Retroelements (LINEs and SINEs) in vole genomes: Differential distribution in the constitutive heterochromatin

The chromosomal distribution of mobile genetic elements is scarcely known in Arvicolinae species, but could be of relevance to understand the origin and complex evolution of the sex chromosome heterochromatin. In this work we cloned two retrotransposon sequences, L1 and SINE-B1, from the genome of Chionomys nivalis and investigated their chromosomal distribution on several arvicoline species. Our results demonstrate first that both retroelements are the most abundant repeated DNA sequences in the genome of these species. L1 elements, in most species, are highly accumulated in the sex chromosomes compared to the autosomes. This favoured L1 insertion could have played an important role in the origin of the enlarged heterochromatic blocks existing in the sex chromosomes of some Microtus species. Also, we propose that L1 accumulation on the X heterochromatin could have been the consequence of different, independent and rapid amplification processes acting in each species. SINE elements, however, were completely lacking from the constitutive heterochromatin, either in autosomes or in the heterochromatic blocks of sex chromosomes. These data could indicate that some SINE elements are incompatible with the formation of heterochromatic complexes and hence are necessarily missing from the constitutive heterochromatin.     

X chromosome painting in <Emphasis Type="Italic">Microtus</Emphasis>: Origin and evolution of the giant sex chromosomes

Sex chromosomes in species of the genus Microtus present some characteristic features that make them a very interesting group to study sex chromosome composition and evolution. M. cabrerae and M. agrestis have enlarged sex chromosomes (known as ‘giant sex chromosomes’) due to the presence of large heterochromatic blocks. By chromosome microdissection, we have generated probes from the X chromosome of both species and hybridized on chromosomes from six Microtus and one Arvicola species. Our results demonstrated that euchromatic regions of X chromosomes in Microtus are highly conserved, as occurs in other mammalian groups. The sex chromosomes heterochromatic blocks are probably originated by fast amplification of different sequences, each with an independent origin and evolution in each species. For this reason, the sex heterochromatin in Microtus species is highly heterogeneous within species (with different composition for the Y and X heterochromatic regions in M. cabrerae) and between species (as the composition of M. agrestis and M. cabrerae sex heterochromatin is different). In addition, the X chromosome painting results on autosomes of several species suggest that, during karyotypic evolution of the genus Microtus, some rearrangements have probably occurred between sex chromosomes and autosomes.     

Meiotic chromosomes and stages of sex chromosome evolution in fish: zebrafish, platyfish and guppy

We describe SC complements and results from comparative genomic hybridization (CGH) on mitotic and meiotic chromosomes of the zebrafish Danio rerio, the platyfish Xiphophorus maculatus and the guppy Poecilia reticulata. The three fish species represent basic steps of sex chromosome differentiation: (1) the zebrafish with an all-autosome karyotype; (2) the platyfish with genetically defined sex chromosomes but no differentiation between X and Y visible in the SC or with CGH in meiotic and mitotic chromosomes; (3) the guppy with genetically and cytogenetically differentiated sex chromosomes. The acrocentric Y chromosomes of the guppy consists of a proximal homologous and a distal differential segment. The proximal segment pairs in early pachytene with the respective X chromosome segment. The differential segment is unpaired in early pachytene but synapses later in an ‘adjustment’ or ‘equalization’ process. The segment includes a postulated sex determining region and a conspicuous variable heterochromatic region whose structure depends on the particular Y chromosome line. CGH differentiates a large block of predominantly male-specific repetitive DNA and a block of common repetitive DNA in that region. This revised version was published online in July 2006 with corrections to the Cover Date.     

Complex evolution of X and Y autosomal translocations in the giant mole-rat, Cryptomys mechowi (Bathyergidae)

Cross-species chromosome painting was used to determine homologous chromosomal regions between two species of mole-rat, the naked mole-rat, Heterocephalus glaber (2n = 60), and the giant mole-rat, Cryptomys mechowi (2n = 40), using flow-sorted painting probes representative of all but two of the H. glaber chromosomal complement. In total 43 homologous regions were identified in the C. mechowi genome. Eight H. glaber chromosomes are retained in toto in C. mechowi, and 13 produce two or more signals in this species. The most striking difference in the karyotypes of the two taxa concerns their sex chromosomes. The H. glaber painting probes identified a complex series of translocations that involved the fractionation of four autosomes and the subsequent translocation of segments to the sex chromosomes and to autosomal partners in the C. mechowi genome. An intercalary heterochromatic block (IHB) was detected in sex chromosomes of C. mechowi at the boundary with the translocated autosomal segment. We discuss the likely sequence of evolutionary events that has led to the contemporary composition of the C. mechowi sex chromosomes, and consider these in the light of prevailing views on the genesis of sex chromosomes in mammals.     

