A comparison of chromosomal and allozymal variation across a narrow hybrid zone in the grasshopper Caledia captiva

A hybrid zone between the Moreton and Torresian taxa of the grasshopper Caledia captiva in S.E. Queensland has been characterised in terms of allozyme and chromosome variation within the same individuals. — On chromosomal criteria (pericentric rearrangements), the zone is asymmetrical with evidence of high levels of introgression of Torresian chromosomes into the Moreton taxon. This is apparent from the analysis of two independent transects across the hybrid zone. Major changes in chromosomal frequency occur over distances of less than 0.5 km. and the level of introgression differs between the two transects, with much higher levels in the northern Moreton populations, characterised by an acrocentric X-chromosome, when compared with the southern metacentric-X Moreton populations. Chromosome analysis of samples taken from the same transect over two years has revealed no major changes in the structure of the zone. Moreover, a Moreton population located only 0.5 km. from the null point was found to be stable over 6 generations with evidence for a new balanced genome having originated following the differential incorportation of Torresian chromosomes. — Contrary to the chromosomal situation, the same hybrid zone was found to be symmetrical with respect to allozyme variation with evidence of movement of diagnostic alleles in both directions across the zone. The alleles are independent and not tightly linked to any of the pericentric rearrangements. Thus these 5 alleles are acting as markers of the background genome and reveal the relatively free movement of genes which are located outside the pericentric rearrangements. — It is proposed that the hybrid zone in Caledia captiva is unstable and is moving slowly in a westerly direction into the Torresian territory. This is due to the ability of the Moreton taxon to incorporate more readily into its genome those Torresian chromosomes or chromosome segments which increase the fitness of the Moreton taxon. On chromosomal criteria, the Torresian taxon does not share the same capacity. — It is suggested that, so long as the two taxa retain their ability to hybridise with subsequent asymmetrical introgression, the zone will continue to move westwards and eventually lead to the selective incorporation of the Torresian genome into the Moreton taxon. This will result in a polymorphic situation with clinal variation in chromosomal frequencies. The structure of the zone is dependent upon a fine balance between genomic reorganisation in recombinant genotypes and the relative dispersal capacities of the two hybridising taxa.     

Biased gene conversion and asymmetrical introgression between subspecies

Data from chromosomal in situ hybridization and restriction fragment length polymorphism (RFLP) of ribosomal DNA (rDNA) obtained from the same individuals of two subspecies of the Australian grasshopper Caledia captiva, suggest the occurrence of biased exchange of DNA sequences in hybrid individuals. Thus, there are a disproportionate number of Torresian individuals that possess Moreton ribosomal RNA gene sequences. This bias correlates with the previously described pattern of asymmetrical introgression of Moreton rDNA into Torresian populations. The present findings demonstrate the potential effect of biased gene conversion on the frequencies of introduced gene variants.     

THE EFFECT OF A PARTIAL BARRIER ON THE MOVEMENT OF A HYBRID ZONE

Computer simulations of clines (Brues, 1972; Endler, 1977) as well as theoretical arguments (Nagylaki, 1975), have shown that steps in gene frequencies will be pulled to partial barriers (areas of reduced gene flow) if they form within approximately a cline width of the partial barrier. The behavior of a hybrid zone between two chromosomal taxa (“Moreton” and “Torresian”) of the acridine grasshopper Caledia captiva in southeast Queensland has been analyzed and found to comform qualitatively with a model of altered gene flow patterns. Clines in four enzyme systems were analyzed for 1983 and 1986 along a transect across the hybrid zone. The clines have shifted towards an area of regenerating forest, while homozygote frequencies have increased at this point. This forest barrier has broken the continuity of the spatial distribution of C. captiva, forming population islands in part of the hybrid zone, and thus reducing the amount of gene flow. The distance between the barrier and the original cline is approximately of the order of a cline width, so that they would be expected to interact. Historical information suggests that the secondary contact between the “Moreton” and “Torresian” taxa occurred very recently (1844–1940), due to the intensive land-clearing activities during the European settlement.     

