomes in A. kawasakiana, two repeat families were present. The presence of three families of centromeric repeats in A. gemmifera and A. kawasakiana indicates that the first step toward homogenization of centromeric repeats occurred at the chromosome level.

Epigenetic control of CACTA transposon mobility in Arabidopsis thaliana

Epigenetic mutation, heritable developmental variation not based on a change in nucleotide sequence, is widely reported in plants. However, the developmental and evolutionary significance of such mutations remains enigmatic. On the basis of our studies of the endogenous Arabidopsis transposon CACTA, we propose that the inheritance of epigenetic gene silencing over generations can function as a transgenerational genome defense mechanism against deleterious movement of transposons. We previously reported that silent CACTA1 is mobilized by the DNA hypomethylation mutation ddm1 (decrease in DNA methylation). In this study, we report that CACTA activated by the ddm1 mutation remains mobile in the presence of the wild-type DDM1 gene, suggesting that de novo silencing is not efficient for the defense of the genome against CACTA movement. The defense depends on maintenance of transposon silencing over generations. In addition, we show that the activated CACTA1 element transposes throughout the genome in DDM1 plants, as reported previously for ddm1 backgrounds. Furthermore, the CACTA1 element integrated into both the ddm1-derived and the DDM1-derived chromosomal regions in the DDM1 wild-type plants, demonstrating that this class of transposons does not exhibit targeted integration into heterochromatin, despite its accumulation in the pericentromeric regions in natural populations. The possible contribution of natural selection as a mechanism for the accumulation of transposons and evolution of heterochromatin is discussed.

Molecular cytogenetic characterization of the Antirrhinum majus genome

As a model system in classical plant genetics, the genus Antirrhinum has been well studied, especially in gametophytic self-incompatibility, flower development biology, and transposon-induced mutation. In contrast to the advances in genetic and molecular studies, little is known about Antirrhinum cytogenetics. In this study, we isolated two tandem repetitive sequences, CentA1 and CentA2, from the centromeric regions of Antirrhinum chromosomes. A standard karyotype has been established by anchoring these centromeric repeats on meiotic pachytene chromosome using FISH. An ideogram based on the DAPI-staining pattern of pachytene chromosomes was developed to depict the distribution of heterochromatin in the Antirrhinum majus genome. To integrate the genetic and chromosomal maps, we selected one or two molecular markers from each linkage group to screen an Antirrhinum transformation-competent artificial chromosome (TAC) library. These genetically anchored TAC clones were labeled as FISH probes to hybridize to pachytene chromosomes of A. majus. As a result, the relationship between chromosomes and the linkage groups (LGs) in Antirrhinum has been established.

Diverse patterns of the tandem repeats organization in rye chromosomes

Although the monomer size, nucleotide sequence, abundance and species distribution of tandemly organized DNA families are well characterized, little is known about the internal structure of tandem arrays, including total arrays size and the pattern of monomers distribution. Using our rye specific probes, pSc200 and pSc250, we addressed these issues for telomere associated rye heterochromatin where these families are very abundant. Fluorescence in situ hybridization (FISH) on meiotic chromosomes revealed a specific mosaic arrangement of domains for each chromosome arm where either pSc200 or pSc250 predominates without any obvious tendency in order and size of domains. DNA of rye-wheat monosomic additions studied by pulse field gel electrophoresis produced a unique overall blot hybridization display for each of the rye chromosomes. The FISH signals on DNA fibres showed multiple monomer arrangement patterns of both repetitive families as well as of the Arabidopsis-type telomere repeat. The majority of the arrays consisted of the monomers of both families in different patterns separated by spacers. The primary structure of some spacer sequences revealed scrambled regions of similarity to various known repetitive elements. This level of complexity in the long-range organization of tandem arrays has not been previously reported for any plant species. The various patterns of internal structure of the tandem arrays are likely to have resulted from evolutionary interplay, array homogenization and the generation of heterogeneity mediated by double-strand breaks and associated repair mechanisms.

Tandemly repeated DNA sequences and centromeric chromosomal regions of Arabidopsis species

Despite their common function, centromeric DNA sequences are not conserved between organisms. Most centromeres of animals and plants so far investigated have now been shown to consist of large blocks of tandemly repeated satellite sequences that are embedded in recombination-deficient heterochromatic regions. This central domain of satellite sequences that is postulated to mediate spindle attachment is surrounded by pericentromeric sequences incorporating various classes of repetitive sequences often including retroelements. The centromeric satellite DNA sequences are amongst the most rapidly evolving sequences and pose some fundamental problems of maintaining function. In this overview, we will discuss work on centromeric repetitive sequences in Arabidopsis thaliana and its relatives, and highlight some of the common features that are emerging when analysing closely related species.

Molecular cytogenetic analysis of recently evolved Tragopogon (Asteraceae) allopolyploids reveal a karyotype that is additive of the diploid progenitors

Tragopogon mirus and T. miscellus (both 2n = 4x = 24) are recent allotetraploids derived from T. dubius × T. porrifolius and T. dubius × T. pratensis (each 2n = 2x = 12), respectively. The genome sizes of T. mirus are additive of those of its diploid parents, but at least some populations of T. miscellus have undergone genome downsizing. To survey for genomic rearrangements in the allopolyploids, four repetitive sequences were physically mapped. TPRMBO (unit size 160 base pairs [bp]) and TGP7 (532 bp) are tandemly organized satellite sequences isolated from T. pratensis and T. porrifolius, respectively. Fluorescent in situ hybridization to the diploids showed that TPRMBO is a predominantly centromeric repeat on all 12 chromosomes, while TGP7 is a subtelomeric sequence on most chromosome arms. The distribution of tandem repetitive DNA loci (TPRMBO, TGP7, 18S-5.8S-26S rDNA, and 5S rDNA) gave unique molecular karyotypes for the three diploid species, permitting the identification of the parental chromosomes in the polyploids. The location and number of these loci were inherited without apparent changes in the allotetraploids. There was no evidence for major genomic rearrangements in Tragopogon allopolyploids that have arisen multiple times in North America within the last 80 yr.

