Avian lampbrush chromosomes: a powerful tool for exploration of genome expression

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.