Evidence for association of chromosomal form and development time from complex clines and geographic races in the grasshopper Caledia captiva (Orthoptera: Acridadae)

Evidence for an association between chromosomal form and development time in the grasshopper Caledia captiva (F.) was obtained through comparison of two geographic taxa and analysis of a complex latitudinal cline within one of the taxa. Northern populations of the Moreton taxon possess a metacentric genome and are slow-developing. In contrast, the Torresian taxon, distributed throughout northern, coastal Australia, a region of pronounced seasonality in rainfall, and southern populations of the Moreton taxon, which inhabit a region of pronounced seasonality in temperature, Soth have an acrocentric genome and are fast-developing. The convergence of chromosomal form and development time between Torresian and southern Moreton populations appears to be driven by convergence in life history. Seasonality limits grasshoppers to one generation per year and favours fast development. The transition between relatively acrocentric southern Moreton populations and relatively metacentric northern Moreton populations is gradual but not monotonic. Instead, a shift to a bivoltine life history in the middle of the transect occurs and is associated with shifts in both development time and chromosomal form. These results imply an adaptive role for chromosomal form, although the causative link between chromosomal variation and variation in development time remains to be established.     

The chromosomal component of reproductive isolation in the grasshopper Caledia captiva

An analysis of the relative viabilities of recombinant and nonrecombinant chromosomes among the surviving embryos from back-crosses involving the Moreton (M) and Torresian (T) taxa has revealed that these embryos do not contain a representative sample of gametes derived from the F₁ hybrid parent. The significant deviations in the hybrid gametic population arise entirely from intrachromosomal effects with no evidence of any between-chromosome interactions. This is interpreted as clear evidence to show that recombinational repatterning within heterozygous bivalents in the F₁ parent is a significant factor in inducing the observed deviant segregation ratios. Furthermore, by using a population which is chromosomally equivalent to the Torresian but genically similar to the Moreton, it has been shown that over 46% of the F₂ embryonic breakdown arises solely from the effects of chromosomal heterozygosity upon recombination repatterning among (Moreton × Torresian) F₁ hybrids. From these data it is proposed that each chromosome is internally coadapted in the sense that it contains balanced blocks of cis-acting acting loci which can be disrupted by recombinational change. Disruption of the linear association of the genes on structurally different chromosomes by recombination repatterning results in novel intrachromosomal associations which may be functionally inadequate and so lead to arrested embryonic development. It is speculated that an important factor in arresting development may involve interactions between the novel recombinant chromosomes of the gamete and maternal factors laid down in the egg during oogenesis which are responsible for the sequential activation of the genomes of the progeny during development. Thus coadaptation is interpreted in terms of the functional intergration of a chromosome with the products of the genome of the previous generation. The assessment of the relative viabilities of recombinant and nonrecombinant chromosomes has shown that the Torresian nonrecombinant chromosomes possess the highest viabilities in the sequence T^N>M^NT^R = M^R where N and R represent nonrecombinant and recombinant classes. This sequence is relevant to the structure of the hybrid zone between the Torresian and Moreton taxa and explains both its asymmetry and the basis of the observed introgression of Torresian chromosomes into the Moreton taxon and the absence of the reverse movement.     

Telomeric satellite DNA functions in regulating recombination

Molecular and cytogenetical analyses of three sibling species of Australian grasshopper, Atractomorpha australis, A. species-1 and A. similis, resolves one of the long standing problems of highly repeated DNA. In this system satellite DNA functions in regulating the level and position of recombination, irrespective of whether the repeated DNA is located in telomeric or centric regions. — Even though the three species do not differ in their euchromatic genome sizes, their relative DNA contents are 1.00/1.10/ 1.41, the difference in genome size being due solely to visible centric or telomeric blocks of heterochromatin. — Antibiotic analytical and preparative ultracentrifugation, in situ hybridization and renaturation kinetic analyses reveal that a large cryptic satellite of A. similis constitutes the heterochromatic telomeric blocks of nearly all autosomes and that the DNA of this satellite is highly repeated. — Comparison of these grasshopper data with published literature of heterochromatic rearrangements in Drosophila and with heterochromatin distribution and recombination patterns in diploid plant species reveals that in every case heterochromatin is implicated in some form of alteration in the meiotic recombination system.     

