Centromeric satellite DNA in the newt Triturus cristatus karelinii and related species: its distribution and transcription on lampbrush chromosomes

Genome editing with CRISPR-Cas nucleases, base editors, transposases and prime editors

The development of new CRISPR-Cas genome editing tools continues to drive major advances in the life sciences. Four classes of CRISPR-Cas-derived genome editing agents-nucleases, base editors, transposases/recombinases and prime editors-are currently available for modifying genomes in experimental systems. Some of these agents have also moved rapidly into the clinic. Each tool comes with its own capabilities and limitations, and major efforts have broadened their editing capabilities, expanded their targeting scope and improved editing specificity. We analyze key considerations when choosing genome editing agents and identify opportunities for future improvements and applications in basic research and therapeutics.

H3K4 methylation at active genes mitigates transcription-replication conflicts during replication stress

Transcription-replication conflicts (TRCs) occur when intensive transcriptional activity compromises replication fork stability, potentially leading to gene mutations. Transcription-deposited H3K4 methylation (H3K4me) is associated with regions that are susceptible to TRCs; however, the interplay between H3K4me and TRCs is unknown. Here we show that H3K4me aggravates TRC-induced replication failure in checkpoint-defective cells, and the presence of methylated H3K4 slows down ongoing replication. Both S-phase checkpoint activity and H3K4me are crucial for faithful DNA synthesis under replication stress, especially in highly transcribed regions where the presence of H3K4me is highest and TRCs most often occur. H3K4me mitigates TRCs by decelerating ongoing replication, analogous to how speed bumps slow down cars. These findings establish the concept that H3K4me defines the transcriptional status of a genomic region and defends the genome from TRC-mediated replication stress and instability.

Transcription-replication conflicts (TRC) can contribute to genome instability. Here the authors reveal that under replication stress H3K4 methylation can play a role in TRC prevention.

Synergistic lethality between PARP-trapping and alantolactone-induced oxidative DNA damage in homologous recombination-proficient cancer cells

PARP1 and PARP2 play critical roles in regulating DNA repair and PARP inhibitors have been approved for the treatment of BRCA1/2-mutated ovarian and breast cancers. It has long been known that PARP inhibition sensitizes cancer cells to DNA-damaging cytotoxic agents independent of BRCA status, however, clinical use of PARP inhibitors in combination with DNA-damaging chemotherapy is limited by the more-than-additive cytotoxicity. The natural compound alantolactone (ATL) inhibits the thioredoxin reductase to induce ROS accumulation and oxidative DNA damage selectively in cancer cells. Here, we showed that nontoxic doses of ATL markedly synergized with the PARP inhibitor olaparib to result in synthetic lethality irrespective of homologous recombination status. Synergistic cytotoxicity was seen in cancer but not noncancerous cells and was reduced by the ROS inhibitor NAC or knockdown of OGG1, demonstrating that the cytotoxicity resulted from the repair of ATL-induced oxidative DNA damage. PARP1 knockdown suppressed the synergistic lethality and olaparib was much more toxic than veliparib when combined with ATL, suggesting PARP-trapping as the primary inducer of cytotoxicity. Consistently, combined use of ATL and olaparib caused intense signs of replication stress and formation of double strand DNA breaks, leading to S and G₂ arrest followed by apoptosis. In vivo, the combination effectively induced regression of tumor xenografts, while either agent alone had no effect. Hence, PARP trapping combined with specific pro-oxidative agents may provide safe and effective ways to broaden the therapeutic potential of PARP inhibitors.

Transcription of highly repetitive tandemly organized DNA in amphibians and birds: A historical overview and modern concepts

Tandemly organized highly repetitive DNA sequences are crucial structural and functional elements of eukaryotic genomes. Despite extensive evidence, satellite DNA remains an enigmatic part of the eukaryotic genome, with biological role and significance of tandem repeat transcripts remaining rather obscure. Data on tandem repeats transcription in amphibian and avian model organisms is fragmentary despite their genomes being thoroughly characterized. Review systematically covers historical and modern data on transcription of amphibian and avian satellite DNA in somatic cells and during meiosis when chromosomes acquire special lampbrush form. We highlight how transcription of tandemly repetitive DNA sequences is organized in interphase nucleus and on lampbrush chromosomes. We offer LTR-activation hypotheses of widespread satellite DNA transcription initiation during oogenesis. Recent explanations are provided for the significance of high-yield production of non-coding RNA derived from tandemly organized highly repetitive DNA. In many cases the data on the transcription of satellite DNA can be extrapolated from lampbrush chromosomes to interphase chromosomes. Lampbrush chromosomes with applied novel technical approaches such as superresolution imaging, chromosome microdissection followed by high-throughput sequencing, dynamic observation in life-like conditions provide amazing opportunities for investigation mechanisms of the satellite DNA transcription.