Exceptional minute sex-specific region in the X0 mammal,Ryukyu spiny rat

The Ryukyu spiny rats (genus Tokudaia) inhabit only three islands in the Nansei Shoto archipelago in Japan, and have the variations of karyotype among the islands. The chromosome number of T. osimensis in Amami-Oshima Island is 2n = 25, and T. tokunoshimensis in Tokunoshima Island is 2n = 45, and the two species have X0 sex chromosome constitution with no cytogenetically visible Y chromosome in both sexes. We constructed the standard ideograms for these species at the 100 and 200 band levels. Comparing the banding patterns between these species, it was suggested that at least 10 times the number of Robertsonian fusions occurred in T. osimensis chromosomes. However, no karyotypic differences were observed between sexes in each species. To detect the sex-specific chromosomal region of these X0 species we applied the comparative genomic hybridization (CGH) method. Although the male- and female-derived gains and losses were detected in several chromosome regions, all of them were located in the heterochromatic and/or telomeric regions. This result suggested that the differences detected by CGH might be caused by the polymorphism on the copy numbers of repeated sequences in the heterochromatic and telomeric regions. Our result indicated that the sex-specific region, where the key to sex determination lies, is very minute in X0 species of Tokudaia.     

A repeat DNA sequence from the Y chromosome in species of the genus <Emphasis Type="Italic">Microtus</Emphasis>

In most mammals, the Y chromosome is composed of a large amount of constitutive heterochromatin. In some Microtus species, this feature is also extended to the X chromosome, resulting in enlarged (giant) sex chromosomes. Several repeated DNA sequences have been described in the gonosomal heterochromatin of these species, indicating that it has heterogeneous and species-specific composition and distribution. We have cloned an AT-rich, 851-bp long, repeated DNA sequence specific for M. cabrerae Y chromosome heterochromatin. The analysis of other species of the genus Microtus indicated that this sequence is also located on the Y chromosome (male-specific) in three species (M. agrestis, M. oeconomus and M. nivalis), present on both Y and X chromosomes and on some autosomes in M. arvalis and absent in the genome of M. guentheri. Our data also suggest that the mechanism of heterochromatin amplification operating on the sex chromosomes could have been different in each species since the repeated sequences of the gonosomal heterochromatic blocks in M. cabrerae and M. agrestis are different. The absence of this sequence in the mouse genome indicates that its evolutionary origin could be recent. Future analysis of the species distribution, localization and sequence of this repeat DNA family in arvicolid rodent species could help to establish the unsolved phylogenetic relationships in this rodent group.     

Analysis of heterochromatic epigenetic markers in the holocentric chromosomes of the aphid <Emphasis Type="Italic">Acyrthosiphon pisum</Emphasis>

Monomethylated-K9 H3 histones (Me9H3) and heterochromatin protein 1 (HP1) are reported as heterochromatin markers in several eukaryotes possessing monocentric chromosomes. In order to confirm that these epigenetic markers are evolutionarily conserved, we sequenced the HP1 cDNA and verified the distribution of Me9H3 histones and HP1 in the holocentric chromosomes of the aphid Acyrthosiphon pisum. Sequencing indicates that A. pisum HP1 cDNA (called ApHP1) is 1623 bp long, including a 170 bp long 5′UTR and a 688 bp long 3′UTR. The ApHP1 protein consists of 254 amino acidic residues, has a predicted molecular mass of 28 kDa and a net negative charge. At the structural level, it shows an N terminal chromo domain and a chromo shadow domain at the C terminus linked by a short hinge region. At the cytogenetic level, ApHP1 is located exclusively in the heterochromatic regions of the chromosomes. The same heterochromatic regions were labelled after immuno-staining with antibodies against Me9H3 histones, confirming that Hp1 and Me9H3 co-localize at heterochromatic chromosomal areas. Surprisingly, aphid heterochromatin lacks DNA methylation and methylated cytosine residues were mainly spread at euchromatic regions. Finally, the absence of DNA methylation is observed also in aphid rDNA genes that have been repeatedly described as mosaic of methylated and unmethylated units in vertebrates.     