The spatial distribution of chromosomes in metaphase neuroblast cells from subspecific F1 hybrids of the grasshopper Caledia captiva

The spatial distribution of chromosomes has been analysed in radial metaphase neuroblast cells in F₁ hybrid embryos generated by crossing individuals of the Moreton and Torresian (TT) chromosomal taxa of the grasshopper Caledia captiva. The Moreton individuals were of two kinds depending on whether they carried an acrocentric X (MAX) or a metacentric X (MMX). No significant associations were detected between any pair of homologous chromosomes in either male or female (MAX x TT) and (MMX x TT) F₁ hybrids. This result was supported by data which showed that the mean separation between homologues is greater, although not significantly so, than the mean separation between non-homologous chromosomes within the two Moreton genomes. Indeed, in a number of cases, genome separation was clearly observed in radial metaphase preparations from these F₁ hybrids. By comparison the analysis of pairwise associations between non-homologous chromosomes within the MMX and MAX Moreton genomes revealed a number of significant associations and dissociations which strongly suggests that at least some chromosomes in these genomes are organised non-randomly at metaphase. Of particular interest was the highly significant X-5 association in the MMX genome since in a previous study X-5 rearrangements were found to occur repeatedly among different backcross progeny involving Moreton x Torresian F₁ hybrids. Additionally a comparison of the organisation of chromosomes in the MAX and MMX genomes, which differ primarily by the type of X chromosome, revealed that in a number of cases pairs of chromosomes are arranged very differently with respect to each other. The distribution of chromosomes on the hollow spindle was also analysed to investigate whether a specific spatial ordering of chromosomes exists within these Moreton genomes based on the association of pairs of short arms and pairs of long arms of most similar length (the Bennett model). The twelve chromosomes in both genomes were uniquely ordered in a single chain. However, because of computing limitations, only the ordered arrangement of chromosomes 1–10 was investigated. An analysis of 48 cells in the MMX and 38 cells in the MAX genomes showed that the predicted order in the ten chromosome sub-set in each genome did not rank in the top 20% of the 181,440 possible orders. This suggests that, although there is a good evidence that some non-homologous chromosomes may be associated non-randomly at metaphase in these genomes, they do not appear to show a specific, ordered arrangement as predicted by the Bennett model. The significance of the observed non-random organisation of chromosomes in the MMX and MAX genomes is discussed in relation to the generation of novel chromosome rearrangements in Moreton x Torresian F₁ hybrids and the evolution of the Moreton and Torresian genomes.     

Two repeated DNA sequences from the heterochromatic regions of rye (Secale cereale) chromosomes

Rye DNA sequences renaturing with a C₀t <0.02 mol·sec/l, are largely undigested by the restriction enzyme HindIII. These HindIII-spared sequences are mostly located in telomeric heterochromatin. When digested with EcoRI* and cloned into the EcoRI site of pBR 325, these sequences yielded clones of two classes when hybridized to a probe of rapidly renaturing DNA. One class contains a DNA sequence which is a major constituent of the telomeric heterochromatic blocks, while the other is a minor component of the highly repeated DNA of the genome. The major component was sequenced, its chromosomal distribution mapped using wheat-rye addition lines and its distribution in meiotic prophase nuclei determined. The minor component is present in significant amounts in wheat as well as in rye and is localized at the terminal heterochromatic regions of three rye chromosomes but not in the major blocks of heterochromatin.     

New families of site-specific repetitive DNA sequences that comprise constitutive heterochromatin of the Syrian hamster (Mesocricetus auratus, Cricetinae, Rodentia)

We molecularly cloned new families of site-specific repetitive DNA sequences from BglII- and EcoRI-digested genomic DNA of the Syrian hamster (Mesocricetus auratus, Cricetrinae, Rodentia) and characterized them by chromosome in situ hybridization and filter hybridization. They were classified into six different types of repetitive DNA sequence families according to chromosomal distribution and genome organization. The hybridization patterns of the sequences were consistent with the distribution of C-positive bands and/or Hoechst-stained heterochromatin. The centromeric major satellite DNA and sex chromosome-specific and telomeric region-specific repetitive sequences were conserved in the same genus (Mesocricetus) but divergent in different genera. The chromosome-2-specific sequence was conserved in two genera, Mesocricetus and Cricetulus, and a low copy number of repetitive sequences on the heterochromatic chromosome arms were conserved in the subfamily Cricetinae but not in the subfamily Calomyscinae. By contrast, the other type of repetitive sequences on the heterochromatic chromosome arms, which had sequence similarities to a LINE sequence of rodents, was conserved through the three subfamilies, Cricetinae, Calomyscinae and Murinae. The nucleotide divergence of the repetitive sequences of heterochromatin was well correlated with the phylogenetic relationships of the Cricetinae species, and each sequence has been independently amplified and diverged in the same genome.     