The DNA sequence of a gypsy element from Gossypium hirsutum L. and characterization of gypsy elements in three Gossypium species

A strategy was developed to isolate a complete gypsy-element from Gossypium hirsutum L. based on the sequence of a RAPD that was polymorphic in near isoganic lines of cotton that varied in leaf shape. A 5998 nt clone was isolated and its gene order and sequence confirmed it was a gypsy-type retroelement. Sequences homologous to the gag portion of this clone were also found in Gossypium herbaceum L. and Gossypium raimondii L. A portion of the open reading frame of the integrase gene was amplified from three Gossypium species under investigation. PCR products of the expected size were amplified from all three Gossypium species. G. raimondii sequences were statistically more degenerate than sequences from either G. hirsutum or G. herbaceum. Finally, analysis of the open reading frames of selected integrase clones from the three Gossypium species revealed a second gypsy element based on similarity of the deduced amino acid sequence.

A molecular view of plant centromeres

Although plants were the organisms of choice in several classical centromere studies, molecular and biochemical studies of plant centromeres have lagged behind those in model animal species. However, in the past several years, several centromeric repetitive DNA elements have been isolated in plant species and their roles in centromere function have been demonstrated. Most significantly, a Ty3/gypsy class of centromere-specific retrotransposons, the CR family, was discovered in the grass species. The CR elements are highly enriched in chromatin domains associated with CENH3, the centromere-specific histone H3 variant. CR elements as well as their flanking centromeric satellite DNA are actively transcribed in maize. These data suggest that the deposition of centromeric histones might be a transcription-coupled event.

DNA and proteins of plant centromeres

In plants, as in all eukaryotes, centromeres are chromatin domains that govern the transmission of nuclear chromosomes to the next generation of cells/individuals. The DNA composition and sequence organization of centromeres has recently been elucidated for a few plant species. Although there is little sequence conservation among centromeres, they usually contain tandem repeats and retroelements. The occurrence of neocentromeres reinforces the idea that the positions of centromeres are determined epigenetically. In contrast to centromeric DNA, structural and transient kinetochoric proteins are highly conserved among eukaryotes. Candidate sequences have been identified for a dozen putative kinetochore protein homologues, and some have been localized to plant centromeres. The kinetochore protein CENH3, which substitutes histone H3 within centromeric nucleosomes, co-immunoprecipitates preferentially with centromeric sequences. The mechanism(s) of centromere assembly and the functional implication of (peri-)centromeric modifications of chromatin remain to be elucidated.

Genomic localization of endogenous mobile CACTA family transposons in natural variants of Arabidopsis thaliana

The differentiation between gene-rich and transposon-rich (gene-poor) regions is a common feature of plant genomes. This may be due to preferential integration of transposons into gene-poor regions or may be due to purifying selection against transposon insertion into gene-rich regions. We examined the distribution of a low-copy-number mobile subfamily of Arabidopsis CACTA transposons in the genomes of 19 natural variants (ecotypes) of A. thaliana, and compared that to the pattern of integrations induced in the laboratory by mutation of the DDM1 (Decrease in DNA Methylation) gene. Sequences similar to mobile CACTA1 copies were distributed among the ecotypes and showed high degrees of polymorphism in genomic localization. Despite the high level of polymorphism, the copy number was low in all the ecotypes examined, and the elements were localized preferentially in pericentromeric and transposon-rich regions. This contrasts with the pattern of transposition induced by the ddm1 mutation, in which the range of integration sites is less biased and the copy number frequently increases. Based on these observations, we discuss the possible contribution of natural selection and chromatin structure to the distribution of transposons.

Divergence of satellite DNA and interspersion of dispersed repeats in the genome of the wild beet Beta procumbens

Several repetitive sequences of the genome of Beta procumbens Chr. Sm., a wild beet species of the section Procumbentes of the genus Beta have been isolated. According to their genomic organization, the repeats were assigned to satellite DNA and families of dispersed DNA sequences. The tandem repeats are 229-246 bp long and belong to an AluI restriction satellite designated pAp11. Monomers of this satellite DNA form subfamilies which can be distinguished by the divergence or methylation of an internal restriction site. The satellite is amplified in the section Procumbentes, but is also found in species of the section Beta including cultivated beet (Beta vulgaris). The existence of the pAp11 satellite in distantly related species suggests that the AluI sequence family is an ancient component of Beta genomes and the ancestor of the diverged satellite subfamily pEV4 in B. vulgaris. Comparative fluorescent in-situ hybridization revealed remarkable differences in the chromosomal position between B. procumbens and B. vulgaris, indicating that the pAp11 and pEV4 satellites were most likely involved in the expansion or rearrangement of the intercalary B. vulgaris heterochromatin. Furthermore, we describe the molecular structure, and genomic and chromosomal organization of two repetitive DNA families which were designated pAp4 and pAp22 and are 1354 and 582 bp long, respectively. The families consist of sequence elements which are widely dispersed along B. procumbens chromosomes with local clustering and exclusion from distal euchromatic regions. FISH on meiotic chromosomes showed that both dispersed repeats are colocalized in some chromosomal regions. The interspersion of repeats of the pAp4 and pAp22 family was studied by PCR and enabled the determination of repeat flanking sequences. Sequence analysis revealed that pAp22 is either derived from or part of a long terminal repeat (LTR) of an Athila-like retrotransposon. Southern analysis and FISH with pAp4 and pAp22 showed that both dispersed repeats are species-specific and can be used as DNA probes to discriminate parental genomes in interspecific hybrids. This was tested in the sugar beet hybrid PRO1 which contains a small B. procumbens chromosome fragment.

