Repeated sequences in the DNA of Drosophila and their localization in giant chromosomes

It is shown by isopycnic density gradient centrifugation that the DNAs of the sibling species Drosophila hydei, Drosophila neohydei and Drosophila pseudoneohydei differ regarding the numbers and proportions of satellite DNA bands. An overwhelming proportion of all repetitive nucleotide sequences of the DNA is contained in these satellite fractions. The majority of the satellites are species specific despite the close phylogenetic and cytological relationship between the three species studied. — By in situ hybridization experiments it is demonstrated that the various satellite sequences occupy different positions within the chromosomes. All types of localization patterns, from a wide spread occurrence in all chromosomes to an apparent restriction to kinetochore regions of single chromosomes, have been observed. Main band DNA, on the other hand, in its hybridization behavior reflects the DNA distribution according to the banding pattern in giant chromosomes. Generally satellite sequences seem to be included in -heterochromatic chromosome regions but no relation to the heterochromatin of the Y-chromosome was found. — Renaturation studies support various evidence that satellite sequences occur in tandemly repetitious units. At least some of this repetitious material seems to be linked to non-satellite DNA sequences or to DNA of other satellites.     

Heterochromatin and highly repeated DNA sequences in rye (Secale cereale)

Secale cereale DNA, of mean fragment length 500 bp, was fractionated by hydroxylapatite chromatography to allow recovery of a very rapidly renaturing fraction (C₀t 0–0.02). This DNA fraction was shown to contain several families of highly repeated sequence DNA. Two highly repeated families were purified; (1) a fraction which renatured to a density of 1.701 g/ cc and comprised 2–4% of the total genome, and (2) polypyrimidine tract DNA which comprised 0.1% of the total genome. The 1.701 g/cc DNA consisted of short sequence repeat units (5–50 bp long) tandemly repeated in blocks 30 kb long, while a portion of the polypyrimidine tract DNA behaved as part of a much larger block of tandemly repeated sequences. The chromosomal location of these sequences was determined by the in situ hybridisation of radioactive, complementary RNA to root tip mitotic chromosomes and showed the 1.701 g/cc sequences to be largely limited to the telomeric blocks of heterochromatin, accounting for 25–50% of the DNA present in these parts of the chromosomes. The polypyrimidine tracts were distributed at interstitial locations with 20–30% of the sequences at three well defined sites. The combined distributions of the 1.701 g/cc DNA sequences and polypyrimidine tracts effectively individualised each rye chromosome thus providing a sensitive means of identifying these chromosomes. The B chromosomes present in Secale cereale cv. Unevita, did not show defined locations for the sequences analysed. — The data are discussed in terms of the structure of the rye genome and the generality of the observed genomic arrangement of highly repeated sequence DNA.     

Polytene chromosomes of Oxytricha: Biochemical and morphological changes during macronuclear development in a ciliated protozoan

After conjugation in the ciliated protozoan, Oxytricha, polytene chromosomes are formed during the development of a macronucleus from a micronucleus. Here we report a microscopic study of these chromosomes and an analysis of their DNA. The polytene chromosomes of Oxytricha bear a strong morphological resemblance to the polytene chromosomes of the Dipteran salivary gland. The nucleus of a developing macronuclear anlage contains 120±2 polytene chromosomes and each chromosome has an average of 81 bands; a total of about 10,000 bands per nucleus. At a later stage in development, the number of bands per chromosome is reduced by a factor of four, presumably due to fusion of adjacent bands. The polytene chromosomes then break up into their constituent bands, each of which is encased in a vesicle. There are about 2,700 vesicles per nucleus. — During the growth of polytene chromosomes, there is a change in the relative proportion of sequences in the DNA. The DNA from polytene nuclei has a buoyant density of 1.695 g/cc, significantly lighter than the density of the original micronuclear DNA (1.698 g/cc to 1.702 g/cc). We interpret this buoyant density change to be the result of differential replication of DNA sequences during polytene chromosome growth. A second change in DNA composition occurs after the polytene stage of development, shown by a shift in buoyant density to 1.701 g/cc in the DNA of the mature macronucleus. During this second process, the molecular weight of the DNA is reduced from greater than 50×10⁶ daltons to about 2×10⁶ daltons.This paper is No. VI in the series, DNA of Ciliated Protozoa.     