The mapping of highly-repeated DNA families and their relationship to C-bands in chromosomes of Secale cereale

The relationship between the chromosomal location of heterochromatin C-bands and of four non-homologous repeated sequence families constituting 8 to 12% of total rye DNA has been investigated in chromosomes of rye (Secale cereale) by in situ hybridisation. Three rye varieties, a set of rye disomic additions to wheat and a triticale were studied. Only centromeric and nucleolar organizer region (NOR) associated C-bands failed to display hybridisation to at least one of the sequences and many telomeric blocks of heterochromatin contained all four repeated sequence families. Both between-variety differences in the chromosomal distribution of repeated sequences, and intravarietal heterozygosities were frequently noted and are probably widespread. — Previously reported deletions of heterochromatin from King II rye chromosomes added to the Holdfast wheat complement were correlated with deletions of some, but not all, of the highly repeated sequence families. A previously unreported loss of some families from King II rye chromosome 4R/7R in a Holdfast wheat genetic background was detected. This loss was not associated with complete deletion of a C-band. A deletion has also probably occurred from the short arm telomere of 4R/7R in the triticale variety Rosner. It is suggested that the families of repeats in rye telomeric heterochromatin which are absent from wheat are selected against in the wheat genetic background.     

The karyotypes of the thorny catfishes <Emphasis Type="Italic">Wertheimeria maculata</Emphasis> Steindachner, 1877 and <Emphasis Type="Italic">Hassar wilderi</Emphasis> Kindle, 1895 (Siluriformes: Doradidae) and their relevance in doradids chromosomal evolution

We studied the karyotypes of two doradids, the rare and endangered Wertheimeria maculata and a derived Amazonian species, Hassar wilderi. Cytogenetic characterization was assessed using conventional staining (Giemsa), C-banding, and NOR banding. Both species had 2n = 58 chromosomes but differed in their chromosome formulae, 24 m + 14sm + 8st + 12a for W. maculata and 32 m + 16sm + 10st for H. wilderi. In W. maculata heterochromatin was mainly telomeric, and three chromosomes had a fully heterochromatic arm; in H. wilderi heterochromatin was also predominantly telomeric and evident in many more chromosomes. Hassar wilderi also presented one pair of homologues with a fully heterochromatic arm. In both species, nucleolar organizer regions were restricted to one pair of subtelocentric chromosomes. Assuming a basal position for W. maculata, we hypothesized that underlying conserved diploid and NOR-bearing chromosome numbers, chromosomal evolution in doradids has involved pericentric inversions and an increase of heterochromatic blocks.     

Human (Homo sapiens) and chimpanzee (Pan troglodytes) share similar ancestral centromeric alpha satellite DNA sequences but other fractions of heterochromatin differ considerably

The euchromatic regions of chimpanzee (Pan troglodytes) genome share approximately 98% sequence similarity with the human (Homo sapiens), while the heterochromatic regions display considerable divergence. Positive heterochromatic regions revealed by the CBG-technique are confined to pericentromeric areas in humans, while in chimpanzees, these regions are pericentromeric, telomeric, and intercalary. When human chromosomes are digested with restriction endonuclease AluI and stained by Giemsa (AluI/Giemsa), positive heterochromatin is detected only in the pericentromeric regions, while in chimpanzee, telomeric, pericentromeric, and in some chromosomes both telomeric and centromeric, regions are positive. The DA/DAPI technique further revealed extensive cytochemical heterogeneity of heterochromatin in both species. Nevertheless, the fluorescence in situ hybridization technique (FISH) using a centromeric alpha satellite cocktail probe revealed that both primates share similar pericentromeric alpha satellite DNA sequences. Furthermore, cross-hybridization experiments using chromosomes of gorilla (Gorilla gorilla) and orangutan (Pongo pygmaeus) suggest that the alphoid repeats of human and great apes are highly conserved, implying that these repeat families were present in their common ancestor. Nevertheless, the orangutan's chromosome 9 did not cross-hybridize with human probe. The euchromatic regions of chimpanzee (Pan troglodytes) genome share approximately 98% sequence similarity with the human (Homo sapiens), while the heterochromatic regions display considerable divergence. Positive heterochromatic regions revealed by the CBG-technique are confined to pericentromeric areas in humans, while in chimpanzees, these regions are pericentromeric, telomeric, and intercalary. When human chromosomes are digested with restriction endonuclease AluI and stained by Giemsa (AluI/Giemsa), positive heterochromatin is detected only in the pericentromeric regions, while in chimpanzee, telomeric, pericentromeric, and in some chromosomes both telomeric and centromeric, regions are positive. The DA/DAPI technique further revealed extensive cytochemical heterogeneity of heterochromatin in both species. Nevertheless, the fluorescence in situ hybridization technique (FISH) using a centromeric alpha satellite cocktail probe revealed that both primates share similar pericentromeric alpha satellite DNA sequences. Furthermore, cross-hybridization experiments using chromosomes of gorilla (Gorilla gorilla) and orangutan (Pongo pygmaeus) suggest that the alphoid repeats of human and great apes are highly conserved, implying that these repeat families were present in their common ancestor. Nevertheless, the orangutan's chromosome 9 did not cross-hybridize with human probe. © 1995 Wiley-Liss, Inc.     

