Nucleotide sequence and genomic organization of a tandem satellite array from the rock vole Microtus chrotorrhinus (Rodentia)

A tandem satellite array (herein named MSAT-160) has been isolated and characterized from the rodent Microtus chrotorrhinus. Sequence data from 15 partial or complete monomers revealed a repeat unit length of 160 bp. This unit length was apparently derived from two shorter sub-motifs, one a tetramer (GAAA), the other a hexamer (CTTTCT), through polymerase slippage and mutation. Collectively, perfect or imperfect variants of these two motifs comprise nearly 60% of the component. Southern blot analyses of genomic DNA digested with 14 different restriction endonucleases indicated that most enzymes yielded either classical type A or type B restriction patterns, while RsaI yielded a pattern that combined features of both the A and B types, and BamHI appeared to lack sites altogether in MSAT-160. An examination of restriction patterns from 16 individuals with three enzymes failed to identify intraspecific variation, while a related study compared 11 species and documented interspecific distinctiveness (Modi, submitted). Fluorescence in situ hybridization indicated that the satellite DNA was located at the centromeres of several autosomes and at sex chromosome heterochromatin (GenBank accession No. M86843).     

Heterogeneity in the concerted evolution process of a tandem satellite array in meadow mice (Microtus)

The evolutionary history of a 160-bp tandem satellite array, originally described from Microtus chrotorrhinus and called MSAT-160, was examined in related species of arvicolid rodents by sequence analyses, quantitative dot blotting, and Southern blotting. Results indicate that MSAT-160 is present in 12 of the 20 species and subspecies of Microtus assayed, but not in species belonging to any of the eight other genera examined. DNA from each species containing MSAT-160 was digested with 12 restriction endonucleases and restriction patterns were obtained reflecting the variable extent of homogenization of any given variant in different species. For example, with MboI digestion, M. chrotorrhinus produced a type A ladder pattern where most monomers contain the restriction site, M. ochrogaster generated a type B pattern where most monomers lack the site, and M. agrestis yielded a pattern intermediate between the A and B types. Further, dot blotting revealed copy-number differences between species. These findings indicate that changes in the periodic structure and amount of satellite DNA have occured since these species last shared a common ancestor. In addition, various species-pacific patterns were documented, illustrating that mechanism other than genomewide homogenization, such as stochastic mutation, out-of-register crossing over, deletion, and random amplification also play a role in structuring tandem arrays. Stochastic mutation and homogenization rates in satellite DNA, levels of species diversity, and magnitudes of chromosomal divergence differ significantly in Microtus, Mus, and Ctenomys, the three rodent lineages examined.     

Organization and chromosomal localization of a B1-like containing repeat of Microtus subarvalis

A repetitive DNA sequence, MS2, was isolated from EcoRI-digested genomic DNA of the vole, Microtus subarvalis. The fragment was cloned and sequenced. Sequence analysis of this 1194-bp fragment revealed a 156-bp region demonstrating a 55% homology with the mouse B1 repeat. The remaining MS2 sequence shows no significant homology with other known GenBank sequences. The results of in situ hybridization of MS2 on vole metaphase chromosomes indicate the fragment is confined to heterochromatin blocks of the sex chromosomes in all but one species (M. arvalis). Distribution of MS2 sequences provides evidence for heterogeneity of the giant heterochromatin blocks of the XY Chromosomes (Chrs) in voles, for the unique cluster-like localization of MS2 within these blocks. Received: 10 October 1995 / Accepted: 30 March 1996     

Giant sex chromosomes retained within the Portuguese lineage of the field vole (<Emphasis Type="Italic">Microtus agrestis</Emphasis>)

The field vole (Microtus agrestis) is characterised by extremely large blocks of heterochromatin on both the X and Y chromosome. Some other Microtus also have blocks of heterochromatin on their sex chromosomes but not as extensive and always of independent origin from the heterochromatic expansion found in M. agrestis. Coupled with evidence of geographic variation in large heterochromatic blocks within other species (e.g. in the western hedgehog Erinaceus europaeus), it might be expected that field voles would show substantial variation in size and disposition of the sex chromosome heterochromatin. In fact, only minor variation has been described up to now. Those studies conducted previously were largely on field voles from central and northern Europe. Here, we describe the karyotype of field voles from Portugal, of interest because recent molecular studies have shown field voles from western Iberia to be a separate evolutionary unit that might be considered a cryptic species, distinct from populations further to the east. The two Portuguese field voles (one female, one male) that we examined also had essentially the same karyotype as seen in other field voles, including the giant sex chromosomes, but with small differences in the structure of the Y chromosome from that described previously. The finding that field voles throughout Europe show relatively little variation in their giant sex chromosomes is consistent with molecular data which suggest a recent origin for this complex of species/near-species.     