Distinct chromosomal distributions of highly repetitive sequences in maize

The majority of genomic DNA in most plant species is made up of repetitive elements including satellites and retrotransposons. The maize genome is intermediate in size and abundance of repetitive elements between small genomes such as Arabidopsis and rice and larger genomes such as wheat. Although repetitive elements are present throughout the maize genome, individual families are non-randomly distributed along chromosomes. In this work we use fluorescence in-situ hybridization (FISH) to examine the distribution of abundant LTR retroelement families and satellites contained in heterochromatic blocks called knobs. Different retroelement families have distinct patterns of hybridization. Prem1 and Tekay, two very closely related elements, both hybridize along the length of all chromosomes but do so with greater intensity near the centromeres, although subtle differences are detectable between the hybridization patterns. Opie, Prem2/Ji, and Huck are enriched away from the centromeres and Grande is distributed uniformly along the chromosomes. Double labeling with proximally and distally enriched elements on pachytene chromosomes produces alternating blocks of element enrichment. The maize elements hybridized in the same general patterns to chromosomes of maize relatives including Zea diploperennis and Tripsacum dactyloides. Additionally, abundant Tripsacum LTR retroelements are enriched in similar chromosomal regions among the different species. The 180 bp knob satellite is present in large blocks at interstitial locations on chromosome arms. With long exposures, smaller sites of hybridization are detected at the ends of chromosomes, adjacent to the telomere tract. This distal position for knob satellites is conserved among Zea and Tripsacum species. Electronic supplementary material Supplementary material to this paper is available in electronic form at and is accessible for authorized users.     

Accumulation of rare sex chromosome rearrangements in the African pygmy mouse, Mus (Nannomys) minutoides: a whole-arm reciprocal translocation (WART) involving an X-autosome fusion

Although sex chromosomes are generally the most conserved elements of the mammalian karyotype, those of African pygmy mice show three extraordinary deviations from the norm: (a) asynaptic sex chromosomes, (b) multiple sex–autosome fusions, and (c) modifications of sex determination in some populations/species. In this study we identified, in two sex-reversed females of Mus (Nannomys) minutoides, a fourth rare sex chromosome change: a spontaneous whole-arm reciprocal translocation (WART) between an autosomal Robertsonian pair Rb(13.16) and the sex–autosome fusion Rb(X.1). This represents one of the very few reported cases of WARTs in natura within mammals, and is the first one to involve sex chromosomes. Hence, this finding offers new insights into the mechanisms of chromosomal differentiation in African pygmy mice, as WARTs may have contributed to the extensive diversity not only of autosomal Robertsonian fusions, but also of sex–autosome translocations. More widely, these results provide additional support to previous studies on the house mouse and the common shrew which indirectly inferred the role of WARTs in their karyotypic evolution, and may even help to understand how the fascinating 10 sex chromosome chain of the platypus might have evolved. This accumulation of rare sex chromosome changes in single specimens is, to our knowledge, exceptional among mammals.     

Compositional mapping of chicken chromosomes and identification of the gene-richest regions

Compositional chromosomal mapping, namely the assessment of the GC level of chromosomal bands, led to the identification, in the human chromosomes, of the GC-richest H3⁺ bands and of the GC-poorest L1⁺ bands, which were so called on the basis of the isochore family predominantly present in the bands. The isochore organization of the avian genome is very similar to those of most mammals, the only difference being the presence of an additional, GC-richest, H4 isochore family. In contrast, the avian karyotypes are very different from those of mammals, being characterized, in most species, by few macrochromosomes and by a large number of microchromosomes. The compositional mapping of chicken mitotic and meiotic chromosomes by in-situ hybridization of isochore families showed that the chicken GC-richest isochores are localized not only on a large number of microchromosomes but also on almost all telomeric bands of macrochromosomes. On the other hand, the GC-poorest isochores are generally localized on the internal regions of macrochromosomes and are almost absent in microchromosomes. Thus, the distinct localization of the GC-richest and the GC-poorest bands observed on human chromosomes appears to be a general feature of chromosomes from warm-blooded vertebrates.     