Meiotic effects of supernumerary heterochromatin in Heteropternis obscurella

The endemic Australian grasshopper Heteropternis obscurella shows considerable variation in respect of both chromosome structure and chromosome behaviour. The structural differences depend upon different patterns of heterochromatin distribution as revealed by C-banding. These involve differences between populations in respect of polytypic variation in the size of paracentromeric C-bands and differences within populations in respect of polymorphisms both for terminal blocks of heterochromatin in autosomes 3 to 8 and a large proximal block of heterochromatin in autosome 7. The behavioural differences stem in part from genotypically determined variation in the chiasma distribution pattern which is markedly localised in a majority of populations but more randomly distributed in populations from the south of Western Australia. Behavioural differences also arise as secondary consequences of the presence of those heterochromatic blocks which occur as polymorphisms. The distal blocks on autosomes 5, 6, 7 and 8 lead to a redistribution of chiasmata to more proximal sites while the proximal block on 7 leads to the virtual abolition of chiasma formation in that bivalent and its replacement by a non-chiasmate mechanism of segregation. This depends upon a persistent proximal heterochromatic association between the pairing partners. The presence of distal C-blocks on bivalents 3 to 8 gives rise to persistent pseudomultiples, formed as a result of heterochromatic associations between these blocks. Such pseudomultiples involve any two or three of these six bivalents, provided they carry distal blocks, and their frequency rises dramatically in the presence of the proximal heterochromatic block on chromosome 7.     

The structure,amount and chromosomal localisation of defined repeated DNA sequences in species of the genus Secale

The structure, copy number and chromosomal location of arrays of four families of highly repeated sequences have been investigated in representative species of the genus Secale. The four unrelated families, previously characterised in Secale cereale, have repeating units of 480, 610, 630 and 120 base pairs respectively. The following general conclusions can be drawn in addition to detailed knowledge of the sequence content of heterochromatin in each accession studied: (1) Every species is unique in its complement or chromosomal distribution or both of the four highly repeated sequence families. S. montanum and S. cereale accessions studied here show the same complement of repeated sequences, but they differ substantially in the amounts they contain of the 610 and 630 base pair (bp) families, and in the distribution over the chromosomes of the 480 bp family. The structure of the repeating unit is also different in many members of the 480 bp family in S. montanum. — (2) The substantial differences between species in the amounts of the most highly repeated DNA sequences exist in the absence of any such conspicuous differences in most other repeated sequences which were detected as fluorescent bands after restriction enzyme digestion and gel electrophoresis. — (3) Each of the different highly repeated families can exist independently of the other families, though all the families have telomeric sites. Also, in the outbreeding species, heteromorphisms are frequent, and are particularly conspicuous in hybridisation detecting the 480 bp sequence family. — (4) The association of the highly repeated sequences with heterochromatin, discussed in the accompanying paper is generally true for other species in the genus, and the lower amounts of heterochromatin in other Secale species compared to S. cereale are associated with lower amounts of specific families of highly repeated DNA sequences. — (5) Analysis of highly repeated sequence families is likely to provide an easy method of identification of new accessions of Secale.     

Euchromatic and heterochromatic domains at Drosophila telomeres.