Non-coding RNA derived from a conservative subtelomeric tandem repeat in chicken and Japanese quail somatic cells

BACKGROUND

Subtelomeres are located close to the ends of chromosomes and organized by tandemly repetitive sequences, duplicated copies of genes, pseudogenes and retrotransposons. Transcriptional activity of tandemly organized DNA at terminal chromosomal regions and the distribution of subtelomere-derived non-coding RNAs are poorly investigated. Here we aimed to analyze transcriptional activity of subtelomeric tandem repeat in somatic tissues and cultured cells of birds. We focused on tissue-specific differences of subtelomeric repeats transcription, structure of the resulting transcripts and the behavior of subtelomere-derived RNA during mitosis.

RESULTS

Transcriptional activity of short subtelomeric PO41 ("pattern of 41 bp") tandem repeat in the somatic and cultured cells of chicken (Gallus gallus domesticus) and Japanese quail (Coturnix coturnix japonica) was examined using RNA fluorescence in situ hybridization approach. We discovered transcripts from both strands of the PO41 repeat in chicken MDCC-MSB1 cells as well as in chicken and Japanese quail somatic tissues, such as tissues of cerebellum, telencephalon, muscles, oviduct, small and large intestine. Normal somatic and transformed cells demonstrate similar distribution of PO41 repeat transcripts in interphase nuclei. We revealed one or two major foci of PO41 repeat transcripts associated with RNA polymerase II, representing nascent RNA, and dispersed PO41 repeat transcripts localized in euchromatin or interchromatin space, representing released RNA. During mitosis PO41 non-coding RNA distribute between condensed chromosomes till anaphase, when they concentrate at the cleavage plane. At telophase, clusters of PO41 RNA surround terminal regions of chromosomes. Treatments with RNases of different substrate specificity indicate that PO41 repeat transcripts are single-stranded RNAs with short double-stranded regions due to appearance of inverted repeats.

CONCLUSION

Transcription of a subtelomeric tandem repeat in avian somatic cells is reported here for the first time. PO41 repeat transcription is conserved among Galliformes and has similar pattern in somatic tissues. We demonstrated redistribution of non-coding PO41 RNA occurring during the cell cycle. Potential regulatory role of the PO41 repeat transcripts in RNA-dependent process of subtelomere heterochromatin maintenance is discussed.

Chromomeres revisited

The history of studies on the chromomeres of lampbrush chromosomes is outlined and evidence for the nature and function of these structures is collected and summarised. Chromomeres and their associated loops on lampbrush chromosomes are not genetic units although in some special cases, they consist of specific families of repeated DNA sequences. The emergence of a chromomeric organisation coincides with the onset and intensification of transcription on lampbrush loops. Modern molecular studies have provided evidence that the chromatin of lampbrush chromomeres differs in several important respects from that of condensed metaphase chromosomes. It is in a highly dynamic state that facilitates localised transcription whilst keeping the chromosome safe from structural changes that might impede its orderly progression up to and through meiotic metaphase 1. Lampbrush chromosomes (LBCs) are a physically induced phenomenon, facilitated by the selective absence of molecular factors that would interfere with their main transcriptional role. LBC morphology is highly dynamic and driven by transcriptive activity.