Location of satellite DNAs in the chromosomes of the kangaroo rat (Dipodomys ordii)

The location of satellite DNA sequences in metaphase chromosomes has been studied in the kangaroo rat by the in situ hybridization technique, staining techniques and phase contrast microscopy. The HS- satellite DNA is located at the kinetochores of all but three chromosome pairs. The HD satellite is located predominantly in the short arms of the chromosomes containing HS- and in the kinetochores of chromosome pairs that lack HS-. The regions that contain the satellite DNA sequences can also be identified by the Giemsa staining technique, and can be visualized with phase contrast microscopy or following Feulgen staining of fixed chromosome preparations.     

Satellite DNA and evolution of sex chromosomes

The satellite DNA (satellite III) which is mainly represented in the female of Elaphe radiata (Ophidia, Colubridae) has been isolated and its buoyant density has been determined (=1.700 g cm^–3). In situ hybridisation of radioactive complementary RNA of this satellite DNA with the chromosomes of different species has revealed that it is mainly concentrated on the W sex chromosome and its sequences are conserved throughout the sub-order Ophidia. From hybridisation studies these sequences are absent from the primitive family Boidae which represents a primitive state of differentiation of sex chromosomes. Chromosome analysis and C-banding have also revealed the absence of heteromorphism and of an entirely heterochromatic chromosome in the species belonging to the primitive family and their presence in the species of highly evolved families. It is suggested that the origin of satellite DNA (satellite III) in the W chromosome is the first step in differentiation of W from the Z in snakes by generating asynchrony in the DNA replication pattern of Z and W chromosomes and thus conceivably reducing the frequency of crossing-over between them which is the prerequisite of differentiation of sex chromosomes. Presence of similar sex chromosome associated satellite DNA in domestic chicken suggests its existence in a wider range of vertebrates than just the snakes.     

The location of DNA homologous to human satellite III DNA in the chromosomes of chimpanzee (Pan troglodytes), gorilla (Gorilla gorilla) and orang utan (Pongo pygmaeus)

Radioactive RNA with sequences complementary to human DNA satellite III was hybridised in situ to metaphase chromosomes of the chimpanzee (Pan troglodytes), the gorilla (Gorilla gorilla) and the orangutan (Pongo pygmaeus). A quantitative analysis of the radioactivity, and hence of the chromosomal distribution of human DNA satellite III equivalent sequences in the great apes, was undertaken, and the results compared with interspecies chromosome homologies based upon Giemsa banding patterns. In some instances DNA with sequence homology to human satellite III is present on the equivalent (homologous) chromosomes in identical positions in two or more species although quantitative differences are observed. In other cases there appears to be no correspondence between satellite DNA location and chromosome homology determined by banding patterns. These results differ from those found for most transcribed DNA sequences where the same sequence is located on homologous chromosomes in each species.     