Noncoding repetitive sequences make up a large portion of eukaryotic genomes, but their function is not well understood. Large blocks of repetitive DNA-forming heterochromatin around the centromeres are required for this region to function properly, but are difficult to analyze. The smaller regions of heterochromatin at the telomeres provide an opportunity to study their DNA and protein composition. Drosophila telomere length is maintained through the targeted transposition of specific non-long terminal repeat retrotransposons to chromosome ends, where they form long tandem arrays. A subterminal telomere-associated sequence (TAS) lies immediately proximal to the terminal-retrotransposon array. Here, we review the experimental support for the heterochromatic features of Drosophila telomeres, and provide evidence that telomeric regions contain 2 distinct chromatin subdomains: TAS, which exhibits features that resemble beta heterochromatin; and the terminal array of retrotransposons, which appears euchromatic. This organization is significantly different from the telomeric organization of other eukaryotes, where the terminal telomerase-generated repeats are often folded in a t-loop structure and become part of the heterochromatin protein complex.     

Organization and dynamics of satellite and telomere DNAs in Ascaris: implications for formation and programmed breakdown of compound chromosomes

In the nematode genus Ascaris the germline genome contains considerable amounts of extra DNA, which is discarded from the somatic founder blastomeres during early cleavage. In Parascaris univalens the haploid germline genome is contained in one large compound chromosome, which consists of a euchromatic region containing the somatic genome flanked by large blocks of heterochromatin. Fluorescence in situ hybridization of fractions of the germline-limited satellite DNA revealed two highly repeated sequence families establishing the entire heterochromatin (HET blocks). The repeats, a pentanucleotide, TTGCA, and a decanucleotide, TTTGTGCGTG, constitute separate segments of the HET blocks. The blocks are polymorphic in length and, hence, in copy number of the repeats, and the arrangement of the segments. The numerous sequence variants of both repeats display a disperse distribution. The type and rate of base substitutions within both repeat units depend on position. Prior to the elimination process in presomatic cells, termed chromatin diminution, the chromosomes undergo differential mitotic condensation. Interstitial 'chromatin linkers' flanking the prospective numerous somatic chromosomes remain entirely decondensed. The somatic chromosomes are released from the plurivalent chromosomes via excision of the linkers at onset of anaphase, followed by exclusion of the akinetic linker chromatin and HET blocks from the daughter nuclei. In Ascaris suum, the germline-limited satellite, which consists of one 123 bp repeat, is scattered throughout the numerous chromosomes in small heterochromatic knobs of variable sizes, residing at chromosomal ends and/or intercalary positions. The programmed breakage, which appears to proceed in a similar manner to that in P. univalens, results in the loss of all heterochromatic knobs, accompanied by an increase in chromosome number. In both species, all germline chromosomes are capped by tracts of TTAGGC repeats. In P. univalens, such telomeric tracts also occur at the termini of the euchromatic intercalary regions. Upon diminution all telomeric tracts are discarded. De novo telomere addition occurs in all somatic cell lineages of both species. The presented data shed light on the evolutionary history of chromosome aggregation and satellite DNA formation, and putative mechanisms involved in the process of site-directed breakage to reestablish stable somatic chromosomes.     

Cytogenetic and molecular characterization of a highly repeated DNA sequence in the peach potato aphid Myzus persicae

Electrophoresis following digestion of Myzus persicae genomic DNA with HindIII showed the presence of a prominent band of approximately 200 bp whereas a faint electrophoretic band corresponding to DNA fragments of about 3000 bp was observed after digestion with ApaI. In situ digestion with restriction enzymes, followed by in situ nick translation, showed that ApaI targets are localized at the nucleolus organizer-bearing X telomeric region, whereas HindIII restriction sites are clustered in intercalary C-positive areas on the same X chromosome. Fluorescent in situ hybridization (FISH) carried out by using digoxygenin-labeled HindIII repeats as probe fully confirmed overlapping between the hybridization sites of this probe and the AT-rich intercalary heterochromatic bands on the X chromosome. These findings, together with published data, allow us to conclude that the M. persicae genome possesses three classes of C-positive heterochromatin: (i) a GC-rich argentophilic band located on one telomere of the X chromosome that contains ApaI targets; (ii) AT-rich intercalary bands located on the X chromosome containing clustered HindIII fragments; (iii) AT-rich telomeric bands, located on autosomes, consisting of HaeIII repeats. Molecular analysis has shown that the length of the HindIII repeat consensus sequence is 189 bp with an AT content of 67%. Southern blotting with HindIII monomers revealed a regular ladder of bands composed of multimers of basic length that are characteristic of satellite DNAs. The HindIII repeat displays other features typical of eukaryotic satellite arrays such as overlapping with heterochromatic bands and a high degree of sequence similarity among monomers (84%–94%). A similarity plot showed that sequences were particularly variable in the 50–100 bp region whereas they proved to be highly conservative in the first 50 bp, thus suggesting that this portion of the repeat might be functionally important. Received: 23 February 1999; in revised form: 21 July 1999 / Accepted: 28 July 1999     