High-resolution mapping and transcriptional activity analysis of chicken centromere sequences on giant lampbrush chromosomes

Exploration into morphofunctional organisation of centromere DNA sequences is important for understanding the mechanisms of kinetochore specification and assembly. In-depth epigenetic analysis of DNA fragments associated with centromeric nucleosome proteins has demonstrated unique features of centromere organisation in chicken karyotype: there are both mature centromeres, which comprise chromosome-specific homogeneous arrays of tandem repeats, and recently evolved primitive centromeres, which consist of non-tandemly organised DNA sequences. In this work, we describe the arrangement and transcriptional activity of chicken centromere repeats for Cen1, Cen2, Cen3, Cen4, Cen7, Cen8, and Cen11 and non-repetitive centromere sequences of chromosomes 5, 27, and Z using highly elongated lampbrush chromosomes, which are characteristic of the diplotene stage of oogenesis. The degree of chromatin packaging and fine spatial organisations of tandemly repetitive and non-tandemly repetitive centromeric sequences significantly differ at the lampbrush stage. Using DNA/RNA FISH, we have demonstrated that during the lampbrush stage, DNA sequences are transcribed within the centromere regions of chromosomes that lack centromere-specific tandem repeats. In contrast, chromosome-specific centromeric repeats Cen1, Cen2, Cen3, Cen4, Cen7, Cen8, and Cen11 do not demonstrate any transcriptional activity during the lampbrush stage. In addition, we found that CNM repeat cluster localises adjacent to non-repetitive centromeric sequences in chicken microchromosome 27 indicating that centromere region in this chromosome is repeat-rich. Cross-species FISH allowed localisation of the sequences homologous to centromeric DNA of chicken chromosomes 5 and 27 in centromere regions of quail orthologous chromosomes.

Cytological maps of lampbrush chromosomes of European water frogs (Pelophylax esculentus complex) from the Eastern Ukraine

Background

Hybridogenesis (hemiclonal inheritance) is a kind of clonal reproduction in which hybrids between parental species are reproduced by crossing with one of the parental species. European water frogs (Pelophylax esculentus complex) represent an appropriate model for studying interspecies hybridization, processes of hemiclonal inheritance and polyploidization. P. esculentus complex consists of two parental species, P. ridibundus (the lake frog) and P. lessonae (the pool frog), and their hybridogenetic hybrid – P. esculentus (the edible frog). Parental and hybrid frogs can reproduce syntopically and form hemiclonal population systems. For studying mechanisms underlying the maintenance of water frog population systems it is required to characterize the karyotypes transmitted in gametes of parental and different hybrid animals of both sexes.

Results

In order to obtain an instrument for characterization of oocyte karyotypes in hybrid female frogs, we constructed cytological maps of lampbrush chromosomes from oocytes of both parental species originating in Eastern Ukraine. We further identified certain molecular components of chromosomal marker structures and mapped coilin-rich spheres and granules, chromosome associated nucleoli and special loops accumulating splicing factors. We recorded the dissimilarities between P. ridibundus and P. lessonae lampbrush chromosomes in the length of orthologous chromosomes, number and location of marker structures and interstitial (TTAGGG)_n-repeat sites as well as activity of nucleolus organizer. Satellite repeat RrS1 was mapped in centromere regions of lampbrush chromosomes of the both species. Additionally, we discovered transcripts of RrS1 repeat in oocytes of P. ridibundus and P. lessonae. Moreover, G-rich transcripts of telomere repeat were revealed in association with terminal regions of P. ridibundus and P. lessonae lampbrush chromosomes.

Conclusions

The constructed cytological maps of lampbrush chromosomes of P. ridibundus and P. lessonae provide basis to define the type of genome transmitted within individual oocytes of P. esculentus females with different ploidy and from various population systems.

Proteins involved in meiotic recombination: a role in male infertility?

Meiotic recombination results in the formation of crossovers, by which genetic information is exchanged between homologous chromosomes during prophase I of meiosis. Recombination is a complex process involving many proteins. Alterations in the genes involved in recombination may result in infertility. Molecular studies have improved our understanding of the roles and mechanisms of the proteins and protein complexes involved in recombination, some of which have function in mitotic cells as well as meiotic cells. Human gene sequencing studies have been performed for some of these genes and have provided further information on the phenotypes observed in some infertile individuals. However, further studies are needed to help elucidate the particular role(s) of a given protein and to increase our understanding of these protein systems. This review will focus on our current understanding of proteins involved in meiotic recombination from a genomic perspective, summarizing our current understanding of known mutations and single nucleotide polymorphisms that may affect male fertility by altering meiotic recombination.