Analysis by atomic force microscopy of morphological changes in barley chromosomes during FISH treatment

We employed atomic force microscopy (AFM) to examine structural changes in barley chromosomes during the four steps of standard FISH processes. Rehydration and dehydration with alcohol accompanying RNase treatment increased chromosome arm width and decreased chromosome height about 50%. Subsequent heat denaturation reduced chromosome height further. These three-dimensional structural changes of the chromosomes were substantial, but the FISH signal produced by the hybridization of fluorescent probes was clear when observed by a fluorescence microscope. In higher-magnification images, we observed granular structures considered to represent the chromatin fiber on the surface of the chromosomes in each FISH protocol step. These our results indicate that FISH treatments result in severe damage of the three-dimensional higher-order structures of the chromosomes, although nano-structures, such as nucleosome and chromatin fibers, remain intact and relatively unaffected.

Distribution of Retroelements in Centromeres and Neocentromeres of Maize

Fluorescent in situ hybridization was used to examine the distribution of six abundant long terminal repeat (LTR) retroelements, Opie, Huck, Cinful-1, Prem-2/Ji, Grande, and Tekay/Prem-1 on maize pachytene chromosomes. Retroelement staining in euchromatin was remarkably uniform, even when we included the structurally polymorphic abnormal chromosome 10 (Ab10) in our analysis. This uniformity made it possible to use euchromatin as a control for quantitative staining intensity measurements in other regions of the genome. The data show that knobs, known to function as facultative neocentromeres when Ab10 is present, tend to exclude retroelements. A notable exception is Cinful-1, which accumulates in TR-1 knob arrays. Staining for each of the six retroelements was also substantially reduced in centromeric satellite arrays to an average of 30% of the staining in euchromatin. This contrasted with two previously described centromere-specific retrotransposable (CR) elements that were readily detected in centromeres. We suggest that retroelements are relatively rare in centromeres because they interrupt the long satellite arrays thought to be required for efficient centromere function. CR elements may have evolved mutualistic relationships with their plant hosts: they are known to interact with the kinetochore protein CENH3 and appear to accumulate in clusters, leaving long satellite arrays intact.

A centromeric tandem repeat family originating from a part of Ty3/gypsy-retroelement in wheat and its relatives

From a wild diploid species that is a relative of wheat, Aegilops speltoides, a 301-bp repeat containing 16 copies of a CAA microsatellite was isolated. Southern blot and fluorescence in situ hybridization revealed that approximately 250 bp of the sequence is tandemly arrayed at the centromere regions of A- and B-genome chromosomes of common wheat and rye chromosomes. Although the DNA sequence of this 250-bp repeat showed no notable homology in the databases, the flanking or intervening sequences between the repeats showed high homologies (>82%) to two separate sequences of the gag gene and its upstream region in cereba, a Ty3/gypsy-like retroelement of Hordeum vulgare. Since the amino acid sequence deduced from the 250 bp with seven CAAs showed some similarity ( approximately 53%) to that of the gag gene, we concluded that the 250-bp repeats had also originated from the cereba-like retroelements in diploid wheat such as Ae. speltoides and had formed tandem arrays, whereas the 300-bp repeats were dispersed as a part of cereba-like retroelements. This suggests that some tandem repeats localized at the centromeric regions of cereals and other plant species originated from parts of retrotransposons.

Chromatin immunoprecipitation reveals that the 180-bp satellite repeat is the key functional DNA element of Arabidopsis thaliana centromeres

The centromeres of Arabidopsis thaliana chromosomes contain megabases of complex DNA consisting of numerous types of repetitive DNA elements. We developed a chromatin immunoprecipitation (ChIP) technique using an antibody against the centromeric H3 histone, HTR12, in Arabidopsis. ChIP assays showed that the 180-bp centromeric satellite repeat was precipitated with the antibody, suggesting that this repeat is the key component of the centromere/kinetochore complex in Arabidopsis.

Molecular and cytological analyses of large tracks of centromeric DNA reveal the structure and evolutionary dynamics of maize centromeres

We sequenced two maize bacterial artificial chromosome (BAC) clones anchored by the centromere-specific satellite repeat CentC. The two BACs, consisting of approximately 200 kb of cytologically defined centromeric DNA, are composed exclusively of satellite sequences and retrotransposons that can be classified as centromere specific or noncentromere specific on the basis of their distribution in the maize genome. Sequence analysis suggests that the original maize sequences were composed of CentC arrays that were expanded by retrotransposon invasions. Seven centromere-specific retrotransposons of maize (CRM) were found in BAC 16H10. The CRM elements inserted randomly into either CentC monomers or other retrotransposons. Sequence comparisons of the long terminal repeats (LTRs) of individual CRM elements indicated that these elements transposed within the last 1.22 million years. We observed that all of the previously reported centromere-specific retrotransposons in rice and barley, which belong to the same family as the CRM elements, also recently transposed with the oldest element having transposed approximately 3.8 million years ago. Highly conserved sequence motifs were found in the LTRs of the centromere-specific retrotransposons in the grass species, suggesting that the LTRs may be important for the centromere specificity of this retrotransposon family.

LTR retrotransposons in the dioecious plant Silene latifolia

Conserved domains of two types of LTR retrotransposons, Tyl-copia- and Ty3-gypsy-like retrotransposons, were isolated from the dioecious plant Silene latifolia, whose sex is determined by X and Y chromosomes. Southern hybridization analyses using these retrotransposons as probes resulted in identical patterns from male and female genomes. Fluorescence in situ hybridization indicated that these retrotransposons do not accumulate specifically in the sex chromosomes. These results suggest that recombination between the sex chromosomes of S. latifolia has not been severely reduced. Conserved reverse transcriptase regions of Ty1-copia-like retrotransposons were isolated from 13 different Silene species and classified into two major families. Their categorization suggests that parallel divergence of the Ty1-copia-like retrotransposons occurred during the differentiation of Silene species. Most functional retrotransposons from three dioecious species, S. latifolia, S. dioica, and S. diclinis, fell into two clusters. The evolutionary dynamics of retrotransposons implies that, in the genus Silene, dioecious species evolved recently from gynodioecious species.

