A molecular description of telometic heterochromatin in secale species

Telomeric motifs are present in a highly repetitive element in the Plasmodium berghei genome

Using as probes the subfragments of the telomeric sequence previously cloned by us from Plasmodium berghei DNA, we identified and cloned a 2.3 kb repeat, largely overlapping the original telomeric insert. Restriction mapping indicated that cloned inserts (2.3 kb in length) represented circularly permutated versions of a rather well conserved repeated element, at least in part organized in tandem. The 2.3 kb repeat family with a copy number of about 300 occupies about 4% of the whole genome. The copies are unevenly distributed among the chromosome-sized molecules revealed by pulsed field gradient electrophoresis. Complete sequence determination of the 2.3 kb element revealed that telomere-related motifs are present with a characteristic pattern in a set of tandem repeats, 27 bp long. The perfect conservation of these motifs as well as the pattern of chromosomal distribution suggest that we are dealing with a specialised structure subject to selective mechanisms of amplification and maintenance.

["Artificial" chromosomes]

The structural elements required for chromosome replication, segregation, and stability are replication origins, centromeres, and telomeres. DNA sequences capable of organizing these three elements have been isolated from yeast chromosomal DNA by means of recombinant DNA techniques and yeast cell transformation. It is now possible to combine these sequences into "artificial" chromosomes for yeast cells to obtain more insight into chromosome structure and function. Evidence is presented that the construction of artificial chromosomes functional in higher eukaryotes will be possible in the near future.

The occurrence of Ds-like sequences in cereal genomes

The occurrence of DNA sequences similar to the Ds-element of sh-m5933 maize (Ds-like sequences) was studied in other representatives of the Gramineae. The approximate number of copies of such sequences found under gentle and stringent conditions of washing was determined by dot-hybridization. It was shown that in the maize genome the number of copies of Ds-like sequences exceeds about ten-fold the content of such sequences found in wheat, rye and barley genomes. Quantitative differences in Ds-like sequences between wheat species with various genomes and ploidies (when estimated per genome) as well as between different H. vulgare varieties was not determined. The various melting points (Tm) of DNA-duplexes formed when the Ds-element is hybridized with wheat, rye and barley DNA respectively do not show significant differences and are essentially lower than the Tm of the Ds-element (by 8°-9°C). Thus, these duplexes have 9-11% of nucleotide substitutions in comparison to Ds sh-m5933. The data obtained permit one to suppose the presence of a series of Ds-like sequences heterogenous for the length and degree of homology to the Ds-element isolated from the shrunken locus (sh-m5933) of maize DNA.

Different AT-rich satellite DNAs in Cucurbita pepo and Cucurbita maxima

The AT-rich highly repeated satellite DNA of Cucurbita pepo (zucchini) and Cucurbita maxima (pumpkin) were cloned and their DNA structure was investigated. DNA sequencing revealed that the repeat length of satellite DNA in Cucurbita pepo is 349-352 base pairs. The percentage of AT-base pairs is about 61%. This satellite is highly conserved in restriction enzyme pattern and DNA sequence; sequence heterogeneity is about 10%. In contrast, the satellite DNA of Cucurbita maxima has a repeat length of 168-169 base pairs. This satellite is also rich in AT-base pairs (64%), existing in at least three different variants as revealed by restriction enzyme analysis and DNA sequencing. The sequence heterogeneity between these variants is about 15%. The two satellite DNAs showed no cross-hybridization to each other and sequence homology is only limited. Nevertheless, we found in the C. pepo genome a high amount of sequences resembling the satellite of C. maxima. In contrast, the satellite repeat of C. pepo is found in the C. maxima DNA only in a few copies. These observations were discussed with respect to satellite DNA evolution and compared to the data received from monocotyledonous species.

Small tandemly repeated DNA sequences of higher plants likely originate from a tRNA gene ancestor

Several monomers (177 bp) of a tandemly arranged repetitive nuclear DNA sequence of Brassica oleracea have been cloned and sequenced. They share up to 95% homology between one another and up to 80% with other satellite DNA sequences of Cruciferae, suggesting a common ancestor. Both strands of these monomers show more than 50% homology with many tRNA genes; the best homologies have been obtained with Lys and His yeast mitochondrial tRNA genes (respectively 64% and 60%). These results suggest that small tandemly repeated DNA sequences of plants may have evolved from a tRNA gene ancestor. These tandem repeats have probably arisen via a process involving reverse transcription of polymerase III RNA intermediates, as is the case for interspersed DNA sequences of mammalians. A model is proposed to explain the formation of such small tandemly repeated DNA sequences.

