Satellite DNA and evolution of sex chromosomes

Individualization and estimation of relatedness in crocodilians by DNA fingerprinting with a Bkm-derived probe

Individual-specific DNA fingerprints of crocodilians were obtained by the use of Bkm-2(8) probe. Pedigree analyses of Crocodylus palustris, C. porosus and Caiman crocodilus revealed that the multiple bands (22-23 bands with Aludigest) thus obtained were inherited stably in a Mendelian fashion. Unique fingerprints permitted us to identify individuals, assign parentage, and reconstruct the DNA profile of a missing parent. Average band sharing between unrelated crocodiles was found to be 0.37. Band sharing between animals of known pedigrees increased predictably with relatedness and provided a basis for distinguishing relatives from non-relatives. Similar results obtained in other species/genera, using the same probe, suggest that this approach may be applicable to all species of crocodilians, and could facilitate genetic studies of wild and captive populations.

Primitive sex chromosomes in poeciliid fishes harbor simple repetitive DNA sequences

The demonstration of the chromosomal mode of sex determination via genetic experiments as well as the absence of heteromorphic sex chromosomes affirm poeciliid fishes as a unique group among vertebrates that are endowed with the most primitive form of sex chromosomes. In many different taxa the evolutionary process involved in the differentiation of advanced sex chromosomes is outlined through sex specifically organized repetitive sequences. In this investigation hybridization of synthetic probes specific to genomic simple repeat motifs uncovers a sex-specific hybridization pattern in certain viviparous fishes of the family Poeciliidae. The hybridization pattern together with specific staining of the constitutive heterochromatin by C-banding reveals heterogamety in males (Poecilia reticulata) as well as in females (P. sphenops). In P. velifera, however, C-banding alone fails to unravel the heterogametic status. The female specific W-chromosome can be detected by simple repetitive sequence probes. Therefore, the principal significance of heterochromatization as a means of generating differentiated sex chromosomes is evident.

Early stages of sex chromosome differentiation in fish as analysed by simple repetitive DNA sequences

Animal sex chromosome evolution has started on different occasions with a homologous pair of autosomes leading to morphologically differentiated gonosomes. In contrast to other vertebrate classes, among fishes cytologically demonstrable sex chromosomes are rare. In reptiles, certain motifs of simple tandemly repeated DNA sequences like (gata)n/(gaca)m are associated with the constitutive heterochromatin of sex chromosomes. In this study a panel of simple repetitive sequence probes was hybridized to restriction enzyme digested genomic DNA of poeciliid fishes. Apparent male heterogamety previously established by genetic experiments in Poecilia reticulata (guppy) was correlated with male-specific hybridization using the (GACA)4 probe. The (GATA)4 oligonucleotide identifies certain male guppies by a Y chromosomal polymorphism in the outbred population. In contrast none of the genetically defined heterogametic situations in Xiphophorus could be verified consistently using the collection of simple repetitive sequence probes. Only individuals from particular populations produced sex-specific patterns of hybridization with (GATA)4. Additional poeciliid species (P. sphenops, P. velifera) harbour different sex-specifically organized simple repeat motifs. The observed sex-specific hybridization patterns were substantiated by banding analyses of the karyotypes and by in situ hybridization using the (GACA)4 probe.