The heterochromatin of grasshoppers from the Caledia captiva species complex

A highly repeated family of sequences from the grasshopper Caledia captiva shows a dispersed distribution at the cytological level. Members of this 185 bp sequence family are not restricted to C-band heterochromatin, but rather are distributed in regions which appear as euchromatin in C-banded chromosomes. Sequence variation in this family is equivalent (14%–16%) at all levels of taxonomic comparison from within a population to between species. However, contiguous repeats demonstrate a much lower level of variation (9%). These, and other data, indicate that the concept of sequence homogeneity within a family of highly repeated sequences must be qualified with respect to the extensive variation between members of a given family. Comparison of the data for the 185 bp family with those from a study of a second highly repeated family, from the same taxon, demonstrates divergent patterns of evolution. Thus, the 185 bp repeats show much greater sequence variation, as well as a seemingly random pattern of incorporation of base pair alterations. The factors which may contribute to the observed pattern of variation include the time since the sequence family originated, its cytological distribution, the frequency of unequal crossing over and gene conversion and natural selection.     

Epigenetic profiling of heterochromatic satellite DNA

Sugar beet (Beta vulgaris) chromosomes consist of large heterochromatic blocks in pericentromeric, centromeric, and intercalary regions comprised of two different highly abundant DNA satellite families. To investigate DNA methylation at single base resolution at heterochromatic regions, we applied a method for strand-specific bisulfite sequencing of more than 1,000 satellite monomers followed by statistical analyses. As a result, we uncovered diversity in the distribution of different methylation patterns in both satellite families. Heavily methylated CG and CHG (H=A, T, or C) sites occur more frequently in intercalary heterochromatin, while CHH sites, with the exception of CAA, are only sparsely methylated, in both intercalary and pericentromeric/centromeric heterochromatin. We show that the difference in DNA methylation intensity is correlated to unequal distribution of heterochromatic histone H3 methylation marks. While clusters of H3K9me2 were absent from pericentromeric heterochromatin and restricted only to intercalary heterochromatic regions, H3K9me1 and H3K27me1 were observed in all types of heterochromatin. By sequencing of a small RNA library consisting of 6.76 million small RNAs, we identified small interfering RNAs (siRNAs) of 24 nucleotides in size which originated from both strands of the satellite DNAs. We hypothesize an involvement of these siRNAs in the regulation of DNA and histone methylation for maintaining heterochromatin.     

Karyotype of Callithrix mauesi (Callitrichidae,Primates) and its relations with those of C. emiliae and C. jacchus

A study was conducted on the most recently described marmoset species, Callithrix mauesi, and the results obtained were compared to those previously reported for the karyotypes of C. jacchus and C. emiliae. No mechanism of chromosome rearrangement differentiates the karyotypes of C. mauesi (2n = 44) and C. emiliae (2n = 44), which diverge from C. jacchus (2n = 46) by a Robertsonian translocation and a paracentric inversion. C. mauesi, like C. emiliae, presents telomeric constitutive heterochromatin in various chromosomes, forming large heterochromatic blocks in some. This does not occur in C. jacchus, which basically presents centromeric constitutive heterochromatin. The karyotype of C. mauesi differs from that of C. emiliae only by the amount and distribution of this telomeric constitutive heterochromatin. One of the chromosomes presenting a heterochromatic block in C. mauesi is chromosome X, a fact not previously reported in the Order Primates. The present chromosome data show that C. mauesi is closer to C. emiliae than to C. jacchus, in agreement with its inclusion in the C. argentata group. In the present paper, we describe for the first time, at the chromosome level, chimerism between fraternal twins of the same sex (XY/XY), with the heterochromatic block of pair 2 being the marker. © 1994 Wiley-Liss, Inc.