Rye cytology, cytogenetics and genetics - current status

Progress in rye karyology is reviewed with respect to chromosome structure, recognition and chromosome nomenclature. Considerable contributions have been brought about by molecular techniques which have even revealed nucleotide sequences of some of the ribosomal DNA. DNA sequence organization correlates with the distribution of major Giemsa C-band regions as well as with N-bands and the binding sites of fluorescent dyes. The several banding patterns permit the classification of rye chromosomes. The increased data and widespread application of banding analysis require a consistent system of chromosome and/or band designation. Therefore, a standard band nomenclature is proposed with reference to the recommendations of the "Paris Conference on Standardization in Human Cytogenetics". In addition, advances in genetics are summarized and discussed. Based on the original accepted standard karyogram and banding patterns of the rye chromosomes, meanwhile, 120 genes determining several characters have been associated with individual chromosomes and/or chromosome arms, including linkage studies for about 19 arrangements. Most results were obtained using wheat-rye addition lines as well as test crosses with defined translocations. Moreover, genetical studies based on appropriate trisomic and telotrisomic material resulted in the localization of 19 genes, including their linkage relationships.

Chromosome alterations in the karyotype of triticale in comparison with the parental forms : 1. Heterochromatic regions of R genome chromosomes

R genome chromosomes were studied in two forms of primary triticales (hexaploid 'TPG-1/1-78' and octoploid 'AD 825') and in their parent rye forms (Secale cereale L. cv. 'Kharkovskaya 60' and 'VSKhI', respectively) using the methods of C-banding and morphometric analysis. The size of some heterochromatic segments was shown to alter in the karyotype of triticale. An increase in size was detected approximately in half of all telomeric C-bands; the size of the other C-bands either decreased or did not change. The frequencies of these alterations were 1∶1. The variability in the size of telomeric C-bands in rye chromosomes diminished in both triticales studied. The two triticale forms inherited variants of R genome chromosome polymorphism predominantly with the medium size range of telomeric C-bands. The centromeric C-bands in both triticale forms either enlarged or did not alter. Possible mechanisms responsible for the observed pattern of alterations are discussed.

Sequence characterization of Tetrahymena macronuclear DNA ends

Tetrahymena is a ciliated protozoan which has two nuclei: a micronucleus, which maintains the genetic continuity of the cell, and the macronucleus which is derived from the micronucleus after sexual conjugation. A macronuclear DNA library was constructed to contain DNA ends. A probe containing C4A2 repeats which are known to be present at macronuclear DNA ends (1) was used to screen the library. Three clones were characterized by sequencing, restriction enzyme mapping and Bal 31 digestion. The data indicate that these three clones represent macronuclear DNA ends which were generated by DNA fragmentation during macronuclear formation. The sequencing data at the C4A2 repeat junction show a conserved sequence of five nucleotides, TTATT. Sequences further away show no obvious homologies except that they are highly enriched in AT. This structure is quite different from the subtelomeric sequences of other organisms.

Wheat specific repetitive DNA sequences - construction and characterization of four different genomic clones

The construction and molecular analysis of four recombinant clones - pTa1, pTa2, pTa7, and pTa8 - is described. The four clones contain different highly repeated sequences of genomic DNA from Triticum aestivum variety 'Chinese Spring'. The wheat specificity has been determined by colony and dot blot hybridization in comparison with total rye DNA (Secale cereale variety 'Petka'). The four clones with a variable degree of specificity were compared by sequence analysis after the recloning of wheat DNA inserts into M13 mp8. Within the sequencing data a tendency can be observed that those repeated sequences which show the highest degree of species specificity contain a significantly increased amount of GC residues.

Organization and evolution of a higher plant alphoid-like satellite DNA sequence

Two recombinant plasmids containing, respectively, three and eight tandem repeats of a 177 base-pair (bp) element from radish nuclear DNA have been isolated. These plasmids were used as probes to investigate the organization and the copy number of this element within the genome. This sequence is present in congruent to 0.6 million copies. Restriction analysis provides evidence for sequence heterogeneity and reveals the occurrence of non-overlapping subfamilies. Nine units were sequenced and found to be remarkably conserved. However, sequences in the two clones clearly belong to two distinct subgroups. Our data suggest that these sequences evolved in a concerted manner and that homogenization mechanisms such as gene conversions certainly took place. The 177 bp sequence is made from three 60 bp blocks that are derived from a common ancestor. Exchanges between the three blocks probably occurred before they became fixed as a patchwork of short sequences, the 177 bp element. This unit of 177 bp was then amplified in several steps. The presence of such a repeated sequence can be detected in other Cruciferae when hybridizations are carried out under low stringency conditions. Direct comparison with a previously published mustard satellite DNA sequence indicates a similar organization and a 75% homology. Homology was also found with shorter regions (congruent to 60 bp) of broad bean and corn satellite DNA. Finally, homology was also found with several animal alphoid sequences, suggesting that this family also occurs in the plant genomes.