Complex structure of B-chromosomes in two mammalian species: Apodemus peninsulae (Rodentia) and Nyctereutes procyonoides (Carnivora)

B-chromosomes (Bs) of two mammalian species, raccoon dog (Nyctereutes procyonoides, Carnivora) and Asian wood mouse (Apodemus peninsulae, Rodentia) were investigated using chromosome segment microdissection and double-colour FISH. In the raccoon dog, all B-chromosomes showed homology with each other but not with the A-chromosomes. Two segment-specific probes (from proximal and distal parts of B) have been localized in corresponding chromosome parts, with significant variation in their sizes. In Asian wood mice, two types of B-specific chromatin were revealed – B1 and B2. Most Bs were either B1 or B2 specific; furthermore, some Bs were found to be composed of both types of chromatin. B-chromosome-specific libraries of A. peninsulae contain sequences homologous to the heterochromatic regions of sex and some A-chromosomes and dispersed repeated sequences. B1-specific probes gave signals on sex chromosomes of Apodemus speciosus and Apodemus agrarius. The origin and evolution of B-chromosomes in mammals are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chromosomal evolution of Arvicolinae (Cricetidae, Rodentia). II. The genome homology of two mole voles (genus Ellobius), the field vole and golden hamster revealed by comparative chromosome painting

Using cross-species chromosome painting, we have carried out a comprehensive comparison of the karyotypes of two Ellobius species with unusual sex determination systems: the Transcaucasian mole vole, Ellobius lutescens (2n = 17, X in both sexes), and the northern mole vole, Ellobius talpinus (2n = 54, XX in both sexes). Both Ellobius species have highly rearranged karyotypes. The chromosomal paints from the field vole (Microtus agrestis) detected, in total, 34 and 32 homologous autosomal regions in E. lutescens and E. talpinus karyotypes, respectively. No difference in hybridization pattern of the X paint (as well as Y paint) probes on male and female chromosomes was discovered. The set of golden hamster (Mesocricetus auratus) chromosomal painting probes revealed 44 and 43 homologous autosomal regions in E. lutescens and E. talpinus karyotypes, respectively. A comparative chromosome map was established based on the results of cross-species chromosome painting and a hypothetical ancestral Ellobius karyotype was reconstructed. A considerable number of rearrangements were detected; 31 and 7 fusion/fission rearrangements differentiated the karyotypes of E. lutescens and E. talpinus from the ancestral Ellobius karyotype. It seems that inversions have played a minor role in the genome evolution of these Ellobius species.     

Cytogenetic differentiation of two sympatric tree shrew taxa found in the southern part of the Isthmus of Kra

Sympatric populations of the genus Tupaia encompassing two cytotypes (cyt60, 2n = 60 and cyt62, 2n = 62) were found in the southern part of the Isthmus of Kra (the middle region of the Malay Peninsula, Thailand). C-bands, location of rDNA, and location of non-essential telomeric repeats (TRs) were investigated in detail for 23 animals captured in the area. Such chromosomal traits definitely reveal that two distinct cytotypes exist in the sympatric population, though the external morphological traits are similar. Hybrid cytotypes were not observed; thus, these two cytotypes appear to be genetically isolated sibling species. Chromosomal results compared with previous data, geographic distribution and morphological data observed with new insight suggest that, in the sympatric population, ‘cyt60’ represents members of Tupaia glis, while ‘cyt62’ identifies individuals of Tupaia belangeri. The cytogenetic information discovered in the present study offers new insight to morphological classification and, further, may provide substantial diagnostic characteristics for the distinction of tree shrew species. This revised version was published online in July 2006 with corrections to the Cover Date.     

FISH mapping and molecular organization of the major repetitive sequences of tomato

This paper presents a bird’s-eye view of the major repeats and chromatin types of tomato. Using fluorescence in-situ hybridization (FISH) with Cot-1, Cot-10 and Cot-100 DNA as probes we mapped repetitive sequences of different complexity on pachytene complements. Cot-100 was found to cover all heterochromatin regions, and could be used to identify repeat-rich clones in BAC filter hybridization. Next we established the chromosomal locations of the tandem and dispersed repeats with respect to euchromatin, nucleolar organizer regions (NORs), heterochromatin, and centromeres. The tomato genomic repeats TGRII and TGRIII appeared to be major components of the pericentromeres, whereas the newly discovered TGRIV repeat was found mainly in the structural centromeres. The highly methylated NOR of chromosome 2 is rich in [GACA]₄, a microsatellite that also forms part of the pericentromeres, together with [GA]₈, [GATA]₄ and Ty1-copia. Based on the morphology of pachytene chromosomes and the distribution of repeats studied so far, we now propose six different chromatin classes for tomato: (1) euchromatin, (2) chromomeres, (3) distal heterochromatin and interstitial heterochromatic knobs, (4) pericentromere heterochromatin, (5) functional centromere heterochromatin and (6) nucleolar organizer region.     