Tandem 41-bp repeats in chicken and Japanese quail genomes: FISH mapping and transcription analysis on lampbrush chromosomes

The chromosomal distribution of 41-bp repeats, known as CNM and PO41 repeats in the chicken genome and BglII repeats in the Japanese quail, was analyzed precisely using giant lampbrush chromosomes (LBC) from chicken, Japanese quail, and turkey growing oocytes. The PO41 repeat is conserved in all galliform species, whereas the other repeats are species specific. In chicken and quail, the centromere and subtelomere regions share homologous satellite sequences. RNA polymerase II transcribes the 41-bp repeats in both centromere and subtelomere regions. Ongoing transcription of these repeats was demonstrated by incorporation of BrUTP injected into oocytes at the lampbrush stage. RNA complementary to both strands of CNM and PO41 repeats is present on chicken LBC loops, whereas strand-specific G-rich transcripts are characteristic of BglII repeats in the Japanese quail. The RNA from 41-bp repeats does not undergo cotranscriptional U snRNP-dependent splicing. At the same time, the ribonucleoprotein matrix of transcription units with C-rich RNA of CNM and PO41 repeats was enriched with hnRNP protein K. Potential promoters for satellite transcription are discussed.

Transcription and Evolutionary Dynamics of the Centromeric Satellite Repeat CentO in Rice

Satellite DNA is a major component of centromeric heterochromatin in most multicellular eukaryotes, where it is typically organized into megabase-sized tandem arrays. It has recently been demonstrated that small interfering RNAs (siRNAs) processed from centromeric satellite repeats can be involved in epigenetic chromatin modifications which appear to underpin centromere function. However, the structural organization and evolution of the centromeric satellite DNA is still poorly understood. We analyzed the centromeric satellite repeat arrays from rice chromosomes 1 and 8 and identified higher order structures and local homogenization of the CentO repeats in these 2 centromeres. We also cloned the CentO repeats from the CENH3-associated nucleosomes by a chromatin immunoprecipitation (ChIP)-based method. Sequence variability analysis of the ChIPed CentO repeats revealed a single variable domain within the repeat. We detected transcripts derived from both strands of the CentO repeats. The CentO transcripts are processed into siRNA, suggesting a potential role of this satellite repeat family in epigenetic chromatin modification.

Variability of DNA methylation pattern in somatic and germ cells in male newt (Amphibia, Urodela) Triturus cristatus carnifex

In a survey of several mammalian genomes, namely humans, rodents and bovines, the differences in the 5-methylcytosine (m5C) content show that repeated DNA sequences from sperm were undermethylated and from various somatic tissues were heavily methylated. This report shows a pattern of methylation in male newt (Amphibia, Urodela) Triturus cristatus carnifex (T. c. c.) unlike that so far described by other authors in mammals. Using methylation sensitive and insensitive enzymes (HpaII and MspI) and successive 3' terminal labelling (fill-in), we found a greater degree of DNA methylation in premeiotic germ and sperm cells compared to somatic tissue such as hepatocytes. Furthermore the degree of total DNA methylation in spermatozoa appears somewhere between premeiotic germ cells and somatic tissue. Blot hybridization shows that two highly conserved repetitive sequences in amphibian T. c. c., pTvm1 and pTvm8, contribute significantly to the degree of DNA methylation, suggesting a function for these sequences, such as a role in transcriptional regulation.

A family of centromeric satellite DNAs from the European brown frog Rana graeca italica

Digestion of Rana graeca italica DNA with Asp 718I produces highly repetitive fragments of 281 and 385 bp that were cloned and sequenced. The shorter fragment corresponds to the unit repeat (RgiS1b) of a satellite DNA. The longer fragment was found to be part of a 494-bp repeat of another satellite DNA (RgiS1a) that was cloned intact as an EcoRV fragment. RgiS1b is 97% homologous to RgiS1a, from which it seems to be derived by a single deletion. Among all species tested, only the related brown frog Rana dalmatina contained homologous repetitive DNA. The overall number of RgiS1a and RgiS1b repeats per R. graeca italica haploid genome was estimated to be 2.7 x 10(5). RgiS1a and RgiS1b repeats are organized in separate arrays, but repetitive units formed by various combinations of the two repeats were also observed on Southern blots. The amount of these extra repeats varies greatly among animals from the same population, representing a rare case of individual variability in the satellite DNA organization. FISH with probes specific for both satellites, or for RgiS1a only, labeled the centromeric and pericentromeric heterochromatin of all chromosomes. This indicated that RgiS1a and RgiS1b are interspersed within the same heterochromatic regions of the chromosomes.