Repetitive DNA and chromosome evolution in plants

Most higher plant genomes contain a high proportion of repeated sequences. Thus repetitive DNA is a major contributor to plant chromosome structure. The variation in total DNA content between species is due mostly to variation in repeated DNA content. Some repeats of the same family are arranged in tandem arrays, at the sites of heterochromatin. Examples from the Secale genus are described. Arrays of the same sequence are often present at many chromosomal sites. Heterochromatin often contains arrays of several unrelated sequences. The evolution of such arrays in populations is discussed. Other repeats are dispersed at many locations in the chromosomes. Many are likely to be or have evolved from transposable elements. The structures of some plant transposable elements, in particular the sequences of the terminal inverted repeats, are described. Some elements in soybean, antirrhinum and maize have the same inverted terminal repeat sequences. Other elements of maize and wheat share terminal homology with elements from yeast, Drosophila, man and mouse. The evolution of transposable elements in plant populations is discussed. The amplification, deletion and transposition of different repeated DNA sequences and the spread of the mutations in populations produces a turnover of repetitive DNA during evolution. This turnover process and the molecular mechanisms involved are discussed and shown to be responsible for divergence of chromosome structure between species. Turnover of repeated genes also occurs. The molecular processes affecting repeats imply that the older a repetitive DNA family the more likely it is to exist in different forms and in many locations within a species. Examples to support this hypothesis are provided from the Secale genus.

Distribution of telomere-associated sequences in yeast

Two middle repetitive DNA sequences called X and Y' are found near the telomeres of many chromosomes in Saccharomyces cerevisiae. Orthogonal field gel electrophoresis (OFAGE) was used to examine the distribution of X and Y' on different yeast chromosomes. Although the distribution of X and Y' varies among different laboratory strains of yeast, most yeast chromosomes in four different strains carry both X and Y'. However, at least one chromosome in each strain lacks the Y' element. This result indicates that Y' is not essential for replication or segregation of at least some yeast chromosomes.

A rapid screening technique for the detection of repeated DNA sequences in plant tissues

DNA sequences cloned from nuclear and mitochondrial chromosomes have been used as hybridisation probes to distinguish different plant genotypes. The probes are hybridised to squashed segments of tissue e.g. root tips. The 'squash-dot' method is rapid and suitable for screening large numbers of individual plants. One probe, specific for a rye repeated sequence family, enables rye chromosomes to be detected in wheat plants. A probe for ribosomal DNA enables plants with high or low numbers of ribosomal RNA genes to be distinguished. A maize mitochondrial DNA probe is used to distinguish plants with N, T or S cytoplasms.

Sequence analysis of Vicia faba highly repeated DNA: the BamHI repeated sequence families

Cleavage of Vicia faba nuclear DNA with the restriction endonuclease BamHI yielded discrete size classes of 250, 850, 900, 990, 1 150, 1 500 and 1 750 bp of highly repetitive DNA. Each of these sequence families comprised about 3% of the total genomic DNA. Some sequence members from each sequence family were cloned in pBR322 and their primary structures determined. Computer analyses of nucleotide sequences suggested the existence of about 60 bp sequence periodicity within the repeating unit of the 990 bp sequence family, though the extent of homology among the surmised shorter subrepeat units was very low. With other BamHI sequence families, however, the data did not show any clear internal sequence periodicity. The repeat units of the 850 bp and 1 750 bp sequence families contained nucleotide sequences homologous to the 250 bp family sequence. No sequence relationship between or among other sequence families was observed. There was 13-25% sequence variation among 6 cloned members of the 250 bp family and probably also among those of other BamHI repeat families. DNA sequences homologous to these V. faba BamHI repeat families were detected in Pisum sativum DNA by Southern blot hybridization. Furthermore, very weak cross-hybridization was observed with plant DNAs from Phaseolus vulgaris, Triticum aestivum, Cucumis sativus and Trillium kamtschaticum.