Integration of Cot analysis, DNA cloning, and high-throughput sequencing facilitates genome characterization and gene discovery

Cot-based sequence discovery represents a powerful means by which both low-copy and repetitive sequences can be selectively and efficiently fractionated, cloned, and characterized. Based upon the results of a Cot analysis, hydroxyapatite chromatography was used to fractionate sorghum (Sorghum bicolor) genomic DNA into highly repetitive (HR), moderately repetitive (MR), and single/low-copy (SL) sequence components that were consequently cloned to produce HRCot, MRCot, and SLCot genomic libraries. Filter hybridization (blotting) and sequence analysis both show that the HRCot library is enriched in sequences traditionally found in high-copy number (e.g., retroelements, rDNA, centromeric repeats), the SLCot library is enriched in low-copy sequences (e.g., genes and "nonrepetitive ESTs"), and the MRCot library contains sequences of moderate redundancy. The Cot analysis suggests that the sorghum genome is approximately 700 Mb (in agreement with previous estimates) and that HR, MR, and SL components comprise 15%, 41%, and 24% of sorghum DNA, respectively. Unlike previously described techniques to sequence the low-copy components of genomes, sequencing of Cot components is independent of expression and methylation patterns that vary widely among DNA elements, developmental stages, and taxa. High-throughput sequencing of Cot clones may be a means of "capturing" the sequence complexity of eukaryotic genomes at unprecedented efficiency.

Comparison of orthologous and paralogous DNA flanking the wheat high molecular weight glutenin genes: sequence conservation and divergence, transposon distribution, and matrix-attachment regions

Extended flanking DNA sequences were characterized for five members of the wheat high molecular weight (HMW) glutenin gene family to understand more of the structure, control, and evolution of these genes. Analysis revealed more sequence conservation among orthologous regions than between paralogous regions, with differences mainly owing to transposition events involving putative retrotransposons and several miniature inverted transposable elements (MITEs). Both gyspy-like long terminal repeat (LTR) and non-LTR retrotransposon sequences are represented in the flanking DNAs. One of the MITEs is a novel class, but another MITE is related to the maize Stowaway family and is widely represented in Triticeae express sequence tags (ESTs). Flanking DNA of the longest sequence, a 20 425-bp fragment including and surrounding the HMW-glutenin Bx7 gene, showed additional cereal gene-like sequences both immediately 5' and 3' to the HMW-glutenin coding region. The transcriptional activities of sequences related to these flanking putative genes and the retrotransposon-related regions were indicated by matches to wheat and other Triticeae ESTs. Predictive analysis of matrix-attachment regions (MARs) of the HMW glutenin and several alpha-, gamma-, and omega-gliadin flanking DNAs indicate potential MARs immediately flanking each of the genes. Matrix binding activity in the predicted regions was confirmed for two of the HMW-glutenin genes.

A microsatellite sequence from the rice blast fungus (Magnaporthe grisea) distinguishes between the centromeres of Hordeum vulgare and H. bulbosum in hybrid plants

A TC/AG-repeat microsatellite sequence derived from the rice blast fungus (Magnaporthe grisea) hybridized to all of the centromeres of Hordeum vulgare chromosomes, but hybridized faintly or not at all to the chromosomes of Hordeum bulbosum. Using this H. vulgare centromere-specific probe, the chromosomes of four F1 hybrids between H. vulgare and H. bulbosum were analyzed. The chromosome constitution in the root tips of the hybrids was mosaic, i.e., 7 (7v, H. vulgare) and 14 (7v + 7b H. bulbosum), or 14 (7v + 7b) and 27 (14v + 13b), or 7 (7v), 14 (7v + 7b), and 27 (14v + 13b). The 27-chromosome tetraploid hybrid cells were revealed to have the NOR (nucleolus organizer region) bearing chromosome of H. bulbosum in a hemizygous state, which might indicate some role for this chromosome in the chromosome instability of the hybrid condition. The chromosomal distribution showed that the chromosomes of H. vulgare were concentric and chromosomes of H. bulbosum were peripheral in the mitotic squash. This non-random chromosome distribution and the centromere-specific repeated DNA differences in the two species were discussed in relation to H. bulbosum chromosome elimination. Meiotic chromosome analyses revealed a high frequency of homoeologous chromosome pairing in early prophase. However, this chromosome pairing did not persist until later meiotic stages and many univalents and chromosome fragments resulted. These were revealed to be H. bulbosum by fluorescence in situ hybridization (FISH) analysis with the H. vulgare centromere-specific probe. Because the chromosome segregation of H. vulgare and H. bulbosum chromosomes at anaphase I of meiosis was random, the possibility for obtaining chromosome substitution lines in diploid barley from the diploid hybrid was discussed.

Identification and chromosomal organization of two rye genome-specific RAPD products useful as introgression markers in wheat

Two rye genome-specific random amplified polymorphic DNA (RAPD) markers were identified for detection of rye introgression in wheat. Both markers were amplified in all of the tested materials that contained rye chromatin such as rye, hexaploid triticale, wheat-rye addition lines, and wheat varieties with 1BL.1RS translocation. Two cloned markers, designated pSc10C and pSc20H, were 1012 bp and 1494 bp, respectively. Sequence analysis showed that both pSc10C and pSc20H fragments were related to retrotransposons, ubiquitously distributed in plant genomes. Using fluorescence in situ hybridization (FISH), probe pSc10C was shown to hybridize predominantly to the pericentromeric regions of all rye chromosomes, whereas probe pSc20H was dispersed throughout the rye genome except at telomeric regions and nucleolar organizing regions. The FISH patterns showed that the two markers should be useful to select or track all wheat-rye translocation lines derived from the whole arms of rye chromosomes, as well as to characterize the positions of the translocation breakpoints generated in the proximal and distal regions of rye arms.