Satellite DNA and cytogenetic evolution

The genusDipodomys (kangaroo rats) exhibits major interspecies variations in the proportions of highly reiterated satellite DNA sequences in the genome as well as in the chromosome number and the proportions of uni-armed and bi-armed chromosomes. For nearly all of the approximately 22 species of the genus and several subspecies, liver DNA was distributed in neutral CsCl buoyant density gradients into four fractions: principal DNA (1.698 g/ml), intermediate-density DNA (1.702 g/ml), MS satellite (1.707 g/ml) and HS (heavy) satellites (1.713 g/ml). The total nuclear DNA content of diploid liver cells measured in eleven species by quantitative cytophotometry, ranged from 6.9 to 10.9 pg. These data were correlated with known features of the karyotypes of individual species. The salient findings were: (1) that interspecies variations in diploid chromosome number cluster at 52–54, 60–64 and 70–72 (2) that high total nuclear DNA was associated with high chromosome number, and with relatively large amounts of satellite DNA (3) that a high ratio of HS satellites to intermediate-density DNA was generally correlated with a predominance of metacentric and submetacentric chromosomes (high fundamental number). The relationships of satellite DNA to karyotype structure reveal a new level of hierarchy in the genome that appears capable of exerting global control over environmental adaptation and the evolution of new species. This mechanism is consistent with recent hypotheses that changes in the macro-structure of the genome are more important than point mutations in facilitating the rapid phases of animal evolution.     

Satellite DNA of Drosophila nasuta nasuta and D. n. albomicana: Localization in polytene and metaphase chromosomes

The DNA from the two Drosophila nasuta races, D. n. nasuta and D. n. albomicana was investigated by CsCl density gradient centrifugation. D. n. nasuta has one major AT-rich satellite DNA sequence with a density of 1.664g/cm³, while D. n. albomicana has at least three satellites with densities of 1.674g/cm³, 1.665g/cm³ and 1.661 g/cm³. The isolated satellite sequences hybridize in situ to all heterochromatic regions of all metaphase chromosomes of both races. In polytene chromosomes the satellite sequences hybridize exclusively to the chromocenter. All chromosomal regions hybridizing with the satellites show also bright quinacrine fluorescence.     

The distribution of repetitive DNAs between regular and supernumerary chromosomes in Species of Glossina (Tsetse): a two-step process in the origin of supernumeraries

Several species of tsetse fly within the Morsitans and Fusca subgenera of Glossina contain supernumerary (B) chromosomes. Previous studies on the meiotic behaviour of chromosomes (Southern and Pell, 1973) and the C-band patterns (Jordan et al., 1977) have indicated a close similarity between the Y chromosome and the supernumeraries. The distributions of the highly abundant families of DNA (satellite DNAs) between the autosomes, sex chromosomes and B chromosomes of G.m. morsitans, G. austeni and G. pallidipes have been examined by in situ hybridisation. In addition, the organisation and sequence homologies of satellite DNAs have been examined by restriction enzymes and heterologous hybridisations in in situ and Southern transfer conditions. The majority of satellite sequences that are homologous between species are distributed in several different arrangements between A and B chromosome telomeres with minority sequences at some centromeric and intercalary locations. There is no extensive satellite DNA similarity between the Y and B chromosomes. We suggest that the Y and B chromosome associations and synchronous allocycly during meiosis are the result of extensive heterochromatinisation of these two chromosome types, that is probably a reflection of two separate stages involved in the generation of the B chromosomes in the genus. The independent evolution of satellites and supernumeraries is discussed.     

Two AT-rich satellite DNAs in the chironomid Glyptotendipes barbipes (Staeger)

Two AT-rich satellite DNAs are present in the genome of Glyptotendipes barbipes. The two satellites have densities of 1.680 g/cm³ (=21% GC) and of 1.673 g/cm³ (=13% GC) in neutral CsCl-density gradients. The main band DNA has a density of 1.691 g/cm³ (=32% GC). This value is in agreement with the 33% GC-content of G. barbipes DNA calculated from thermal denaturation (T_M=83° C). — In brain DNA as well as in salivary gland DNA the two satellite sequences together comprise 12–15% of the total G. barbipes DNA. Comparisons of the density profiles of DNA extracted from polytene and non-polytene larval tissue gave no hints for underreplication of the satellite DNAs during polytenization. — The two satellite DNAs have been isolated from total DNA by Hoechst 33258-CsCl density centrifugation and then localized in the polytene salivary gland chromosomes by in situ hybridization. Both satellite sequences hybridize to all heterochromatic centromere bands of all four chromosomes of G. barbipes. Satellite I (1.673 g/cm³) hybridizes mainly with the middle of the heterochromatin, satellite II (1.680 g/cm³) hybridizes with two bands at the margin of the heterochromatin. In situ hybridization with polytene chromosomes of Chironomus thummi revealed the presence of G. barbipes satellite sequences also in the Ch. thummi genome at various locations, mainly the centromere regions.     