Transfer of yeast telomeres to linear plasmids by recombination

We have characterized two types of recombination events between linear plasmids and yeast chromosomal telomere-adjacent sequences (Y' elements). In one type of event, a linear plasmid restriction-cut within a Y' element regains the missing Y' DNA, and may also acquire additional Y' elements. This process is similar to the healing of broken chromosomes by recombination. In a second type of event, terminally added C1-3A sequences on the linear plasmid interact with C1-3A sequences located just internal to the chromosomal Y' elements, resulting in the addition of one to four Y' elements to the plasmid. Similar recombination events occurring between different chromosome ends could lead to the dispersal and amplification of telomere-adjacent sequences.

mu-2: mutator gene in Drosophila that potentiates the induction of terminal deficiencies

An x-ray-dependent mutator on chromosome 3 of Drosophila melanogaster is described that specifically increases the recovery of deletions for chromosomal tip regions. Such deficiencies can be induced on any chromosome. More centromere proximal mutations, as assayed by the sex-linked recessive lethal test, are not increased over the wild-type control. As far as can be determined by genetic, cytological, and molecular assays, the deletions extend to the very end of the chromosome involved. In addition, the frequency of these deletions is directly proportional to x-ray dose, suggesting that they are one-break rearrangements. It is proposed that the mutator is blocked in a major pathway for the repair of DNA double-strand breaks, and that a minor repair pathway is responsible for the addition of new telomeres under these conditions.

Sequence analysis of Vicia faba repeated DNA, the FokI repeat element

A type of highly repeated DNA sequences present in the genome of Vicia faba was detected by digestion its nuclear DNA with FokI endonuclease and fractionating the digests on polyacrylamide gels. Four fragments of 59, 108, 177 and 246 bp of the FokI repeated sequences were collected from the gels and their primary structures were determined by the method of Maxam and Gilbert. These repeated DNA sequences were shown to be a multiple tandem array of a 59 bp sequence element. And its nucleotide sequence was almost completely conserved among all the sequence members of each the size class and also among these classes. This sequence element consists of a duplet of an about the duplet has an incomplete dyad symmetrical structure.

Conservation of repetitive DNA sequences in deer species studied by southern blot transfer

The Cervidae show one of the largest variations in chromosome number found within a mammalian family. The five species of the deer family which are the subject of this study vary in chromosome number from 2n = 70 to 2n = 6. Digestion with the restriction enzymes EcoRI, HpaII, HaeIII and MspI reveals that there is a series of highly repetitive sequences forming similar band patterns in the different species. To obtain information on the degree of homology among these conserved sequences we isolated a HpaII restriction fragment of approximately 990 base pairs from reindeer DNA. This DNA sequence was 32P-labelled and hybridized by the Southern blot technique to DNAs cleaved with HpaII and HaeIII from the reindeer and four other Cervidae species. Hybridization to specific restriction fragments was recorded in all species. The patterns of hybridization showed a higher degree of similarity between reindeer, elk and roe deer than between reindeer and the Asiatic species (fallow deer and muntjac). Homologies are still present between the highly repetitive sequences of the five species despite the drastic reorganization that led to a change in chromosome number from 6 to 70.

Organization and evolution of repeated DNA sequences in closely related plant genomes

In common with many other eukaryotic species, the genomes of species in the genus Allium contain a high proportion of repeated DNA sequences, which may be implicated in the considerable differences in genome size that are seen between even very closely related species. The gross organization of repetitive sequences within the genome of Allium sativum and of some other related species has been investigated using DNA/DNA hybridization studies. Such studies show that there has been much modulation in the amounts of different repeated DNA families during the evolution of the genus Allium; these repetitive elements are interspersed in all species with sequences of low repetition. The organization and distribution of one particular repetitive family within the genus has been examined using a cloned hybridization probe. Hybridization of this probe to DNA from related genomes reveals that this element is present in all Allium species examined, but with large-scale modulation of its abundance, and some considerable changes in its sequence environment. The evolution of such genome-specific arrangements of common repetitive elements and the possible mechanisms by which they might be maintained are discussed.

Characterization of the multigene family coding for HMW glutenin subunits in wheat using cDNA clones

cDNA clones encoding wheat HMW glutenin subunits have been isolated from a cDNA bank made to poly A(+) RNA from developing wheat endosperm var. Chinese Spring. One such clone, pTag 1290, has enabled us to identify the HMW glutenin mRNA species. The DNA sequence of this clone has been partially determined and it contains several tandem DNA repeats. The sequence is discussed in relation to the generation of the HMW glutenin subunit gene family. Analysis of the organization of the HMW glutenin sequences in the wheat genome revealed that the genes encoding HMW glutenin subunits exist in low copy number and are located on the long arm of each of the homoeologous group 1 chromosomes.

