Lampbrush chromosomes (LBCs) are highly extended bivalents that function in the growing oocytes of many animals. Due to their distinctive chromomere-loop organization and intense transcriptional activity of lateral loops the LBCs, mainly amphibian ones, have served as a powerful system for exploring the general principles of chromosome organization and function. The exploitation of avian LBCs has considerably broadened the opportunities for comparative genome research and for cytogenetic analysis of domestic species. In this review we highlight the advantages of avian LBCs for research in different areas including integration of genome organization studies with studies on gene activity in vivo, analysis of co-transcriptional events occurring on nascent transcripts and investigation of chromosome-associated intranuclear domains. Recent findings concerning the organization of transcriptionally active and silent chromatin together with involvement of cohesin and condensin complexes into maintenance of structural integrity of LBCs are presented. The biological significance of the LBC phenomenon is discussed. The intensive transcription on LBCs shows some specific features: very long transcription units, deregulated termination, and transcription of non-coding satellite repeats. Here, based on the modern view on a role of RNA interference machinery in regulation of genome expression, we suggest a mechanism of initiation of satellite DNA transcription and offer a novel interpretation of the ‘classical’ hypothesis that sought to explain the significance of widespread transcription during oocyte growth.

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.

Over the past two decades, chromosome microdissection has been widely used in diagnostics and research enabling analysis of chromosomes and their regions through probe generation and establishing of chromosome- and chromosome region-specific DNA libraries. However, relatively small physical size of mitotic chromosomes limited the use of the conventional chromosome microdissection for investigation of tiny chromosomal regions. In the present study, we developed a workflow for mechanical microdissection of giant transcriptionally active lampbrush chromosomes followed by the preparation of whole-chromosome and locus-specific fluorescent in situ hybridization (FISH)-probes and high-throughput sequencing. In particular, chicken (Gallus g. domesticus) lampbrush chromosome regions as small as single chromomeres, individual lateral loops and marker structures were successfully microdissected. The dissected fragments were mapped with high resolution to target regions of the corresponding lampbrush chromosomes. For investigation of RNA-content of lampbrush chromosome structures, samples retrieved by microdissection were subjected to reverse transcription. Using high-throughput sequencing, the isolated regions were successfully assigned to chicken genome coordinates. As a result, we defined precisely the loci for marker structures formation on chicken lampbrush chromosomes 2 and 3. Additionally, our data suggest that large DAPI-positive chromomeres of chicken lampbrush chromosome arms are characterized by low gene density and high repeat content. The developed technical approach allows to obtain DNA and RNA samples from particular lampbrush chromosome loci, to define precisely the genomic position, extent and sequence content of the dissected regions. The data obtained demonstrate that lampbrush chromosome microdissection provides a unique opportunity to correlate a particular transcriptional domain or a cytological structure with a known DNA sequence. This approach offers great prospects for detailed exploration of functionally significant chromosomal regions.

Background: The epigenetic regulation of genome is crucial for implementation of the genetic program of ontogenesis through establishing and maintaining differential gene expression. Thus mapping of various epigenetic modifications to the genome is relevant for studying the regulation of gene expression. Giant transcriptionally active lampbrush chromosomes are an established tool for high resolution physical mapping of the genome and its epigenetic modifications. This study is aimed at characterizing the epigenetic status of compact chromatin domains (chromomeres) of chicken lampbrush macrochromosomes.Results: Distribution of three epigenetic modifications – 5-methylcytosine, histone H3 trimethylated at lysine 9 and hyperacetylated histone H4 – along the axes of chicken lampbrush chromosomes 1–4, Z and W was analyzed in details. Enrichment of chromatin domains with the investigated epigenetic modifications was indicated on the cytological chromomere-loop maps for corresponding chicken lampbrush chromosomes. Heterogeneity in the distribution of 5-methylcytosine and histone H3 trimethylated at lysine 9 along the chromosome axes was revealed.Conclusions: On examples of certain chromomeres of chicken lampbrush chromosomes 1, 3, 4 and W we demonstrated that a combination of immunofluorescent staining and fluorescence in situ hybridization allows to relate the epigenetic status and a DNA sequence context of individual chromomeres.