A new family of satellite DNA sequences as a major component of centromeric heterochromatin in owls (Strigiformes)

We isolated a new family of satellite DNA sequences from Hae III- and Eco RI-digested genomic DNA of the Blakistons fish owl ( Ketupa blakistoni). The repetitive sequences were organized in tandem arrays of the 174 bp element, and localized to the centromeric regions of all macrochromosomes, including the Z and W chromosomes, and microchromosomes. This hybridization pattern was consistent with the distribution of C-band-positive centromeric heterochromatin, and the satellite DNA sequences occupied 10% of the total genome as a major component of centromeric heterochromatin. The sequences were homogenized between macro- and microchromosomes in this species, and therefore intraspecific divergence of the nucleotide sequences was low. The 174 bp element cross-hybridized to the genomic DNA of six other Strigidae species, but not to that of the Tytonidae, suggesting that the satellite DNA sequences are conserved in the same family but fairly divergent between the different families in the Strigiformes. Secondly, the centromeric satellite DNAs were cloned from eight Strigidae species, and the nucleotide sequences of 41 monomer fragments were compared within and between species. Molecular phylogenetic relationships of the nucleotide sequences were highly correlated with both the taxonomy based on morphological traits and the phylogenetic tree constructed by DNA-DNA hybridization. These results suggest that the satellite DNA sequence has evolved by concerted evolution in the Strigidae and that it is a good taxonomic and phylogenetic marker to examine genetic diversity between Strigiformes species.An erratum to this article can be found at Communicated by Y. Hiraoka     

A high-resolution karyotype of <Emphasis Type="Italic">Brassica rapa</Emphasis> ssp. <Emphasis Type="Italic">pekinensis</Emphasis> revealed by pachytene analysis and multicolor fluorescence in situ hybridization

A molecular cytogenetic map of Chinese cabbage (Brassica rapa ssp. pekinensis, 2n=20) was constructed based on the 4-6-diamino-2-phenylindole dihydrochloride-stained mitotic metaphase and pachytene chromosomes and multicolor fluorescence in situ hybridization (McFISH), using three repetitive DNA sequences, 5S rDNA, 45S rDNA, and C11-350H. The lengths of mitotic metaphase chromosomes ranged from 1.46 m to 3.30 m. Five 45S and three 5S rDNA loci identified were assigned to different chromosomes. The C11-350H loci were located on all the mitotic metaphase chromosomes, except chromosomes 2 and 4. The pachytene karyotype consisted of two metacentric (chromosomes 1 and 6), five submetacentric (chromosomes 3, 4, 5, 9 and 10), two subtelocentric (chromosomes 7 and 8), and one acrocentric (chromosome 2) chromosome(s). The mean lengths of ten pachytene chromosomes ranged from 23.7 m to 51.3 m, with a total of 385.3 m, which is 17.5-fold longer than that of the mitotic metaphase chromosomes. In the proposed pachytene karyotype, all the chromosomes of B. rapa ssp. pekinensis can be identified on the basis of chromosome length, centromere position, heterochromatin pattern, and the location of the three repetitive sequences. Moreover, the precise locations of the earlier reported loci of 5S rDNA, 45S rDNA, and Chinese cabbage tandem DNA repeat C11-350H were established using McFISH analysis. We also identified a 5S rDNA locus on the long arm of pachytene bivalent 7, which could not be detected in the mitotic metaphase chromosomes in the present and earlier studies. The deduced karyotype will be useful for structural and functional genomic studies in B. rapa.     