The evolution of heteromorphic sex chromosomes

The facts and ideas which have been discussed lead to the following synthesis and model. 1. Heteromorphic sex chromosomes evolved from a pair of homomorphic chromosomes which had an allelic difference at the sex-determining locus. 2. The first step in the evolution of sex-chromosome heteromorphism involved either a conformational or a structural difference between the homologues. A structural difference could have arisen through a rearrangement such as an inversion or a translocation. A conformational difference could have occurred if the sex-determining locus was located in a chromosomal domain which behaved as a single control unit and involved a substantial segment of the chromosome. It is assumed that any conformational difference present in somatic cells would have been maintained in meiotic prophase. 3. Lack of conformational or structural homology between the sex chromosomes led to meiotic pairing failure. Since pairing failure reduced fertility, mechanisms preventing it had a selective advantage. Meiotic inactivation (heterochromatinization) of the differential region of the X chromosome in species with heterogametic males and euchromatinization of the W in species with heterogametic females are such mechanisms, and through them the pairing problems are avoided. 4. Structural and conformational differences between the sex chromosomes in the heterogametic sex reduced recombination. In heterogametic males recombination was reduced still further by the heterochromatinization of the X chromosome, which evolved in response to selection against meiotic pairing failure. 5. Suppression of recombination resulted in an increase in the mutation rate and an increased rate of fixation of deleterious mutations in the recombination-free chromosome regions. Functional degeneration of the genetically isolated regions of the Y and W was the result. In XY males this often led to further meiotic inactivation of the differential region of the X chromosome, and in this way an evolutionary positive-feedback loop may have been established. 6. Structural degeneration (loss of material) followed functional degeneration of Y or W chromosomes either because the functionally degenerate genes had deleterious effects which made their loss a selective advantage, or because shorter chromosomes were selectively neutral and became fixed by chance. 7. The evolutionary routes to sex-chromosome heteromorphism in groups with female heterogamety are more limited than in those with male heterogamety. Oocytes are usually large and long-lived, and are likely to need the products of X- or Z-linked genes. Meiotic inactivation of these chromosomes is therefore unlikely. In the oocytes of ZW females, meiotic pairing failure is avoided through euchromatinization of the W rather than heterochromatinization of the Z chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

A novel avian W chromosome DNA repeat sequence in the lesser black-backed gull (Larus fuscus)

The phenol emulsion reassociation technique was used to isolate and clone a female specific, repetitive DNA sequence from Larus fuscus. The repeat, designated P2000-17, is restricted to the W chromosome, although related sequences occur elsewhere in the genome of L. fuscus. Similar sequences were detected in the genome of six other bird species from outside the genus Laridae, but the sequence occurs less frequently and to a similar extent in both sexes. The 298 bp DNA sequence of P2000-17 was determined and found to have extensive sequence identity to the rabbit dihydropyridine (DHP) receptor calcium channel. P2000-17 is represented once within a larger 8.6 kb tandem repeat (LfW-1), which has a complex internal DNA sequence. LfW-1 is highly conserved between repeat motifs and may comprise 3% of the female genome. The possible evolutionary origin of LfW-1 is discussed in relation to the repeat types found on the W and Y chromosomes of other species.

Schistosoma mansoni: chromosomal localization of DNA repeat elements by in situ hybridization using biotinylated DNA probes

Localization of the SM alpha family of repeated DNA and the rDNA repeat on the chromosomes of Schistosoma mansoni by in situ hybridization is presented. Biotinylated DNA was hybridized to target chromosomes and hybridization was detected using either alkaline phosphatase-labeled avidin or fluorescein-labeled avidin and biotinylated anti-avidin antibody. Hybridization detection using a fluorescein conjugate was more specific and sensitive with less background noise than detection with alkaline phosphatase conjugates. SM alpha hybridizing sequences were found dispersed throughout the genome, hybridizing to the sex chromosomes and autosomes. The SM alpha probe showed specific hybridization to the euchromatic gap region within the large heterochromatic block of the short arm of the W chromosome. This specific hybridization coupled with the lack of chiasma formation in this region of the ZW bivalent (presumably due to the heterochromatinization of this region) may explain the pattern of sex-specific hybridization reported for the SM alpha family. The rDNA repeat was localized to the secondary constriction of the short arm of chromosome 3. Specifically, the rDNA probe hybridized with the stalk of the secondary constriction and with parts of both side regions, the satellite and the short arm proper.

A DNA probe from Schistosoma mansoni allows rapid determination of the sex of larval parasites

A DNA clone representing a 0.4 kb degenerative repeat has been isolated. The DNA sequence is present only in the genome of female Schistosoma mansoni at different stages of the life cycle, at a frequency of approximately 75 copies per adult female genome. The sequence is not expressed and probably represents satellite DNA in the heterochromatin region of the W chromosome. It is demonstrated that the DNA clone may be used for the rapid determination of the sex of cercariae without the need for DNA isolation or Southern blotting.