Transcription of lampbrush chromosomes of a centromerically localized highly repeated DNA in pigeon (Columba) relates to sequence arrangement

A highly repetitive, centromerically localized DNA sequence (PR1) has been isolated from the genomic DNA of two species of pigeon (Columba livia and C. palumbus). PR1 is approximately 900 bp long. It includes a sequence that is similar to the CENP-B box of mammals. It represents about 5% of the genome in C. livia and 2% in C. palumbus. In both species, tandem arrays of PR1 form part of larger repeating units. The organization of PR1 repeats and the larger repeating units is strikingly different in the two species. The large repeating units in C. livia include long (at least 14 units) tandem arrays of PR1 interspersed with relatively short intervening sequences. The large repeats of C. palumbus have much shorter (4 units or fewer) PR1 arrays interspersed with longer sections of non-PR1 DNA. PR1 is transcribed on short lampbrush loops in the centromeric regions of all lampbrush bivalents of C. palumbus. In C. livia, it is not transcribed at any of the major pericentromeric sites at which it is known to be present, although it is transcribed at one minor centromeric site on chromosome 2. It is proposed that transcription of the noncoding PR1 sequence on lampbrush chromosomes of pigeons relates to its genomic organization. The proposal is discussed with regard to the 'read-through' hypothesis for transcription on lampbrush loops.

Molecular characterization of a centromeric satellite DNA in the hemiclonal hybrid frog Rana esculenta and its parental species

Hybrid water frogs Rana esculenta reproduce by hybridogenesis: one parental genome (of Rana lessonae) is excluded in the germ line, the other (of Rana ridibunda) is clonally transmitted to haploid gametes. The two parental species differ in that the amount of centromeric heterochromatin revealed by differential staining is much higher in Rana ridibunda. An abundant, tandemly arrayed, centromeric satellite DNA, designated RrS1, is revealed in Rana ridibunda genomes by the restriction endonuclease Stul, which generates a major repetitive sequence fragment of 300 and a minor one of 200 bp. This AT-rich (68%) satellite family is located at the centromeres of the five largest chromosomes (1-5) and of a medium to small heterobrachial one (8 or 9); it thus constitutes only part of the centromeric heterochromatin that characterizes all Rana ridibunda chromosomes. RrS1 represents about 2.5% of the genome of Rana ridibunda; it may represent as little as 0.2% of the genome of Rana lessonae, and cannot be detected in Xenopus laevis frogs or Salamandra salamandra and Triturus carnifex salamanders. Segments of the satellite sequence are similar to sequences of yeast centromeric DNA element CDEIII and of the mammalian CENP-B box. A role for RrS1 and other centromeric satellite DNAs in the germ line genome exclusion of the hybridogenetic frog hybrids, although suggested, has not yet been demonstrated.

Cytoplasmic localization of transcripts of a complex G+C-rich crab satellite DNA

The primary sequence and higher order structures of a G+C-rich satellite DNA of the Bermuda land crab Gecarcinus lateralis have been described previously. The repeat unit of the satellite is approximately 2.1 kb. In exploring a possible function for this satellite, we asked whether it is transcribed. As a probe for transcripts, we used a segment of DNA amplified from a 368 bp EcoRI fragment from the very highly conserved 3' end of the satellite DNA. During polymerase chain reaction (PCR) amplification, the probe was simultaneously either radiolabeled or biotinylated. Tissue- and stage-specific transcripts were observed when blots of poly(A)+ mRNAs recovered from polysomes isolated from crab tissues [including midgut gland (hepatopancreas), limb bud, and claw muscle] were probed with the satellite DNA fragment. The presence of satellite transcripts in polysomal mRNAs is strong evidence that the transcripts had reached the cytoplasm. To corroborate the presence of transcripts in the cytoplasm, we investigated in situ hybridization of satellite probes with RNAs in tissue sections. Biotinylated satellite DNA probes were applied to sections of midgut gland, limb bud papilla, ovary, or testis of anecdysial crabs. Retention of RNAs in tissue sections was improved by UV-irradiation prior to hybridization. Transcripts were abundant in the cytoplasm of all tissues except testis. Sections of crab midgut gland treated with RNase A prior to hybridization and sections of mouse pancreatic tumor served as controls; neither showed any signals with the probe.