A tandem repetitive sequence located in the centromeric region of common wheat (Triticum aestivum) chromosomes

Although Tail-family sequences are present in the subtelomeric region of Leymus racemosus, it became apparent in the present study that such sequences are also present in the centromeric region of common wheat (Triticum aestivum). These sequences hybridized to all chromosomes with various degrees of signal strength. FISH using Tail and Ty3/gypsy, a conservative sequence in cereal centromeres, revealed a complicated arrangement of both sequences in all wheat chromosomes at once. Unlike the Arabidopsis centromeres characterized by massive tandem arrays of 180-bp family with flanking paracentromeric retrotransposons in all chromosomes, wheat chromosomes showed various arrangement patterns of Tail and Ty3/gypsy sequences depending on the chromosome; Tail-family sequences were scattered in many wheat centromeres as isolated colonies instead of forming uninterrupted solid tandem arrays. This pattern may have resulted from retrotransposon insertion within pre-existing Tail-tandem arrays or a two-step amplification mechanism of the Tail family where each Tail colony was amplified to form arrays independently after the insertion of Tail-family sequences along the entire centromere. Although sequence analysis of centromeric Tail repeats in wheat and subtelomeric Tail repeats in L. racemosus showed variable and conservative regions between the two repeats, they did not show a distinctive difference phylogenically. The widespread presence of tandem repetitive sequences in the eucaryotic centromere suggests a significant role for them in centromeric formation.

Sequence organization of barley centromeres

By sequencing, fingerprinting and in situ hybridization of a centromere-specific large insert clone (BAC 7), the sequence organization of centromeric DNA of barley could be elucidated. Within 23 kb, three copies of the Ty3/gypsy-like retroelement cereba were present. Two elements of approximately 7 kb, arranged in tandem, include long terminal repeats (LTRs) (approximately 1 kb) similar to the rice centromeric retrotransposon RIRE 7 and to the cereal centromeric sequence family, the primer binding site, the complete polygene flanked by untranslated regions, as well as a polypurine tract 5' of the downstream LTR. The high density (approximately 200 elements/centromere) and completeness of cereba elements and the absence of internally deleted elements and solo LTRs from the BAC 7 insert represent unique features of the barley centromeres as compared to those of other cereals. Obviously, the conserved cereba elements together with barley-specific G+C-rich satellite sequences constitute the major components of centromeric DNA in this species.

The detection, cloning, and characterisation of WIS 2-1A retrotransposon-like sequences in Triticum aestivum L. and ×Triticosecale Wittmack and an examination of their evolution in related Triticeae

Retrotransposons and other mobile elements are major components of the repeated DNA fraction in higher-plant genomes. They have undoubtedly played an important role in higher plant genome evolution. The present work details the detection and characterisation of a WIS 2-1A related sequence in direct wheat relatives, and discusses the prevalence and evolution of its copy number in their genomes. An increase in copy number is detected when following the natural hybridisation processes that gave rise to bread and durum wheats. However, the opposite is observed in the development of triticale, a synthetic hybrid.Key words: retrotransposons, WIS 2-1A, Triticeae, triticale, FISH.

The pericentromeric heterochromatin of the grass Zingeria biebersteiniana (2n = 4) is composed of Zbcen1-type tandem repeats that are intermingled with accumulated dispersedly organized sequences

DNA reassociation and hydroxyapatite chromatography were used to isolate high-copy DNA of the grass Zingeria biebersteiniana (2n = 4). In situ hybridization demonstrated that the DNA isolated was enriched for pericentromere-specific repetitive sequences. One abundant pericentromere-specific component is the differentially methylated tandem-repeat family Zbcen1. Other sequences isolated, Zb46 and Zb47A, are dispersed and display similarity to parts of the gypsy- and copia-like retrotransposable elements of other grasses. In situ hybridization with the copia-like sequence Zb47A resulted in dispersed labelling along the chromosome arms, with a significant signal accumulation in the pericentromeric region of all chromosomes. It is concluded that the pericentromeric heterochromatin of Z. biebersteiniana is composed of members of the Zbcen1 tandem repeat family and that these tandem arrays are intermingled with accumulated putative copia-like retrotransposon sequences. An observed Rab1 interphase orientation suggests that the length of the chromosomes rather than the genome size is the determining factor of the Rab1 phenomenon.

Construction and characterization of a BAC library for the molecular dissection of a single wild beet centromere and sugar beet (Beta vulgaris) genome analysis

We have constructed a sugar beet bacterial artificial chromosome (BAC) library of the chromosome mutant PRO1. This Beta vulgaris mutant carries a single chromosome fragment of 6-9 Mbp that is derived from the wild beet Beta procumbens and is transmitted efficiently in meiosis and mitosis. The library consists of 50 304 clones, with an average insert size of 125 kb. Filter hybridizations revealed that approximately 3.1% of the clones contain mitochondrial or chloroplast DNA. Based on a haploid genome size of 758 Mbp, the library represents eight genome equivalents. Thus, there is a greater than 99.96% probability that any sequence of the PRO1 genome can be found in the library. Approximately 0.2% of the clones hybridized with centromeric sequences of the PRO1 minichromosome. Using the identified BAC clones in fluorescence in situ hybridization experiments with PRO1 and B. procumbens chromosome spreads, their wild-beet origin and centromeric localization were demonstrated. Comparative Southern hybridization of pulsed-field separated PRO1 DNA and BAC inserts indicate that the centromeric region of the minichromosome is represented by overlapping clones in the library. Therefore, the PRO1 BAC library provides a useful tool for the characterization of a single plant centromere and is a valuable resource for sugar beet genome analysis.Key words: Beta vulgaris, BAC library, Beta procumbens minichromosome, centromere, FISH.