Northern analyses using single-stranded probes do not support a role for GATA/GACA repeats in sex determination in mice and men

The possible role of GATA/GACA repeated sequences in mammalian sex determination was investigated using Northern analyses of mouse and human RNA. Brain, liver, and gonadal RNA from three developmental stages of mice of both sexes and also human fetal RNA from various tissues were hybridized to both sense and antisense Bkm riboprobes as well as to the synthetic oligonucleotide (GATA)5. At low levels of stringency, putative transcripts of various sizes were observed in all tissue samples with all probes. At high stringency, only a putative transcript of approximately 12 kb was observed, but this was later shown to consist of contaminating DNA. No sex-specific differences were observed in any tissue or developmental stage. Thus, we find no evidence that the GATA/GACA repeated sequences are specifically expressed in quantities detectable by Northern analyses in a manner important to mammalian sex determination.

Bkm sequences from the human X chromosome contain large clusters of GATA/GACA repeats

In order to determine whether the regional localizations of Bkm repeats detected on the human X chromosome consisted of typical GATA/GACA repeats, clones were isolated, mapped, and sequenced. Nine Bkm-hybridizing clones from Kunkel's fluorescent-activated, cell-sorted X-chromosome library were all unique. Five were mapped in detail with restriction enzymes and the Bkm-hybridizing segments were localized. Confirmation of X chromosomal homology was obtained for 2 of the clones and Bkm segments from these 2 clones were sequenced. Seventeen contiguous GATA repeats were found in each clone and the overall repeat arrangement showed relatively few differences from previously sequenced Bkm sequences. These are the first sequences of human Bkm repeats. The results, when compared with previously published results, suggest that there may be significant differences between the organization of Bkm repeats on the human X and on the human Y chromosome.

On the evolution of sex determination

Female mice reject skin grafts from intrastrain males because of the H-Y transplantation antigen. Those females produce antibodies that recognize a male-specific cell-surface antigen in serological tests. The serological antigen has also been called 'H-Y', but there is evidence that the two antigens are distinct. We therefore refer to the transplantation antigen as H-Yt, or transplantation H-Y, and to the serological antigen as serological H-Y, or simply H-Y, without prejudice whether these are the same or related or separate antigens. In this study, sex-specific expression of serological H-Y antigen was found in 25 new vertebrate species representing each of seven major vertebrate classes. There was a strong correlation between expression of H-Y and occurrence of the heterogametic-type gonad, although unusual patterns of H-Y expression were noted in cases of temperature-influenced sex determination and in systems representing possible transition from one mode of heterogamety to the other. Male and female heterogamety are found side-by-side in certain freshwater toothed carps; and distinct sex chromosomes have been recognized in certain amphibians, even though they are not apparent in certain reptiles and primitive birds. In seven ophidian species, in which the female is the heterogametic sex, H-Y was detected in the female; and in three species of Ranidae in which the male is heterogametic, it was detected in the male. In three species of cartilaginous fish and in one of the cyclostomes, in which heterogamety has not been ascertained, H-Y was detected in the male, suggesting that those primitive fishes are male-heterogametic. Evidently, then, heterogamety and sex-chromosome heteromorphism are polyphyletic, although certain sex-determining genes may be held in common among the diverse taxonomic groups.

Further evidence for early sex chromosome differentiation of anuran species

Chromosome banding and meiotic evidence show that XX/XY systems found in two Eupsophus species (Amphibia-Leptodactylidae) represent early stages of sex chromosome differentiation. Pair 14 is heteromorphic in E. migueli males and represents the heterochromosomes. In E. roseus this pair is metacentric and does not show heteromorphism. Paracentromeric constitutive heterochromatin is present in all chromosomes except in the E. migueli and E. roseus metacentric Y chromosomes. Constitutive heterochromatin loss is the structural modification responsible for Y chromosome differentiation. Pericentric inversions may have modified the morphology of the X chromosome of Eupsophus species.