Molecular cytogenetics of the equidae. II. purification and cytological localization of a (G + C)-rich satellite DNA from Equus hemionus onager and cross-species hybridization to E. asinus chromosomes

A (G + C)-rich satellite DNA component (p = 1.716 g/ml) has been fractionated from the total DNA of the Iranian subspecies of the Asiatic wild ass, Equus hemionus onager, by successive dactinomycin-CsCl and netropsin sulfate-CsCl isopycnic gradients. Complementary 3H-RNA (cRNA) transcribed from the satellite DNA hybridized predominantly to the centromeric and telomeric constitutive heterochromatic regions of onager chromosomes. These studies have suggested that satellite DNA's with similar sequences are present in the centromeric, as well as telomeric, heterochromatic regions of some onager chromosomes. The centromeric region of the fusion metacentric t(23;24) of the onager is deficient in sequences homologous to the onager 1.716 g/ml satellite DNA, indicating a loss of satellite DNA during fusion or an amplification of the satellite DNA in the centromeric regions of the acrocentric chromosomes 23 and 24 subsequent to fission. Sequences complementary to onager 1.716 g/ml satellite DNA show extensive hybridization to the constitutive heterochromatin of the feral donkey (E. asinus) karyotype, consistent with a view of conservation and amplification of similar or identical sequences in the two species.     

Three euchromatic DNA sequences under-replicated in polytene chromosomes of Drosophila are localized in constrictions and ectopic fibers

We examined three regions of under-represented euchromatic DNA sequences (histone, Ubx, and 11 A), for their possible correlation with euchromatic constrictions in polytene chromosomes of Drosophila melanogaster. Cloned sequences were hybridized to filters and to chromosomes prepared for light microscopy. Under-represented sequences hybridized to DNA within constrictions and in ectopic fibers. In contrast, adjacent sequences that were fully endoreplicated in the Ubx and 11A regions in polytene cells hybridized to sites just adjacent to their respective constrictions. For one region (Ubx), sequences under-represented in salivary gland cells were fully endoreplicated in fat body cells. For this particular region, the morphology of the polytene chromosomes differs between these two cell types in that the specific constriction is absent at this region in fat body polytene chromosomes, thus strengthening the correlation between under-representation and chromosome constrictions. Although all three sequences are in regions that have been classified by others as intercalary heterochromatin, we detect no common functional or sequence organizational feature for these examples of under-represented DNA. We suggest that the lower efficiencies of the replication origins, or special regions of termination at these sites, are the primary cause of the under-replication, and that this under-replication is sufficient to confer the properties of intercalary heterochromatin.     

Studies of He-T DNA sequences in the pericentric regions of Drosophila chromosomes

He-T DNA is a complex set of repeated DNA sequences with sharply defined locations in the polytene chromosomes of Drosophila melanogaster. He-T sequences are found only in the chromocenter and in the terminal (telomere) band on each chromosome arm. Both of these regions appear to be heterochromatic and He-T sequences are never detected in the euchromatic arms of the chromosomes (Young et al. 1983). In the study reported here, in situ hybridization to metaphase chromosomes was used to study the association of He-T DNA with heterochromatic regions that are under-replicated in polytene chromosomes. Although the metaphase Y chromosome appears to be uniformly heterochromatic, He-T DNA hybridization is concentrated in the pericentric region of both normal and deleted Y chromosomes. He-T DNA hybridization is also concentrated in the pericentric regions of the autosomes. Much lower levels of He-T sequences were found in pericentric regions of normal X chromosomes; however compound X chromosomes, constructed by exchanges involving Y chromosomes, had large amounts of He-T DNA, presumably residual Y sequences. The apparent co-localization of He-T sequences with satellite DNAs in pericentric heterochromatin of metaphase chromosomes contrasts with the segregation of satellite DNA to alpha heterochromatin while He-T sequences hybridize to beta heterochromatin in polytene nuclei. This comparison suggests that satellite sequences do not exist as a single block within each chromosome but have interspersed regions of other sequences, including He-T DNA. If this is so, we assume that the satellite DNA blocks must associate during polytenization, leaving the interspersed sequences looped out to form beta heterochromatin. DNA from D. melanogaster has many restriction fragments with homology to He-T sequences. Some of these fragments are found only on the Y. Two of the repeated He-T family restriction fragments are found entirely on the short arm of the Y, predominantly in the pericentric region. Under conditions of moderate stringency, a subset of He-T DNA sequences cross-hybridizes with DNA from D. simulans and D. miranda. In each species, a large fraction of the cross-hybridizing sequences is on the Y chromosome.     