Organization of the 378-bp Satellite Repeat in Terminal Heterochromatin of Allium fistulosum

Telomeres, DNA–protein structures, are important elements of the eukaryotic chromosome. Telomeric regions of the majority of higher plants contain heptanucleotides TTTAGGG arranged into a tandem repeat. However, some taxa have no such repeats. These are some species of Liliaceae and Alliaceae. For example, terminal regions of chromosomes of bunching onion (Allium fistulosum) contain satellite DNA whose unit repeats are 380 bp in length, and the short arm of its chromosome 8 contains rDNA repeats. This study deals with the terminal heterochromatin and organization of the satellite repeat in A. fistulosum. Fluorescent in situ hybridization (FISH) was used to locate the satellite DNA on chromosomes and on extended DNA of A. fistulosum.Nonsatellite DNA was found in the structure of telomeric repeat. Polymerase chain reaction (PCR) and Southern hybridization were used for analysis of terminal heterochromatin. Various rearrangements were found in the satellite repeat. The roles of retrotransposons and microsatellites in the formation of terminal heterochromatin are discussed.     

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.     

A Novel MS7 Repeat Specific for Heterochromatin of Sex Chromosomes in Voles of Genus Microtus, Group arvalis: Genomic Organization and Chromosomal Localization

The tandemly arranged MS4 repeat with monomeric units of 4.1 kb is species-specifically distributed in heterochromatin of sex chromosomes of four common vole species of genus Microtus, group arvalis. In this work, we studied the genomic organization of the MS4 homolog in euchromatin of the X chromosome of M. arvalis. It has been shown by analyzing the phage genomic clones that one MS4 copy makes a part of a monomeric unit exceeding 8.5 kb that also includes a new MS7 repeat and, possibly, LINE fragments. MS7 is located together with MS4 in heterochromatin of common vole sex chromosomes, but in a substantially lesser amount. Probably, as a result of an evolutionary transition of an original repeat from euchromatin of the X chromosome to heterochromatin of the Y chromosome, MS4 underwent multiple amplification, and MS7 spread throughout heterochromatin, being surrounded by the MS4 tandem arrays.     

Evolution of ZZ/ZW and XX/XY sex-determination systems in the closely related medaka species, <Emphasis Type="Italic">Oryzias hubbsi</Emphasis> and <Emphasis Type="Italic">O. dancena</Emphasis>

A DM-domain gene on the Y chromosome was identified as the sex-determining gene in the medaka, Oryzias latipes, and named DMY (also known as dmrt1bY). However, this gene is absent in most Oryzias fishes, suggesting that closely related species have another sex-determining gene. In fact, it has been demonstrated that the Y chromosome in O. dancena is not homologous to that in O. latipes, whereas both species have an XX/XY sex-determination system. Through a progeny test of sex-reversed fish and a linkage analysis of isolated sex-linked DNA markers, we show that O. hubbsi, which is one of the most closely related species to O. dancena, has a ZZ/ZW system. In addition, genetic and fluorescence in situ hybridization mapping of the sex-linked markers revealed that sex chromosomes in O. hubbsi and O. dancena are not homologous, indicating different origins of these ZW and XY sex chromosomes. Furthermore, we found that O. hubbsi has morphologically heteromorphic sex chromosomes, in which the W chromosome has 4,6-diamidino-2-phenylindole (DAPI)-positive heterochromatin blocks and is larger than the Z chromosome, although such differentiated sex chromosomes have not been observed in other Oryzias species. These findings suggest that a variety of sex-determining mechanisms and sex chromosomes have evolved in Oryzias.     

