Heterochromatic regions in Japanese quail chromosomes: comprehensive molecular-cytogenetic characterization and 3D mapping in interphase nucleus

Chromosomes of Japanese quail (Coturnix coturnix japonica, 2n=78), a galliform domestic species closely related to chicken, possess multiple heterochromatic segments. Due to the difficulties in careful analysis of such heterochromatic regions, there is a lack of data on their DNA composition, epigenetic status, as well as spatial distribution in interphase nucleus. In the present study, we applied giant lampbrush chromosome (LBC) microdissection for high-resolution analysis of quail centromeric regions of macrochromosomes and polymorphic short arms of submetacentric microchromosomes. FISH with the dissected material on mitotic and meiotic chromosomes indicated that in contrast to centromeres of chicken macrochromosomes, which are known to harbor chromosome-specific and, in some cases, tandem repeat-free sequences, centromeres of quail macroautosomes (CCO1-CCO11) have canonical organization. CCO1-CCO11 centromeres possess massive blocks of common DNA repeats demonstrating transcriptional activity at LBC stage. These repeats seem to have been subjected to chromosome size-correlated homogenization previously described primarily for avian microchromosomes. In addition, comparative FISH on chicken chromosomes supported the previous data on centromere repositioning events during galliform karyotype evolution. In interphase nucleus of different cell types, repetitive elements specific for microchromosome short arms constitute the material of prominent centrally located chromocenters enriched with markers of constitutive heterochromatin and rimmed with clusters of microchromosomal centromeric BglII-repeat. Thus, clustering of such repeats is responsible for the peculiar architecture of quail interphase nucleus. In contrast, centromere repeats of the largest macrochromosomes (CCO1 and CCO2) are predominantly localized in perinuclear heterochromatin. The possible involvement of the isolated repeats in radial genome organization is discussed.