Karyotype analysis ofLilium longiflorumandLilium rubellumby chromosome banding and fluorescence in situ hybridisation

Detailed karyotypes of Lilium longiflorum and L. rubellum were constructed on the basis of chromosome arm lengths, C-banding, AgNO3staining, and PI-DAPI banding, together with fluorescence in situ hybridisation (FISH) with the 5S and 45S rDNA sequences as probes. The C-banding patterns that were obtained with the standard BSG technique revealed only few minor bands on heterologous positions of the L. longiflorum and L. rubellum chromosomes. FISH of the 5S and 45S rDNA probes on L. longiflorum metaphase complements showed overlapping signals at proximal positions of the short arms of chromosomes 4 and 7, a single 5S rDNA signal on the secondary constriction of chromosome 3, and one 45S rDNA signal adjacent to the 5S rDNA signal on the subdistal part of the long arm of chromosome 3. In L. rubellum, we observed co-localisation of the 5S and 45S rDNA sequences on the short arm of chromosomes 2 and 4 and on the long arms of chromosomes 2 and 3, and two adjacent bands on chromosome 12. Silver staining (Ag-NOR) of the nucleoli and NORs in L. longiflorum and L. rubellum yielded a highly variable number of signals in interphase nuclei and only a few faint silver deposits on the NORs of mitotic metaphase chromosomes. In preparations stained with PI and DAPI, we observed both red- and blue-fluorescing bands at different positions on the L. longiflorum and L. rubellum chromosomes. The red-fluorescing or so-called reverse PI-DAPI bands always coincided with rDNA sites, whereas the blue-fluorescing DAPI bands corresponded to C-bands. Based on these techniques, we could identify most of chromosomes of the L. longiflorum and L. rubellum karyotypes.Key words: fluorescence in situ hybridisation, FISH, 5S rDNA, 45S rDNA, C-banding, reverse PI-DAPI banding.

Chromosomal distribution of reverse transcriptase-containing retroelements in two Triticeae species

A large portion of plant and particularly cereal genomes consist of repetitive DNA families, many of which are likely to be or to have evolved from retroelements. Molecular evidence suggests that repeated DNA sequences, although perhaps originating as innocuous or 'selfish' elements, can have dramatic effects on genome organization and function. Knowledge of chromosomal distribution of retroelements is important for understanding plant chromosome structure/functional organization, and could shed light on the dynamics of retroelements and their role in the evolutionary process. In the present study we aim to find a possible correlation between physical location of the regions with species-specific sequences and the distribution of conserved RT domains of the Ty1-copia, Ty3-gypsy and LINE groups of retroelements on the chromosomes of two diploid species that belong to the different branches of the tribe Triticeae, namely Aegilops speltoides Tausch (2n=2x=14) and Hordeum spontaneum L (2n=2x=14). All three groups of retroelements were found in large quantities in the genomes of the tested species. They are cluster-distributed, and the important role of these elements in the formation of terminal heterochromatin is shown. We found that there was a predominance of Ty1-copia and LINE elements in the chromosome regions with preferential content of species-specific sequences.

The centromere1 (CEN1) region of Arabidopsis thaliana: architecture and functional impact of chromatin

We have analysed the centromere 1 (CEN1) of Arabidopsis thaliana by integration of genetic, sequence and fluorescence in situ hybridisation (FISH) data. CEN1 is considered to include the centromeric core and the flanking left and right pericentromeric regions, which are distinct parts by structural and/or functional properties. CEN1 pericentromeres are composed of different dispersed repetitive elements, sometimes interrupted by functional genes. In contrast the CEN1 core is more uniformly structured harbouring only two different repeats. The presented analysis reveals aspects concerning distribution and effects of the uniformly shaped heterochromatin, which covers all CEN1 regions. A lethal mutation tightly linked to CEN1 enabled us to measure recombination frequencies within the heterochromatin in detail. In the left pericentromere, the change from eu- to heterochromatin is accompanied by a gradual change in sequence composition but by an extreme change in recombination frequency (from normal to 53-fold decrease) which takes place within a small region spanning 15 kb. Generally, heterochromatin is known to suppress recombination. However, the same analysis reveals that left and right pericentromere, though similar in sequence composition, differ markedly in suppression (53-fold versus 10-fold). The centromeric core exhibits at least 200-fold if not complete suppression. We discuss whether differences in (fine) composition reflect quantitative and qualitative differences in binding sites for heterochromatin proteins and in turn render different functional properties. Based on the presented data we estimate the sizes of Arabidopsis centromeres. These are typical for regional centromeres of higher eukaryotes and range from 4.4 Mb (CEN1) to 3.55 Mb (CEN4).

Diversity, origin, and distribution of retrotransposons (gypsy and copia) in conifers

We examined the diversity, evolution, and genomic organization of retroelements in a wide range of gymnosperms. In total, 165 fragments of the reverse transcriptase (RT) gene domain were sequenced from PCR products using newly designed primers for gypsy-like retrotransposons and well-known primers for copia-like retrotransposons; representatives of long interspersed nuclear element (LINE) retroposons were also found. Gypsy and copia-like retroelements are a major component of the gymnosperm genome, and in situ hybridization showed that individual element families were widespread across the chromosomes, consistent with dispersion and amplification via an RNA intermediate. Most of the retroelement families were widely distributed among the gymnosperms, including species with wide taxonomic separation from the Northern and Southern Hemispheres. When the gymnosperm sequences were analyzed together with retroelements from other species, the monophyletic origin of plant copia, gypsy, and LINE groups was well supported, with an additional clade including badnaviral and other, probably virus-related, plant sequences as well as animal and fungal gypsy elements. Plant retroelements showed high diversity within the phylogenetic trees of both copia and gypsy RT domains, with, for example, retroelement sequences from Arabidopsis thaliana being present in many supported groupings. No primary branches divided major taxonomic clades such as angiosperms, monocotyledons, gymnosperms, or conifers or (based on smaller samples) ferns, Gnetales, or Sphenopsida (Equisetum), suggesting that much of the existing diversity was present early in plant evolution, or perhaps that horizontal transfer of sequences has occurred. Within the phylogenetic trees for both gypsy and copia, two clearly monophyletic gymnosperm/conifer clades were revealed, providing evidence against recent horizontal transfer. The results put the evolution of the large and relatively conserved genome structure of gymnosperms into the context of the diversity of other groups of plants.

Identification of Bilby, a diverged centromeric Ty1-copia retrotransposon family from cereal rye (Secale cereale L.)