An accumulation of tandem DNA repeats on the Y chromosome in Silene latifolia during early stages of sex chromosome evolution

Sex chromosomes in mammals are about 300 million years old and typically have a highly degenerated Y chromosome. The sex chromosomes in the dioecious plant Silene latifolia in contrast, represent an early stage of evolution in which functional X–Y gene pairs are still frequent. In this study, we characterize a novel tandem repeat called TRAYC, which has accumulated on the Y chromosome in S. latifolia. Its presence demonstrates that processes of satellite accumulation are at work even in this early stage of sex chromosome evolution. The presence of TRAYC in other species of the Elisanthe section suggests that this repeat had spread after the sex chromosomes evolved but before speciation within this section. TRAYC possesses a palindromic character and a strong potential to form secondary structures, which could play a role in satellite evolution. TRAYC accumulation is most prominent near the centromere of the Y chromosome. We propose a role for the centromere as a starting point for the cessation of recombination between the X and Y chromosomes.     

Origin and Differentiation of a Sex Chromosome System in Parodon hilarii (Pisces, Parodontidae). Satellite DNA, G- and C-banding

A satellite DNA sequence of Parodon hilarii (named pPh2004) was isolated, cloned and sequenced. This satellite DNA is composed of 200 bp, 60% AT rich. In situ hybridization (FISH) results revealed that the satellite DNA pPh2004 is located in the terminal regions of several chromosomes, forming highly evident blocks in some and punctual marks in others. The comparison between the FISH and C-banding results showed that the location of this satellite DNA coincides with that of most terminal heterochromatins. However, some regions are only marked by FISH whereas other regions are only marked by C-banding. The possible existence of more than one satellite DNA family could explain these partial differences. The in situ hybridization with the satellite DNA and the G- and C-bandings confirmed the presence of a sex chromosome system of the ZZ/ZW type in P. hilarii, as well as the correct identification of the Z chromosome in the karyotype. This chromosome displays a segment of terminal heterochromatin in the long arm, similar to the segment observed in the short arm of the W chromosome, also showing a G-banding pattern similar to that of the short arm and part of the long arm of the W chromosome. A hypothesis on the origin of the W chromosome from an ancestral chromosome similar to the Z chromosome is presented.     

Molecular and cytogenetic characterization of an AT-rich satellite DNA family in <Emphasis Type="Italic">Urvillea chacoensis</Emphasis> Hunz. (Paullinieae,Sapindaceae)

Urvillea chacoensis is a climber with 2n = 22 and some terminal AT-rich heterochromatin blocks that differentiate it from other species of the genus. The AT-rich highly repeated satellite DNA was isolated from U. chacoensis by the digestion of total nuclear DNA with HindIII and XbaI and cloned in Escherichia coli. Satellite DNA structure and chromosomal distribution were investigated. DNA sequencing revealed that the repeat length of satDNA ranges between 721 and 728 bp, the percentage of AT-base pairs was about 72–73% and the studied clones showed an identity of 92.5–95.9%. Although this monomer has a tetranucleosomal size, direct imperfect repetitions of ~180 bp subdividing it in four nucleosomal subregions were observed. The results obtained with FISH indicate that this monomer usually appears distributed in the terminal regions of most chromosomes and is associated to heterochromatin blocks observed after DAPI staining. These observations are discussed in relation to the satellite DNA evolution and compared with other features observed in several plant groups.     

Satellite DNA and chromosomes in Neotropical fishes: methods,applications and perspectives

Constitutive heterochromatin represents a substantial portion of the eukaryote genome, and it is mainly composed of tandemly repeated DNA sequences, such as satellite DNAs, which are also enriched by other dispersed repeated elements, including transposons. Studies on the organization, structure, composition and in situ localization of satellite DNAs have led to consistent advances in the understanding of the genome evolution of species, with a particular focus on heterochromatic domains, the diversification of heteromorphic sex chromosomes and the origin and maintenance of B chromosomes. Satellite DNAs can be chromosome specific or species specific, or they can characterize different species from a genus, family or even representatives of a given order. In some cases, the presence of these repeated elements in members of a single clade has enabled inferences of a phylogenetic nature. Genomic DNA restriction, using specific enzymes, is the most frequently used method for isolating satellite DNAs. Recent methods such as C₀t–1 DNA and chromosome microdissection, however, have proven to be efficient alternatives for the study of this class of DNA. Neotropical ichthyofauna is extremely rich and diverse enabling multiple approaches with regard to the differentiation and evolution of the genome. Genome components of some species and genera have been isolated, mapped and correlated with possible functions and structures of the chromosomes. The 5SHindIII‐DNA satellite DNA, which is specific to Hoplias malabaricus of the Erythrinidae family, has an exclusively centromeric location. The As51 satellite DNA, which is closely correlated with the genome diversification of some species from the genus Astyanax, has also been used to infer relationships between species. In the Prochilodontidae family, two repetitive DNA sequences were mapped on the chromosomes, and the SATH 1 satellite DNA is associated with the origin of heterochromatic B chromosomes in Prochilodus lineatus. Among species of the genus Characidium and the Parodontidae family, amplifications of satellite DNAs have demonstrated that these sequences are related to the differentiation of heteromorphic sex chromosomes. The possible elimination of satellite DNA units could explain the genome compaction that occurs among some species of Neotropical Tetraodontiformes. These topics are discussed in the present review, showing the importance of satellite DNA analysis in the differentiation and karyotype evolution of Actinopterygii.     