Quantitative variation of “Mus musculus-like” constitutive heterochromatin and satellite DNA-sequences in the genus Mus

The extent of conservation of constitutive heterochromatin in three species of Mus viz. M. musculus, M. booduga and M. dunni, with shared cytological properties and homologous DNA sequences has been studied. The cytological properties were investigated by doing fluorescence staining and condensation inhibition of their chromosomes with Hoechst 33258. Both the parameters indicate the occurrence of a reduced quantum of M. musculus like heterochromatin at specific sites in the other two genomes. In situ hybridization of the nick translated ³H-labelled M. musculus satellite DNA with M. booduga and M. dunni chromosomes, also corroborates our Hoechst 33258 findings and comparable variation in the amount and site of occurrence of sequences homologous to M. musculus satellite DNA in these species are noticed. The study thus provides a good example of a gradual quantitative variation of a particular type of heterochromatin and in turn of the repetitive DNA constituting it in different related species. Further since the heterochromatin in M. booduga and M. dunni is expected to contain different repetitive DNA sequences in addition to those homologous to M. musculus satellite DNA, it is proposed that a change in the balance between two or more repetitive sequences in heterochromatin may be more crucial in its evolutionary consequences rather than a mere increase or decrease of a homogeneous repetitive sequence.     

Satellite DNA associated with heterochromatin in Rhynchosciara

The DNA of Rhynchosciara hollaenderi was examined using isopycnic centrifugation in neutral CsCl. Two low density minor bands (collectively termed satellite DNA) were detected in addition to the main band DNA. Main band DNA has a buoyant density of 1.695 g/cm³. The larger of the two minor bands has a buoyant density of 1.680 g/cm³ while the smaller of the two minor bands has a buoyant density of about 1.675 g/cm³. Thermal denaturation studies have confirmed the presence of the two minor classes of DNA.—The satellite and main band DNAs were isolated in relatively pure form and were transcribed in vitro using DNA-dependent RNA polymerase from Escherichia coli. Annealing of the two complementary RNAs (cRNAs) with main band and satellite DNA was examined using filter hybridization techniques.—The chromosomal distribution of the satellite DNA was determined by in situ molecular hybridization of satellite-cRNA with Rhynchosciara salivary gland chromosomes. Satellite-cRNA hybridized with the centromeric heterochromatin of each of the four chromosomes (A, B, C, and X) and with certain densely staining bands in the telomere regions of the A and C chromosomes. Main band-cRNA annealed with many loci scattered throughout the chromosomes including areas containing satellite DNA.     

Heteromorphic sex chromosomes of lizardfish (Synodontidae): focus on the ZZ-ZW1W2 system in Trachinocephalus myops

The karyotype and DNA content of four lizardfish species (family Synodontidae), that is, Saurida elongata, Synodus ulae, Synodus hoshinonis and Trachinocephalus myops, were analyzed. The karyotype of T. myops significantly differed from that of the other three species having diploid chromosome number of 48 with mainly acrocentric chromosomes and the ZZ-ZW sex chromosome system. The chromosome number of male T. myops was 2n=26, while that of female T. myops was 2n=27. The karyotype consisted of 11 pairs of metacentrics, one pair of acrocentrics and, in addition, two large metacentrics in the male and a single large metacentric, a distinctly small subtelocentric and a microchromosome in the female. C-banding demonstrated that in the female the subtelocentric chromosome and the microchromosome were heterochromatic. The karyotype of T. myops was thought to be derived from a 48 chromosome type synodontid fish through the involvement of Robertsonian rearrangement; the rearrangement of the sex chromosomes proceeded during karyotype evolution. Among the chromosomes, the large metacentrics were determined to be neo-Z (a fusion of the original Z and an autosome), the microchromosomes the W₁ (originally W), and the subtelocentric chromosomes the W₂ (derived from an autosome pair). The miniaturization of W₁ and W₂ chromosomes and their heterochromatinization suggested that sex chromosomes in this species have been already highly differentiated. The findings on DNA content implied that the karyotype of T. myops evolved by centric fusion events without loss in DNA amount.     