Genus specificity and extensive methylation of the W chromosome-specific repetitive DNA sequences from the domestic fowl, Gallus gallus domesticus

Two female-specific repeating DNA units of 0.6 kilobase pairs (kb) and 1.1 kb, produced by digesting the genomic DNA of the White Leghorn chicken with Xho I, were cloned by inserting them into the Xho I site of an Escherichia coli plasmid vector pACYC177. Two such recombinant plasmids, pAGD0601 and pAGD1101, containing a single 0.6-kb and 1.1-kb sequence, respectively, were used as molecular probes. In situ hybridization of the 3H-probes to the metaphase chromosomes from the female White Leghorn embryos revealed their localization in the W chromosome. Semiquantitative Southern blot hybridization with 32P-probes in excess indicated that the 0.6-kb unit and 1.1-kb unit were repeated approximately 14,000 and 6,000 times, respectively, in the W chromosome. The two units comprised about 46% of the W chromosomal DNA. These two repeating units were found in the female genomes of every line of Gallus g. domesticus tested and in the female genomes of three jungle fowl species (G. gallus, G. sonneratii, and G. varius) but not in three species belonging to other genera in the suborder Galli. Hha I sites in the 0.6-kb and 1.1-kb repeating units were shown to be extensively methylated and a significant fraction of the Hpa II sites in the 0.6-kb repeating units were also shown to be methylated in the female genome of the White Leghorn. Methylation patterns of Hpa II sites in or around the 0.6-kb repeating units examined by the Msp I digestion were similar in the various lines of domestic fowls and the two species of jungle fowls, but G varius (black or green jungle fowl) produced a different pattern of digestion with Msp I.

Chromosome banding in Amphibia. VIII. An unusual XY/XX-sex chromosome system in Gastrotheca riobambae (Anura, Hylidae)

The mitotic and meiotic chromosomes of the marsupial frog Gastrotheca riobambae were analysed with various banding techniques. The karyotype of this species is distinguished by considerable amounts of constitutive heterochromatin and unusual, heteromorphic XY sex chromosomes. The Y chromosome is considerably larger than the X chromosome and almost completely heterochromatic. The analysis of the banding patterns obtained with GC- and AT-base-pair-specific fluorochromes shows that the constitutive heterochromatin in the Y chromosome consists of at least three different structural categories. The only nucleolus organizer region (NOR) of the karyotype is localized in the short arm of the X chromosome. This causes a sex-specific difference in the number of NOR: female animals have two NORs in diploid cells, male animals one. No cytological indications were found for the inactivation of one of the two X chromosomes in the female cells. In male meiosis, the heteromorphic sex chromosomes form a characteristic sex-bivalent by pairing their telomeres in an end-to-end arrangement. The significance of the XY/XX sex chromosomes of G. riobambae for the study of X-linked genes in Amphibia, the evolution of sex chromosomes and their specific DNA sequences, and the significance of the meiotic process of sex chromosomes are discussed.

On evolutionarily conserved simple repetitive DNA sequences: do "sex-specific" satellite components serve any sequence dependent function?

The nuclear genomes of eukaryotes contain DNA of varying degrees of repetition. Highly repetitious DNA and simple repetitive sequences as a fraction thereof appear to be distributed in a non-random fashion in the genome. There are arguments for and against functional roles of simple repetitive sequences, and the reasons for their evolutionary conservation are not at all clear. In order to learn more about the biologic role of simple repetitive sequences in the context of their evolutionary history, we report here the following results from studies of sex-specific snake satellite DNA: 1) The snake simple repeat sequence is 5'-GATAGACA-3' and it is strictly conserved throughout vertebrate evolution. 2) The simple repeat sequence is intimately interspersed with single-copy DNA throughout the mouse genome. 3) The simple repeat is transcribed into RNA in several animal systems and it is translatable in bacterial test systems. 4) The simple repeat sequence is sex-specifically arranged in vertebrates. 5) In snake DNA, the simple repeat is adjacent to a single-copy sequence which singles out a male-specific putative mRNA in mouse polysomal poly (A)+ RNA. Thus even if this snake simple repetitive sequence is not involved in a basic cellular function such as sex-determination, it is nevertheless a valuable tool to approach those problems.