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

Sex chromosome associated satellite DNAs 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.     

Simple GA C T A repeats characterize the X chromosomal heterochromatin of Microtus agrestis,European field vole (Rodentia,Cricetidae)

The sex chromosomes of Microtus agrestis are extremely large due to the accumulation of constitutive heterochromatin. We have identified two prominent satellite bands of 2.0 and 2.8 kb in length after HaeIII and HinfI restriction enzyme digestion of genomic DNA, respectively. These satellites are located on the heterochromatic long arm of the X chromosome as shown using Microtus x mouse somatic cell hybrids. By in-gel hybridization with oligonucleotide probes, the organization of the two satellites was studied: among the many copies of the simple tandem tetranucleotide repeat GATA are interspersed rare single GACA tetramers. One of the satellites also harbours related GGAT simple tandem repeats. In situ hybridizations with plasmid-carried or oligonucleotide GA _C ^T A probes show clustered silver grains on the long and short arm of the X chromosome. Interspersion of differently organized (GATA)_n elements is also demonstrable in the autosomal complement and on the Y chromosome. These results are discussed in the context of the evolution of vertebrate sex chromosomes in relation to heterochromatin and simple repetitive DNA sequences.     

Evolution of diploid chromosome number,sex‐determining systems,and heterochromatin in Western Mediterranean and Canarian species of the genus Pimelia (Coleoptera: Tenebrionidae)

A compilation of the diploid chromosome numbers and karyotype formulae of 30 species of the genus Pimelia from Morocco, Iberian Peninsula, Balearic and Canary Islands is presented. All species show a conservation of diploid numbers and karyotype formulae 2n = 18 (8 + Xy_p) except for Pimelia cribra, Pimelia elevata, and Pimelia interjecta 2n = 20 (9 + Xy_p) and Pimelia sparsa sparsa 2n = 18 (8 + neoXY). The ancestral state for the genus Pimelia is suggested to be 2n = 18 (8 + Xy_p) in accordance with a previously described phylogeny of these species based on mitochondrial and nuclear DNA. The derived state 2n = 20 (9 + Xy_p) is present in a monophyletic clade, which originated about 2.5–5 Mya. The male meiotic formula 8 + neoXY found in P. sparsa sparsa seems to have originated by the reorganization of the Xy_p pair resulting in two homomorphic sexual chromosomes and the lost of most of the heterochromatin from the former X chromosome. In all chromosomes C‐banding revealed conspicuous pericentromeric heterochromatic blocks, except in the Y chromosome in most of the species, and in situ hybridization of satellite DNA probes revealed the correspondence between heterochromatin and satellite DNA. Finally, the possible role of heterochromatin and satellite DNA is discussed in relation to the uniformity of the Tenebrionidae α‐karyology.     