A diminutive rye chromosome (midget) in wheat was used as a model system to isolate a highly reiterated centromeric sequence from a rye chromosome. Fluorescence in situ hybridization (FISH) shows this sequence localized within all rye centromeres and no signal was detected on wheat chromosomes. DNA sequencing of the repetitive element has revealed the presence of some catalytic domains and signature motifs typical of retrotransposon genes and has been called the Bilby family, representing a diverged family of retrotransposon-like elements. Extensive DNA database searching revealed some sequence similarity to centromeric retrotransposons from wheat, barley, and centromeric repetitive sequences from rice. Very low levels of signal were observed when Bilby was used as a probe against barley, and no signal was detected with rice DNA during Southern hybridization. The abundance of Bilby in rye indicates that this family may have diverged from other distantly related centromeric retrotransposons or incorporated in the centromere but rapidly evolved in rye during speciation. The isolation of a rye retrotransposon also allowed the analysis of centromeric breakpoints in wheat-rye translocation lines. A quantitative analysis shows that the breakpoint in IDS.1RL and 1DL.1RS and recombinant lines containing proximal rye chromatin have a portion of the rye centromere that may contribute to the normal function of the centromeric region.

Localization of single-copy T-DNA insertion in transgenic shallots (Allium cepa) by using ultra-sensitive FISH with tyramide signal amplification

The sensitivity of fluorescence in situ hybridization (FISH) for mapping plant chromosomes of single-copy DNA sequences is limited. We have adapted for plant cytogenetics a new signal-amplification method termed tyramide-FISH (Tyr-FISH). Until present this technique has only been applied to human chromosomes. The method is based on enzymatic deposition of fluorochrome-conjugated tyramide. With Tyr-FISH it was possible to detect target T-DNA sequences on plant metaphase chromosomes as small as 710 bp without using a cooled CCD camera. Short detection time and high sensitivity, in combination with a low background, make the Tyr-FISH method very suitable for routine application in plant cytogenetic research. With Tyr-FISH we analysed the position of T-DNA inserts in transgenic shallots. We found that the inserts were preferentially located in the distal region of metaphase chromosomes. Sequential fluorescence in situ hybridization with a 375 bp satellite sequence suggested that a specific T-DNA insert was located within the satellite sequence hybridization region on a metaphase chromosome. Analysis of less-condensed prophase and interphase chromosomes revealed that the T-DNA was integrated outside the satellite DNA-hybridization region in a more proximal euchromatin region.

Neocentric activity of rye 5RL chromosome in wheat

The neocentric activity of a constriction located on the long arm of rye 5R chromosome (5RL) was analysed. It is not observed in normal rye but it is unusually stretched in bivalents involving 5RL telosomes in wheat-ditelosomic 5RL addition lines. In 20% of metaphase I cells, the 5RL bivalent presents the centromeres oriented to one pole and the constrictions oriented towards the opposite pole with a strong tension. In 5% of the cells, the constriction was able to orient the bivalent to the poles without tension in the centromeres. Sister chromatid cohesion, which is one of the distinct features of centromeric function, is persistent at the constriction in delayed 5RL chromosomes at anaphase I. Neither the elongation of the constriction nor the neocentric activity was observed at second meiotic division or mitosis. FISH studies showed that the 5RL constriction lacked detectable quantities of two repetitive DNA sequences, CCS1 and the 180-bp knob repeat, present at cereal centromeres and neocentromeres, respectively. We propose that, under special conditions, such as the wheat background, the normally non-centromeric DNA present at this region of 5RL acquires a specific chromatin structure, differentiated as an elongated constriction, which is able to function as a centromere.

Physical arrangement of retrotransposon-related repeats in centromeric regions of wheat

Cereal centromeres commonly contain many repetitive sequences that are derived from Ty3/gypsy retrotransposon. FISH analysis using a large DNA insert library of wheat identified a 67-kb clone (R11H) that showed strong hybridization signals on the centromeres. The R11H clone contains Ty3/gypsy retrotransposon-related sequences; both integrase and CCS1 family sequences were identified. Subsequently, we isolated additional 23 large-insert clones which also contained the integrase and CCS1 sequences. Based on the number of the integrase repeats in the clones determined by DNA gel blot analysis, we concluded that the retrotransposon-like sequences are tandemly repeated in wheat centromeres in ca. 55-kb interval on average. This conclusion is consistent with the results of FISH analysis on the extended DNA fibers.

The large-scale organization of the centromeric region in Beta species

In higher eukaryotes, the DNA composition of centromeres displays a high degree of variation, even between chromosomes of a single species. However, the long-range organization of centromeric DNA apparently follows similar structural rules. In our study, a comparative analysis of the DNA at centromeric regions of Beta species, including cultivated and wild beets, was performed using a set of repetitive DNA sequences. Our results show that these regions in Beta genomes have a complex structure and consist of variable repetitive sequences, including satellite DNA, Ty3-gypsy-like retrotransposons, and microsatellites. Based on their molecular characterization and chromosomal distribution determined by fluorescent in situ hybridization (FISH), centromeric repeated DNA sequences were grouped into three classes. By high-resolution multicolor-FISH on pachytene chromosomes and extended DNA fibers we analyzed the long-range organization of centromeric DNA sequences, leading to a structural model of a centromeric region of the wild beet species Beta procumbens. The chromosomal mutants PRO1 and PAT2 contain a single wild beet minichromosome with centromere activity and provide, together with cloned centromeric DNA sequences, an experimental system toward the molecular isolation of individual plant centromeres. In particular, FISH to extended DNA fibers of the PRO1 minichromosome and pulsed-field gel electrophoresis of large restriction fragments enabled estimations of the array size, interspersion patterns, and higher order organization of these centromere-associated satellite families. Regarding the overall structure, Beta centromeric regions show similarities to their counterparts in the few animal and plant species in which centromeres have been analyzed in detail.