Two dispersed highly repeated DNA families of Triturus vulgaris meridionalis (Amphibia, Urodela) are widely conserved among Salamandridae

Two BamHI families of repeated sequences were characterized from the genome of the Italian smooth newt, Triturus vulgaris meridionalis (Amphibia, Urodela). The first family, which is divided into subfamilies, consists of tandemly arranged arrays whose basic repeat is around 398 bp long; these arrays are dispersed throughout the entire chromosome sets of the various species of Triturus tested. Moreover the family is widely conserved among Salamandridae, being detected by genomic DNA blotting of Notophthalmus viridescens, Taricha granulosa, Salamandrina terdigitata and Euproctus platycephalus. The second BamHI family is represented by a cloned sequence of 419 bp, which is dispersed in the chromosome set of several species of Triturus. The sequence is also conserved in S. terdigitata and in E. platycephalus but is not detectable in N. viridescens or T. granulosa. The cloned sequence is most probably only part of a longer unit interspersed within the Triturus genome.

Characterisation of a short, highly repeated and centromerically localised DNA sequence in crested and marbled newts of the genus Triturus

A 32-33 bp highly repeated DNA sequence, TkS1, has been isolated from genomic DNA of the newt Triturus karelini digested with the restriction endonucleases HaeIII or AluI. TkS1 is known to be localised in the centromeric heterochromatin of all the chromosomes in T. karelini and the related species T. cristatus. TkS1 has been shown to be present in varying amounts in the genomic DNA of a range of species of Triturus, including representatives of the two main subgenera Triturus and Palaeotriton. A programme of sequencing of monomers, dimers and trimers of TkS1 was carried out in order to determine the level of conservation of the sequence within and between species of Triturus. Altogether 204 monomer (32/33 bp) clones were made of TkS1 from three individuals of T. karelini, and one individual each of T. cristatus, T. carnifex, T. dobrogicus and T. marmoratus, all members of the subgenus Triturus and the cristatus species group. A number of dimer (64 bp) and trimer (96 bp) clones were also made from DNA of a single specimen of T. karelini digested with HaeIII or AluI. Three distinct types of TkS1 were identified in all species examined, except for T. marmoratus where only two of the types were found. The types were distinguished on the basis of certain recurring divergent patterns in monomers sequenced from T. karelini. Type 1 is mainly characterised by the presence of an AluI site at positions 24-27 and type 3 mainly by the presence of an additional base (C) at position 14. Type 2 normally lacks the AluI site and the C at position 14, as well as having a number of other distinguishing features. TkS1 and its three types have remained remarkably constant in sequence since before the divergence of T. marmoratus from other species in the cristatus species group, about 10 million years ago. Examination of all 204 monomer clones and comparison with consensus sequences for the three types shows less than 5% divergence at any one position in the sequence. There is good evidence from examination of dimer and trimer clones of TkS1 that the different types are intermingled with each other, and all three types are likely to be present on all chromosomes. Dimeric (64 bp) TkS1 clones constructed from AluI fragments of T. karelini DNA show evidence of a trimeric (96 bp) "supertype" with the pattern type 1-type 3-type 1 that is much more common than would be expected on a random basis.(ABSTRACT TRUNCATED AT 400 WORDS)

Lampbrush and mitotic chromosomes of the hemiclonally reproducing hybrid Rana esculenta and its parental species

Mitotic chromosomes of the European water frogs Rana ridibunda and Rana lessonae, the parental species of Rana esculenta, differ significantly in their centromeric regions: when C-banded or when made fluorescent, the centromeres of R. ridibunda (and of ridibunda chromosomes in R. esculenta) are visible as a conspicuous dark granule or as a conspicuous fluorescent spot; the centromeres of R. lessonae (and of the lessonae chromosomes in R. esculenta) are inconspicuous or not fluorescent. Lampbrush chromosomes of these three taxa are described in detail for the first time; those of R. ridibunda and R. lessonae differ significantly in morphostructural characters such as conspicuousness of centromeres and number, form, and location of giant loops as well as in chiasma frequency. Chromosomes of the two parental species can thus be distinguished when present in lampbrush complements of hybrids. Reproduction in both sexes of natural R. esculenta lineages is hemiclonal: only the unrecombined genome of one parental species, usually R. ridibunda, is transmitted to haploid gametes (hybridogenesis). In 18 hybrids from natural populations of Poland, somatic tissues had allodiploid complements with chromosomes from each parental species. In contrast, spermatocytes I of five males and oocytes I of seven of eight females (221 of 222 oocytes) were autodiploid and contained only R. ridibunda chromosomes that formed n bivalents. These 12 hybrids thus were hybridogenetic. A single female hybrid had oocytes I (33 of 34) with genomes of both parental species; they showed various disturbances including tetraploidy, reduced number of chiasmata, and incomplete synapsis resulting in univalents. This individual thus was not hybridogenetic. The irregular lampbrush patterns indicate that such hybrids will have severely reduced fertility and most of their successful gametes will result in allotriploid progeny.