Evolutionary conservation of sex specific DNA sequences

A family of DNA sequences which appears to be limited to eukaryotes is concentrated on the sex-determining chromosomes of species as widely separated evolutionarily as snakes and mammals. The significance of this distribution is presently seen in terms of the function of these sequences in cycles of chromosome condensation and decondensation involved in the control of gene expression. Thus, their concentration on the sex chromosomes is interpreted in the context of the evolution of such chromosomes which, it is hypothesised, involves the superimposition of the controls of the dominant sex-determining gene(s) upon the entire linkage group. This would result in the prevention of the large majority of its genes from having effects upon the phenotype and, in consequence, lead to their mutation to functionlessness at the maximum rate.

Demonstration of W chromosome-specific repetitive DNA sequences in the domestic fowl, Gallus g. domesticus

Evidence is presented to demonstrate the presence of W chromosome-specific repetitive DNA sequences in the female White Leghorn chicken, Gallus g. domesticus, based on two different experimental approaches. First, 3H-labelled, female chicken DNA was hybridized with excess, unlabelled, mercurated, male DNA, and unhybridized single-stranded 3H-DNA (3H-SHU-DNA) was recovered by SH-Sepharose and hydroxyapatite column chromatography. Approximately 24% of the hybridizable 3H-SHU-DNA was female-specific and localized on the W chromosome. The second approach was to examine female-specific DNA fragments among the digests of chicken DNA with various restriction endonucleases. Among them, we found that digestion with XhoI produced two prominent female-specific brands of 0.60 kb (= kilobase pairs) and 1.1 kb. The 0.60 kb fragment was isolated and 3H-labelled by nick-translation. Female-specificity of the 3H-XhoI--0.60 kb DNA was judged to be at least 95% under the conditions of hybridization with membrane filter-bound DNA. Presence of amplified XhoI--0.60 kb DNA on the W chromosome seems to be limited to different lines of G.g. domesticus and no such repeat was detected in three species belonging to other genera in the order Galliformes and in three species belonging to other avian orders.

Base sequence of a cloned snake W-chromosome DNA fragment and identification of a male-specific putative mRNA in the mouse

A 2.5-kilobase fragment of a sex-specific satellite DNA from the Colubrid snake species Elaphe radiata has been cloned, and its sequence has been determined. It contains 26 and 12 copies, respectively, of two base quadruplets, G-A-T-A and G-A-C-A, as its sole highly repetitious elements. Southern hybridization experiments with genomic DNA of the chicken, the mouse, and man indicated male sex-specific conservation of at least parts of this cloned DNA. In situ hybridization experiments with metaphase chromosomes of the mouse showed that elements that can cross-hybridize with parts of the cloned snake DNA are concentrated in the pericentric region of the Y chromosome. In blot hybridization experiments with liver poly(A)+ polysomal RNAs of male and female mice, a probe consisting of the first 1,224 bases of the cloned snake DNA singled out a male-specific RNA of 1,250-1,400 bases. Inasmuch as the proximal end of this probe contained an open reading frame (44 consecutive amino acid-specifying codons), the male-specific putative mRNA so detected may specify H-Y antigen. By contrast, a probe consisting of bases 1,480-1,906, containing the simple repeats of the quadruplets, singled out a shorter (approximately 1,000-base) RNA from males and females alike. Although this RNA is poly(A)+, we have yet to establish its attachment to ribosomes.

Karyotype evolution and sex chromosome differentiation in Schistosomes (Trematoda, Schistosomatidae)

The morphology of C-banded metaphase chromosomes has been studied in two hermaphroditic and ten gonochoristic digenetic trematodes (schistosomes). Comparison of numbers and morphology of chromosomes indicates that the karyotype of primitive trematodes probably was composed of 10 (or 11) pairs of telocentric or subtelocentric chromosomes, and reduction of chromosome numbers in advanced species resulted from centromeric fusion rather than elimination of chromosomes. Observation of heteromorphic chromosomes in a hermaphroditic trematode (Spirorchis) suggested a differentiation of "pre-sex" chromosomes in species ancestral to dioecious trematodes which possess distinctly differentiated sex chromosomes. Our results indicate that differentiation of Z and W chromosomes in the gonochoristic trematodes resulted from: (a) partial constitutive heterochromatinization of the W chromosome (Schistosoma mansoni and S. haematobium complexes, African schistosomes), (b) deletion of part of the W (S. japonicum and S. mekongi, Asian schistosomes), and (c) translocation of part of one sex chromosome onto another (Schistosomatium douthitti and Heterobilharzia americana, American schistosomes) with subsequent heterochromatinization of the W in H. americana.