Karyotype diversity and the origin of grapefruit

Grapefruit is a group of citrus of recent origin, probably resulting from a cross between pummelo and sweet-orange. Aiming to investigate this putative origin and the genetic variability among grapefruit cultivars, the karyotype of six grapefruits, two pummelos, and one tangelo cultivar (grapefruit × tangerine) were analyzed using sequential CMA/DAPI double staining and FISH with rDNA probes. The karyotypes of grapefruit ‘Duncan’ and ‘Foster’ differ from those of ‘Flame’, ‘Henderson’, ‘Marsh’ and ‘Rio Red’. The former have two chromosomes with a single CMA⁺ band in both terminal regions (C type chromosome) and six chromosomes with only one CMA⁺ terminal band (D type), whereas the latter have three C and five D type chromosomes. All accessions investigated exhibited two chromosomes with 5S rDNA but a variable number of 45S rDNA. The two former grapefruits displayed four 45S rDNA sites, whereas the remaining grapefruit cultivars had five. The two pummelos showed identical karyotypes, homozygous for CMA⁺ bands and their four rDNA sites. From each pummelo chromosome pair one chromosome seems to be present in grapefruit karyotypes. The different grapefruit karyotypes might result from independent crosses between pummelos of different karyotypic constitution and sweet-oranges. The chromosome markers found in the tangelo ‘Orlando’ and the position of their two 45S rDNA confirm the grapefruit ‘Duncan’ and the tangerine ‘Dancy’ as their parents.     

Molecular structure and chromosome distribution of three repetitive DNA families in Anemone hortensis L. (Ranunculaceae)

The structure, abundance and location of repetitive DNA sequences on chromosomes can characterize the nature of higher plant genomes. Here we report on three new repeat DNA families isolated from Anemone hortensis L.; (i) AhTR1, a family of satellite DNA (stDNA) composed of a 554–561 bp long EcoRV monomer; (ii) AhTR2, a stDNA family composed of a 743 bp long HindIII monomer and; (iii) AhDR, a repeat family composed of a 945 bp long HindIII fragment that exhibits some sequence similarity to Ty3/gypsy-like retroelements. Fluorescence in-situ hybridization (FISH) to metaphase chromosomes of A. hortensis (2n = 16) revealed that both AhTR1 and AhTR2 sequences co-localized with DAPI-positive AT-rich heterochromatic regions. AhTR1 sequences occur at intercalary DAPI bands while AhTR2 sequences occur at 8–10 terminally located heterochromatic blocks. In contrast AhDR sequences are dispersed over all chromosomes as expected of a Ty3/gypsy-like element. AhTR2 and AhTR1 repeat families include polyA- and polyT-tracks, AT/TA-motifs and a pentanucleotide sequence (CAAAA) that may have consequences for chromatin packing and sequence homogeneity. AhTR2 repeats also contain TTTAGGG motifs and degenerate variants. We suggest that they arose by interspersion of telomeric repeats with subtelomeric repeats, before hybrid unit(s) amplified through the heterochromatic domain. The three repetitive DNA families together occupy ∼10% of the A. hortensis genome. Comparative analyses of eight Anemone species revealed that the divergence of the A. hortensis genome was accompanied by considerable modification and/or amplification of repeats. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The topology of early- and late-replicating chromatin in differentially decondensed chromosomes

In this study we used a novel technique to reveal both longitudinal and transverse differentiation within mammalian mitotic chromosomes. Structural changes in chromosomes that we term ‘differential decondensation’ were produced in cells that were first incubated in hypotonic medium (15% Hanks’ solution), then adapted to normotonic conditions and thereafter exposed to a second short hypotonic shock. Such a double hypotonic treatment (DHT) is not critical for cell viability, but considerably elongates the G2 phase of the cell cycle. Giemsa staining of differentially decondensed chromosomes corresponds to standard G-banding, but does not need the standard post-fixation treatment. Using ‘dynamic’ BrdU banding, we show that such ‘differential’ staining is a result of differential resistance of the R- and G-bands to DHT. Thus, early-replicating foci, markers of R-bands, are localized in the peripheral chromatin halo, whereas late-replicating foci, corresponding to G-bands, remain associated with the axial regions of chromatids. Remarkably, despite these major changes in the structure of the chromosomal bands, the replication foci still preserve their discrete structure.     