Heterochromatic DNA in Triturus (Amphibia, Urodela) II. A centromeric satellite DNA

The MspI family of highly repeated sequences is a centromeric satellite DNA representing about 1% of the genome of the Italian smooth newt, Triturus vulgaris meridionalis. We have studied the structure, genomic organization, chromosomal localization and conservation across species of this family. MspI sequences are around 197 bp long, as shown by sequencing of three cloned units. The family is organized in large clusters of tandemly arrayed units, present at almost all the centromeres of T.v. meridionalis, and is well conserved in the T.v. vulgaris subspecies. Conserved MspI sequences are also present in the related species T. helveticus, where they appear to be clustered at the centromeres of only a few chromosomes. MspI sequences are not found in other Triturus species analysed. The correlation of these sequences with the overall distribution pattern of heterochromatin and the extent of their conservation within the genus Triturus, are discussed.

Transcriptional activity of constitutive heterochromatin in the mammal Microtus agrestis (Rodentia, Cricetidae)

Two cytological approaches were applied to the analysis of genetic activity in euchromatin and heterochromatin in the European field vole Microtus agrestis. The first is based on the transcriptional activity of prematurely condensed chromosomes, the second on in situ hybridization of labeled cellular RNA to metaphase chromosomes. The results show that in cultivated fibroblasts, the transcriptional activity of constitutive heterochromatin is in the same order of magnitude as that of euchromatin; its chromosomal pattern is nonrandom. It is concluded that--as has already been shown for insects and amphibians--also in mammals, constitutive heterochromatin is transcribed in its decondensed state but is genetically inactive when forming chromocenters at interphase.

Quantitative variation and chromosomal location of satellite DNAs

A model of the evolutionary accumulation of highly repeated DNA (HRDNA) is proposed. The accumulation of HRDNA sequences, which are organized largely in tandem arrays and whose functional significance is obscure, is explained here as a consequence of the action of the forces of amplification (promoting increase in copy numbers) and unequal crossing over, random drift and natural selection (controlling copy numbers). This model provides a general framework (i) to study the chromosomal location of satellite DNAs present in the genomes of all higher eukaryotes, and (ii) to explain the significant variation in the amounts of satellites which is frequently found among closely related species, but only rarely within a species. A review of the relevant data is included and open questions are identified.

Heterochromatic DNA in Triturus (Amphibia, Urodela). I. A satellite DNA component of the pericentric C-bands

We have studied the structure, genome organization, chromosomal location, conservation across species and transcription on lampbrush chromosomes, of an AT-rich satellite DNA component of the newt, Triturus vulgaris meridionalis. The satellite (Sat G), originally isolated by gradient centrifugation, represents about 2% of the vulgaris genome and comprises a highly repetitive sequence family (HindIII family), whose monomers have been cloned. The repeat units are about 330 bp long, as measured on gels, and a cloned unit (pTvm1) is 310 bp long, as shown by sequencing. Abundant clusters of the HindIII family sequences are located within the pericentric heterochromatin (i.e. the C-bands placed at both sides of, and at a certain distance from, the centromeres) in most chromosomes. Both the sequence family and its overall pattern of chromosomal distribution are conserved within the genus Triturus, despite a few species-specific differences. The great majority of the HindIII family sequences are unexpressed on lampbrush chromosomes; they reside within pericentric, condensed segments of the chromosome axis ("loopless bars"). Only a few sequences are transcribed on some loops, suggesting that transcription promotion does not depend on the satellite sequences themselves.