Chromosome banding in Amphibia. VI. BrdU-replication patterns in anura and demonstration of XX/XY sex chromosomes in Rana esculenta

A modified BrdU-Hoechst-Giemsa technique permitted the demonstration of easily reproducible replication patterns in the somatic chromosomes of Amphibia. These banding patterns allow for the first time a precise identification of all chromosomes and the analysis of the patterns of replication in the various stages of S-phase in Amphibia. Several possibilities for the use of this technique were demonstrated on three frog species of the family Ranidae, all differing greatly in their DNA-content. With this method, the homomorphic chromosome pair No. 4 in Rana esculenta could be identified as sex-specific chromosomes of the XX/XY-type. All male animals exhibit an extremely late replicating region in the Y-chromosome, which is lacking in the X-chromosome in the female animals, both X-chromosomes replicate synchronously. These sex-specific chromosomes cannot be distinguished by other banding techniques. In the highly heteromorphic ZZ/ZW-sex chromosome system of Pyxicephalus adspersus a synchronous replication of the two Z-chromosomes of male animals and a very late replication of the short arm of the W-chromosomes of male animals was demonstrated. These results support the assumption that there is no dosage compensation for Z-linked or X-linked genes by the sex chromosome inactivation mechanism in the sex chromosomes of Amphibia.

Linear differentiation of the C-band pattern of the W chromosome in snakes and birds

Evidence is presented from C-banding studies that the W chromosome of eleven species of snakes is not homogeneous in nature but is differentiated linearly into alternating lighter and darker C positive regions. The same is true of the W chromosome of at least some birds. There is evidence from the literature indicating a similar differentiation of the Y chromosome of some mammals and here the intermediate C positive regions are deficient in highly repetitive DNA. The significance of heterochromatinization as a means of generating differentiated sex chromosomes is discussed in the light of these findings.

Satellite DNA of Anopheles stephensi Liston (Diptera: Culicidae). Chromosomal location and under-replication in polytene nuclei

Four satellite DNAs in the Anopheles stephensi genome have been defined on the basis of their banding properties in Hoechst 33258-CsCl density gradients. Two of these satellites, satellites I and II, are visible on neutral CsCl density gradients as a light density peak forming approximately 15% of total cellular DNA. Hoechst-CsCl density gradient profiles of DNA extracted from polytene tissues indicates that these satellites are underreplicated in larval salivary gland cells and adult female Malpighian tubules and possibly also in ovarian nurse cells. The chromosomal location of satellite I on mitotic and polytene chromosomes has been determined by in situ hybridisation. Sequences complementary to satellite I are present in approximately equal amounts on a heterochromatic arm of the X and Y chromosomes and are also present, in smaller amounts, at the centromere of chromosome 3. A quantitative analysis of the in situ hybridisation experiments indicates that sequences complementary to satellite I at these two sites differ in their replicative behaviour during polytenisation: heterosomal satellite I sequences are under-replicated relative to chromosome 3 sequences in polytene larval salivary gland and ovarian nurse cell nuclei.

Mechanisms of gonadal differentiation

Sex differentiation is the result of the translation of genetic sex into gonadal sex. Without recognizable masculinizing signals the embryonic gonad will undergo ovarian differentiation. The main determinant of gonadal differentiation appears to be the presence or absence of a cell surface antigen, called H-Y antigen. The regulation of H-Y antigen expression is complex and involves the interaction between regulatory sites on the Y chromosome, the X chromosome, and possibly the autosomes.