Retrotransposon evolution in diverse plant genomes

Retrotransposon or retrotransposon-like sequences have been reported to be conserved components of cereal centromeres. Here we show that the published sequences are derived from a single conventional Ty3-gypsy family or a nonautonomous derivative. Both autonomous and nonautonomous elements are likely to have colonized Poaceae centromeres at the time of a common ancestor but have been maintained since by active retrotransposition. The retrotransposon family is also present at a lower copy number in the Arabidopsis genome, where it shows less pronounced localization. The history of the family in the two types of genome provides an interesting contrast between "boom and bust" and persistent evolutionary patterns.

Characterization and repeat analysis of the compact genome of the freshwater pufferfish Tetraodon nigroviridis

Tetraodon nigroviridis is a freshwater pufferfish 20-30 million years distant from Fugu rubripes. The genome of both tetraodontiforms is compact, mostly because intergenic and intronic sequences are reduced in size compared to other vertebrate genomes. The previously uncharacterized Tetraodon genome is described here together with a detailed analysis of its repeat content and organization. We report the sequencing of 46 megabases of bacterial artificial chromosome (BAC) end sequences, which represents a random DNA sample equivalent to 13% of the genome. The sequence and location of rRNA gene clusters, centromeric and subtelocentric satellite sequences have been determined. Minisatellites and microsatellites have been cataloged and notable differences were observed in comparison with microsatellites from Fugu. The genome contains homologies to all known families of transposable elements, including Ty3-gypsy, Ty1-copia, Line retrotransposons, DNA transposons, and retroviruses, although their overall abundance is <1%. This structural analysis is an important prerequisite to sequencing the Tetraodon genome.

CEREAL CHROMOSOME STRUCTURE, EVOLUTION, AND PAIRING

▪ Abstract  The determination of the order of genes along cereal chromosomes indicates that the cereals can be described as a single genetic system. Such a framework provides an opportunity to combine data generated from the studies on different cereals, enables chromosome evolution to be traced, and sheds light on key structures involved in cereal chromosome pairing. Centromeric and telomeric regions have been highlighted as important in these processes.

Plant retrotransposons

Retrotransposons are mobile genetic elements that transpose through reverse transcription of an RNA intermediate. Retrotransposons are ubiquitous in plants and play a major role in plant gene and genome evolution. In many cases, retrotransposons comprise over 50% of nuclear DNA content, a situation that can arise in just a few million years. Plant retrotransposons are structurally and functionally similar to the retrotransposons and retroviruses that are found in other eukaryotic organisms. However, there are important differences in the genomic organization of retrotransposons in plants compared to some other eukaryotes, including their often-high copy numbers, their extensively heterogeneous populations, and their chromosomal dispersion patterns. Recent studies are providing valuable insights into the mechanisms involved in regulating the expression and transposition of retrotransposons. This review describes the structure, genomic organization, expression, regulation, and evolution of retrotransposons, and discusses both their contributions to plant genome evolution and their use as genetic tools in plant biology.

De novo evolution of satellite DNA on the rye B chromosome

The most distinctive region of the rye B chromosome is a subtelomeric domain that contains an exceptional concentration of B-chromosome-specific sequences. At metaphase this domain appears to be the physical counterpart of the subtelomeric heterochromatic regions present on standard rye chromosomes, but its conformation at interphase is less condensed. In this report we show that the two sequence families that have been previously found to make up the bulk of the domain have been assembled from fragments of a variety of sequence elements, giving rise to their ostensibly foreign origin. A single mechanism, probably based on synthesis-dependent strand annealing (SDSA), is responsible for their assembly. We provide evidence for sequential evolution of one family on the B chromosome itself. The extent of these rearrangements and the complexity of the higher-order organization of the B-chromosome-specific families indicate that instability is a property of the domain itself, rather than of any single sequence. Indirect evidence suggests that particular fragments may have been selected to confer different properties on the domain and that rearrangements are frequently selected for their effect on DNA structure. The current organization appears to represent a transient stage in the evolution of a conventional heterochromatic region from complex sequences.

Identification and phylogenetic analysis of gypsy-type retrotransposons in the plant kingdom

PCR was performed with degenerate primers which hybridized to the homologous sequences in the reverse transcriptase (rt) genes of gypsy-type retrotransposons from rice (RIRE3, RIRE8 and RIRE2), using total DNA samples from various plants (monocots, dicots, pine, ginkgo, horsetail, liverwort and algae) as templates. Cloning and sequencing showed that the amplified fragments had various degrees of homology to the rt sequences of rice retrotransposons. Phylogenetic analysis showed that these retrotransposon homologues and some additional gypsy-type retrotransposons previously identified from plants could be classified into two families, A and B. In each family, the retrotransposons were further classifiable into several subfamilies. Interestingly, retrotransposons from a single or related plant species were clustered in each subfamily. This indicates that sequence divergence during vertical transmission has been a major influence on the evolution of gypsy-type retrotransposons in plants. The retrotransposons isolated from one plant species could often be classified into the two families. This indicates that the gypsy-type retrotransposons of a family evolved independently within a species without affecting the evolution of retrotransposons of the other family. Retrotransposons in each subfamily are characterized by the lengths of LTR, by the nucleotide sequences in the terminal regions of LTRs, and by the PBS (primer binding site) sequence complementary to the 3' sequence of a particular tRNA species.

Short communication: the cell cycle dependent phosphorylation of histone H3 is correlated with the condensation of plant mitotic chromosomes

Mitotically dividing cells of Secale cereale, Hordeum vulgare and Vicia faba were studied by indirect immunofluorescence using an antibody recognizing phosphorylated histone H3. The study revealed the following features: (i) the H3 phosphorylation starts at prophase and ends at telophase in the pericentromeric chromatin, is associated with the condensation of mitotic chromosomes and is independent of the distribution of late replicating heterochromatin. (ii) Compared with other chromosome regions, the pericentromeric chromatin is histone H3 hyperphos- phorylated. (iii) The study of a semi-dicentric chromo- some revealed that only at intact centromeres is the chromatin hyperphosphorylated at H3.