Caenorhabditis elegans Ce-rdh-1/rad-51 functions after double-strand break formation of meiotic recombination

During meiotic prophase 1, homologous recombination is accompanied by dynamic chromosomal changes. The Ce-rdh-1/rad-51 gene is the only bacterial recA-like gene in the nematode C. elegans genome. Upon depletion of Ce-rdh-1/rad-51 using the RNA interference method, abnormal kinked chromosomes can be observed in mature oocytes at diakinesis, whereas synapsis between homologous chromosomes during the pachytene stage is normal. Following fertilization, Ce-rdh-1/rad-51-depleted embryos die early in embryogenesis, and their nuclei exhibit abnormal chromosome fragments and bridges. From epistasis analyses with Ce-spo-11 defective mutant and ionizing radiation, it is indicated that Ce-rdh-1/rad-51 functions after double-strand break (DSB) formation of meiotic recombination. Under the Ce-chk-2 defective condition, whose meiotic synapsis and meiotic recombination between homologous chromosomes are completely inhibited, the Ce-rdh-1/rad51 is normally expressed in the gonadal cells. Moreover, it seems that exogenous DSBs in the Ce-chk-2 defective nuclei at the pachytene stage can be repaired between sister chromatids in a Ce-rdh-1/rad-51-dependent manner. These results indicate that Ce-rdh-1/rad51 functions after both endogenous and exogenous DSB formation during meiosis, but not as pairing centers for meiotic synapsis.     

Chromosomal study of a lamprey (<Emphasis Type="Italic">Lampetra zanandreai</Emphasis> Vladykov, 1955) (Petromyzonida: Petromyzontiformes): conventional and FISH analysis

Karyotype and other chromosomal characteristics in the Adriatic brook lamprey Lampetra zanandreai, representative of one of the most ancestral group of vertebrates, were examined using conventional (Ag-staining, C-banding as well as CMA₃ and DAPI fluorescence) and molecular (FISH with 18/28S rDNA and EcoRI satDNA as probes) protocols with metaphase chromosomes derived from whole blood cultures. The chromosome complement had a modal diploid chromosome number of 2n = 164, as in other petromyzontid lamprey species. Ag-staining and CMA₃ fluorescence, as well as FISH with 18/28S rDNA probes, detected nucleolar organizer regions (NORs) close to the centromeres of the biarmed chromosomes of pairs 1 and 2, the largest chromosome pairs of the complement. In addition to NORs, CMA₃ fluorescence revealed positive signals in approximately 40 other chromosomes. DAPI stained mostly centromeric regions of many chromosomes as well as conspicuously massive blocks overlapping NOR sites. C-banding evidenced a large amount of constitutive heterochromatin in somatic chromosomes, with approximately 40 C-positive acrocentric elements completely heterochromatic, corresponding with the 40 CMA₃+ chromosomes and positive heterochromatic blocks in pericentromeric regions of chromosome pairs 1 and 2. Polymerase chain reaction (PCR)-based cloning of satellite DNA with primers derived from Petromyzon marinus centromeric sequences was successful for L. zanandreai genomic DNA. The sequence was AT-rich (59%) and characterized by short consensus motifs similar to other centromeric satellite motifs. FISH using satDNA clones as a probe produced a fluorescent signal on a single pair of small chromosomes. This sequence was PCR-amplified also in L. planeri and P. marinus genomic DNA, and the evolution of this repetitive element in the above species was analysed.     

PRINS Analysis of the Telomeric Sequence in Seven Lemurs

We examined the chromosomal localization of the telomeric sequence, (TTAGGG)_n, in seven species of the lemurs and one greater galago, as an outgroup, using the primed in-situ labeling (PRINS) technique. As expected, the telomeric sequence was identified at both ends of all chromosomes of the eight prosimians. However, six species showed a signal at some pericentromeric regions involving constitutive heterochromatin as well. The pericentromeric region of chromosome 1 of Verreaux's sifaka (Propithecus verreauxi verreauxi) was labeled with a large and intense signal. The range of the signal considerably exceeded the area of DAPI positive heterochromatin. On the other hand, in the five lemurs, a large signal was detected also in the short arm of acrocentric chromosomes. Acquisition of the large block of the telomeric sequence into the acrocentric short arm might be interpretable in terms of the tandem growth of the heterochromatic short arm and the reciprocal translocation between heterochromatic short arms involving the telomeric sequence. Subsequently, it was postulated that meta- or submetacentric chromosomes possessing the telomeric sequence at the pericentromeric region could be formed by centric fusion between such acrocentric chromosomes.