Sequence organization of animal nuclear DNA

Animal nuclear genomes contain DNA sequences of various degrees of repetition. These sequences are organized in highly ordered fashions; repetitive and nonrepetitive sequences either alternate in short periods, i.e., short [0.2-0.4 kilobases (kb) long] repeats are flanked by nonrepetitive sequences less than 2 kb long, or in longer periods, with repetitive and/or nonrepetitive sequences extending for several kilobases. There are two main categories of genome organization, namely those exhibiting short-period interspersion and those that do not. There are arguments for and against a regulatory role of short interspersed repetitive sequences. Besides the merely 'statistical' kinetic approach by conventional reassociation kinetics, sequence organization has been studied by restriction endonuclease mapping and nucleotide sequencing. Such studies have revealed some general features of the organization of the eukaryotic gene and its transcripts, namely possible 'promoters', 'leaders', 'introns', 'exons', 'flanking sequences', 'caps', ribosome-binding sites, and poly(A) sequences. This paper discusses how these elements of a gene might serve regulatory roles in its expression.

Sex chromosome associated satellite DNA: evolution and conservation

Satellites visible in female but not in male DNA were isolated from the snakes Elaphe radiata (satellite IV, p = 1.708 g x cm-3) and Bungarus fasciatus (BK1 minor, p = 1.709 g x cm-3). The satellites cross hybridize. Hybridization of 3H labelled nick translated BK minor satellite DNA with the total male and female DNA and/or chromosomes in situ of different species of snakes revealed that its sequences are conserved throughout the snake group and are mainly concentrated on the W chromosomes. Snakes lacking sex chromosomes do possess related sequences but there is no sex difference and visible related satellites are absent. The following conclusions have been reached on the basis of these results. 1. The W chromosome associated satellite DNA is related to similar sequences scattered in the genome. 2. The origin and increment in the number of the W satellite DNA sequence on the W chromosome is assoicated with the heterochromatinization of the W. 3. Satellite sequences have become distributed along the length of the W and resulted in morphological differentiation of sex chromosomes. 4. Evolutionary conservation of W satellite DNA strongly suggests that functional constraints may have limited sequence divergence.

Cytological studies of heterochromatin function in the Drosophila melanogaster male: autosomal meiotic paring

In Drosophila melanogaster it is now documented that the different satellite DNA sequences make up the majority of the centromeric heterochromatin of all chromosomes. The most popular hypothesis on this class of DNA is that satellite DNA itself is important to the pairing processes of chromosomes. Evidence in support of such a hypothesis is, however, circumstantial. This hypothesis has been evaluated by direct cytological examination of the meiotic behaviour of heterochromatically and/or euchromatically rear-ranged autosomes in the male. It was found that neither substantial deletions nor rearrangements of the autosomal heterochromatin cause any disruption of meiotic pairing. Autosomal pairing depends on homologs retaining sufficient euchromatic homology. This is the first clear demonstration that the highly repeated satellite DNA sequences in the heterochromatin of the second, third and fourth chromosomes are not important in meiotic pairing, but rather than some euchromatic homology in the autosome is essential to ensure a regular meiotic process. These results on the autosomes, when taken in conjunction with our previous studies on sex chromosome pairing, clearly indicate that satellite DNA is not crucial for male meiotic chromosome pairing of any member of the D. melanogaster genome.

Behaviour of sex chromosome associated satellite DNAs in somatic and germ cells in snakes

Sex chromosome associated satellite DNAs is isolated from the snakes Elaphe radiata (sat III) (Singh et al., 1976) and Bungarus fasciatus (Elapidae) (minor satellite) are evolutionarily conserved throughout the suborder Ophidia. An autosome limited satellite DNA (B. fasciatus major satellite) is not similarly conserved. Both types of satellites have been studied by in situ hybridisation in various somatic tissues and germ cells where it has been observed that the W sex chromosome remains condensed in interphase nuclei. In growing oocytes however, the W chromosome satellite rich heterochromatin decondenses completely whilst the autosomal satellite rich regions remain condensed. Later, the cycle is reversed and the W chromosome condenses whilst the autosomal satellite regions decondense. In a primitive snake (Eryx johni johni) where the sex chromosomes are not differentiated and where there is no satellite DNA specific to them, these phenomena are absent. - The differential behaviour of autosomal and sex chromosome associated satellite DNAs is discussed in the light of gene regulation.