Heterochromatin (C-bands) in somatic and male germ cells in three species ofMicrotinae

The C-banding patterns in the chromosomes ofMicrotus oeconomus, M. arvalis andM. ochrogaster demonstrate differences in the amount and distribution of heterochromatin. Autosomal centromeric heterochromatin appears as conspicuous blocks or as small dots, and in several chromosomes no heterochromatin was detected; interstitial heterochromatin was observed in one autosome pair ofM. ochrogaster. The sex chromosomes also demonstrate differences in the C-banding pattern. InM. oeconomus, the X chromosome exhibits a block of centromeric heterochromatin which is larger than that of the autosomes; this characteristic helps to recognize the X chromosomes in the karyotype. InM. arvalis no heterochromatin was appreciated in the sex chromosomes. The Y chromosomes ofM. ochrogaster andM. oeconomus are entirely heterochromatic. During male meiosis heterochromatin shows condensation, association and chiasma prevention; the sex chromosomes pair end to end in the three species. At pairing, the Y chromosome ofM. arvalis is despiralized, but it appears condensed again shortly before separation of the bivalent.     

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.     

Molecular cytogenetic characterization of <Emphasis Type="Italic">Rumex papillaris</Emphasis>, a dioecious plant with an XX/XY<Subscript>1</Subscript>Y<Subscript>2</Subscript> sex chromosome system

Rumex papillaris Boiss, & Reut., an Iberian endemic, belongs to the section Acetosa of the genus Rumex whose main representative is R. acetosa L., a species intensively studied in relation to sex-chromosome evolution. Here, we characterize cytogenetically the chromosomal complement of R. papillaris in an effort to enhance future comparative genomic approaches and to better our understanding of sex chromosome structure in plants. Rumex papillaris, as is common in this group, is a dioecious species characterized by the presence of a multiple sex chromosome system (with females 2n = 12 + XX and males 2n = 12 + XY₁Y₂). Except for the X chromosome both Y chromosomes are the longest in the karyotype and appear heterochromatic due to the accumulation of at least two satellite DNA families, RAE180 and RAYSI. Each chromosome of pair VI has an additional major heterochromatin block at the distal region of the short arm. These supernumerary heterochromatic blocks are occupied by RAE730 satellite DNA family. The Y-related RAE180 family is also present in an additional minor autosomal locus. Our comparative study of the chromosomal organization of the different satellite-DNA sequences in XX/XY and XX/XY₁Y₂ Rumex species demonstrates that of active mechanisms of heterochromatin amplification occurred and were accompanied by chromosomal rearrangements giving rise to the multiple XX/XY₁Y₂ chromosome systems observed in Rumex. Additionally, Y₁ and Y₂ chromosomes have undergone further rearrangements leading to differential patterns of Y-heterochromatin distribution between Rumex species with multiple sex chromosome systems.     

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

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 rearranged 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 that some euchromatic homology in the autosomes 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.     

Genetic studies on heterochromatin in Drosophila melanogaster and their implications for the functions of satellite DNA

In Drosophila melanogaster the centromeric heterochromatin of all chromosomes consists almost entirely of several different satellite DNA sequences. In view of this we have examined by genetic means the meiotic consequences of X chromosomes with partial deletions of their heterochromatin, and have found that the amount and position of recombination on each heterochromatically deleted X is substantially different from that of a normal X. It appears that the amount of heterochromatin is important in modifying the centromere effect on recombination. — In all the deleted Xs tested, chromosome segregation is not appreciably altered from that of a nondeleted control chromosome. Thus satellite DNA does not appear to be an important factor in determining the regular segregation of sex chromosomes in Drosophila. Additionally, since X chromosomes with massive satellite DNA deficiencies are able to participate in a chromocenter within salivary gland nuclei, a major role of satellite DNA in chromocenter formation in this tissue is also quite unlikely. — In order to examine the mechanisms by which the amount of satellite DNA is increased or decreased in vivo, we have measured cytologically the frequency of spontaneous sister chromatid exchanges in a ring Y chromosome which is entirely heterochromatic and consists almost exclusively of satellite DNA. In larval neuroblast cells the frequency of spontaneous SCE in this Y is approximately 0.3% per cell division. Since there is no meiotic recombination in D. melanogaster males and since meiotic recombination in the female does not occur in heterochromatin, our results provide a minimum estimate of the in vivo frequency of SCE in C-banded heterochromatin (which is predominantly simple sequence DNA), without the usual complications of substituted base analogs, incorporated radioactive label or substantial genetic content. — We emphasise that: (a) satellite DNA is not implicated in any major way in recognition processes such as meiotic homologue recognition or chromocenter formation in salivaries, (b) there is likely to be continuous variation in the amount of satellite DNA between individuals of a species; and (c) the amount of satellite DNA can have a crucial functional role in the meiotic recombination system.