Satellite DNA properties of the germ line limited DNA and the organization of the somatic genomes in the nematodesAscaris suum andParascaris equorum

During the early cleavage divisions in some Ascarids, parts of the chromosomes are eliminated from the somatic blastomeres (chromatin diminution, Boveri, 1887) while the chromosomes in the germ line cells maintain their integrity. To characterize the germ line and soma genome, DNA was isolated from gametes and embryonic somatic cells of two Ascarid species,Parascaris equorum var. univalens andAscaris suum. It was shown that the germ line limited DNAs of these species have the same density and almost identical reassociation kinetics: in CsCl the predominant component of the germ line limited DNA ofP. equorum andA. suum has the buoyant density of 1.697g/cm³, while soma DNA of both species bands at 1.700 g/cm³. InP. equorum there is a small additional germ line limited satellite DNA component with the density of 1.690 g/cm³, identical to that of mitochondrial DNA of both organisms. Comparison of the reassociation kinetics of germ line and soma DNA demonstrates for both species that the eliminated DNA sequences are highly repetitive. In contrast to these similarities between the germ line limited DNAs ofP. equorum andA. suum the analysis of their base composition revealed differences (40% guanine plus cytosine inP. equorum and 36% inA. suum). The only very fast reassociating DNA sequences which we could isolate from soma DNA was demonstrated to be foldback DNA. The reassociation kinetics of totalA. suum soma DNA was investigated by hydroxylapatite chromatography. Least squares analysis of the data revealed about 10% of intermediate repetitive DNA sequences. Their interspersion between single copy DNA sequences was analyzed by comparing the reassociation kinetics of DNA fragments 0.35 and 7.2 kilobases long. Thus the DNA sequence arrangement ofAscaris does not follow the short period interspersion pattern observed in most organism.     

A highly repetitive DNA component common to all Cervidae: its organization and chromosomal distribution during evolution

In recent work we have isolated and characterized a highly repetitive DNA (MMV satellite IA) from Muntiacus muntjak vaginalis, the species with the most reduced karyotype in the Cervidae family. We have now analysed the genomes of nine related species for the presence of MMV satellite IA components, and have determined their organization and chromosomal distribution. Repetitive satellite IA type DNA is present in all species of the Cervidae, and also in the bovine, but not in a species of the Tragulidae suggesting that these sequences were generated after the phylogenetic separation of Bovidae and Tragulidae. Studies on the organization of the satellite IA DNA in the various species revealed three main repeat lengths: 1400, 1000 and 807 bp. The relative proportion of satellite IA sequences present in any one of the three registers is strikingly different within the various species and can be correlated with the phylogeny of the Cervidae. The chromosomal locations of the satellite IA sequences were determined in seven species by in situ hybridization. It turned out that the chromosomal rearrangements leading to the reduction in the number of chromosomes during karyotype evolution have led to the elimination of satellite I DNA at most locations. In all tandem fusions, the satellite IA sequences located at the centromeres of the ancestral acrocentric chromosomes are lost. In contrast, during the centric fusion that generates the M. m. vaginalis X chromosome satellite IA sequences are amplified. Sequence motifs, which are known to be involved in recombinational events are present in the satellite IA and might have contributed to the unique karyotype variation in the Cervidae.