Complex organization of a cryptic satellite DNA in the genome of the marine invertebrate Rapana thomasiana Grosse (Gastropoda)

Isopicnic centrifugation in Cs2SO4-Ag+ gradients at pH 7.0 reveals that the genome of the marine snail Rapana thomasiana Grosse (Gastropoda) contains an AT-rich satellite fraction comprising 5% of the DNA. Restriction enzyme analysis shows that the satellite DNA is composed of a number of related subsets arranged in tandem arrays. They have evolved from the segmental amplification of an 1460 bp long monomer unit with a complex inner organization. Most probably, the present basic repeat originates from an ancestral 400-500 bp long sequence in which some insertions and/or deletions have occurred.

The structural organization and DNA sequence of a wheat histone H4 gene

Some wheat histone H4 genes have been cloned from a Charon 4 wheat genomic DNA library using sea urchin histone H4 DNA as a probe. DNA sequence analysis of a cloned gene showed that the deduced amino acid sequence of wheat histone H4 protein was identical to that of pea. The 5' end of wheat histone H4 mRNA was mapped on the cloned gene by the S1-procedure. Southern blotting analysis of the genomic DNA indicated that histone H4 genes were reiterated 100 to 125 times per hexaploid wheat genome.

Occurrence and evolution of homogeneously staining regions may be due to breakage-fusion-bridge cycles following telomere loss

Chromosomes with homogeneously staining regions (HSR) were analysed in a subclone of the H4 rat hepatoma cell line, where they represent amplification of the ribosomal RNA (rRNA) genes. Detailed G-band analysis of the subclone revealed that an HSR on the short arm of chromosome 3 became unstable and changed its position within the chromosome. The evolution of this marker chromosome was associated with the terminal deletion of the normal long arm of the HSR-bearing chromosome 3 and may have involved ring formation as a result of fusion between the HSR on the short arm and the broken end of the long arm. Evidence was obtained for breakage at different sites within the ring, producing chromosomes with HSRs located terminally on either the long arms or both arms. The terminally located HSR underwent elongation in some cells presumably as a result of a breakage-fusion-bridge cycle characteristic of instability due to telomeric loss. It is suggested that terminally located HSRs may generally occur this way.

An inverted repeat borders a fivefold amplification in satellite DNA

One variant of a complex satellite DNA of the Bermuda land crab is significantly longer than the average repeat unit of the satellite. The extra DNA in the variant is accounted for by a fivefold tandem amplification of a 0.142-kilobase sequence. The amplified sequence is bounded by a tetranucleotide inverted repeat; the upstream arm of the inverted repeat is missing from two other variants of the satellite. The latter variants contain only one copy of a sequence that is closely related to the amplified sequence. By contrast, in several satellite DNA's of other organisms, extra DNA is inserted.

Telomere regions in Drosophila share complex DNA sequences with pericentric heterochromatin

A 12 kb fragment of Drosophila melanogaster DNA cloned in a lambda phage, lambda T-A, is shown by in situ hybridization to contain sequences homologous to DNA at the extreme ends of each of the polytene chromosomes and to the pericentric sequences present in the beta heterochromatin. This pattern of hybridization is seen for each of the four D. melanogaster stocks that have been studied. Most of the sequence in lambda T-A shows no detectable homology to DNA within the banded chromosome arms. (The only exception is what appears to be a short mobile element inserted in lambda T-A. This portion of lambda T-A hybridizes with internal arm sites that vary from stock to stock). Analysis of restriction fragments of genomic DNA indicates that the sequences in lambda T-A are homologous to complex sets of repeated sequences that differ from stock to stock in D. melanogaster. Some, but not all, of the members of these sets are underreplicated during polytenization in D. melanogaster. The sequences in lambda T-A are also homologous to complex sets of repeated sequences in the genomes of other Drosophila species belonging to the melanogaster species group. Pericentric and telomeric localization is conserved in these related species, although analysis of DNA fragments shows marked changes in sequence organization on a finer scale. The constancy of the localization of lambda T-A-homologous sequences to telomeric and pericentric regions suggests that these sequences serve a function in those regions.

Growth of chromosome ends in multiplying trypanosomes

Some of the genes for the variant surface glycoproteins of trypanosomes are located close to a discontinuity in the DNA, presumably a chromosome end. We show here that DNA fragments containing these telomeres increase in length in multiplying trypanosomes at a rate of about 10 base pairs per division. We argue that chromosome growth may not be restricted to trypanosomes and could explain the heterogeneity of telomeric DNA fragments observed in some other organisms.

Tandemly repeated DNA sequences from Xenopus laevis. I. Studies on sequence organization and variation in satellite 1 DNA (741 base-pair repeat)

A family of tandemly repeated sequences, having a basic repeating unit of 741 base-pairs, has been identified in Xenopus laevis DNA and designated satellite 1. Apart from its rather long repeat unit, the characteristics of this DNA appear to be quite similar to those of complex satellite DNAs from other organisms. The nucleotide sequence of a cloned repeat unit shows no evidence of simpler internal repeats, and there is no obvious suggestion of reasonable RNA- or protein-coding regions. Transcripts homologous to this DNA could not be demonstrated in liver, embryo or oocyte RNAs. By blot-hybridization, satellite 1 has been shown to exist in the genome chiefly as tandem repeats of the 741 base-pair sequence. However, there are a number of repeats that differ from the normal sequence (as judged by loss or gain of restriction sites) and some that differ in length. Similar variants are often, but not always, clustered. Characterization of genomic clones of this satellite has confirmed the tandem organization and clustering of variants. The nature of some variants has been elucidated in more detail. Some regions of the basic repeat seem to be more prone to variation than are others.

Novel repetitive sequence families showing size and frequency polymorphism in the genomes of mice

A middle repetitive sequence, PR1, originally found in mouse rDNA appeared as satellite-like bands when EcoRI and BglII digests of genomic DNA were subjected to Southern blot hybridization using PR1 as probe. The copy number and sizes of PR1-related satellite-like bands, designated as PR1 families, differed remarkably among the subspecies and laboratory strains of mice when the EcoRI digests of genomic DNAs were compared. These bands were not detected in rat and human DNAs. A unit of PR1 sequence was determined by examining cloned EcoRI 3.5 kb (kb, 10(3) bases) fragment and 6.6 kb rDNA by cross-hybridization and sequence analysis: 3.5 kb and 6.6 kb DNAs are composed of homologous PR1 regions and the flanking non-homologous sequences. The results indicate that amplification of different sequences containing PR1 has occurred in different subspecies and strains of mice, and that the segments of satellite-like bands are likely to have been created by recombination of the PR1 sequence with other DNA segments before amplification. The chromosomal distribution of the 3.5 kb PR1 family was studied by back-crossing the female F1 between BALB/c and DDD/1 to male DDD/1. The segregation data strongly suggest that most, if not all, of this family are located on a single chromosome. The stability of these PR1 families in the genomes of cultured cells of a given strain was also examined. An extra band homologous to PR1 appeared in their genomes, but was not detected in other tissues, indicating that some PR1 families may change even during cell propagation.

The wheat ribosomal DNA spacer region: Its structure and variation in populations and among species

The wheat rDNA clone pTA250 was examined in detail to provide a restriction enzyme map and the nucleotide sequence of two of the eleven, 130 bp repeating units found within the spacer region. The 130 bp units showed some sequence heterogeneity. The sequence difference between the two 130 bp units analysed (130.6 and 130.8) was at 7 positions and could be detected as a 4 °C shift in Tm when heterologous and homologous hybrids were compared. This corresponded to a 1.2% change in nucleotide sequence per ΔTm of 1 °C. The sensitivity of the Tm analysis using cloned sequences facilitated the analysis of small sequence variations in the spacer region of different Triticum aestivum cultivars and natural populations of T. turgidum ssp. dicoccoides (referred to as T. dicoccoides). In addition spacer length variation was assayed by restriction enzyme digestion and hybridization with spacer sequence probes.Extensive polymorphism was observed for the spacer region in various cultivars of T. aestivum, although within each cultivar the rDNA clusters were homogeneous and could be assigned to particular chromosomes. Within natural populations of T. dicoccoides polymorphism was also observed but, once again, within any one individual the rDNA clusters appeared to be homogeneous. The polymorphism, at the sequence level (assayed by Tm analysis), was not so great as to prevent the use of spacer sequence variation as a probe for evolutionary relationships. The length variation as assayed by restriction enzyme digestion did not appear to be as useful in this regard, since its range of variation was extensive even within populations of a species.