Heterochromatin of the scarab beetle, Bubas bison (Coleoptera: Scarabaeidae) II. Evidence for AT-rich compartmentalization and a high amount of rDNA copies

An unexpected result arising from a previous characterization of the scarab beetle Bubas bison (Coleoptera: Scarabaeidae) heterochromatin was its unusual homogeneous reaction to different staining methods. In particular, silver stainability of heterochromatic ends of all chromosomes prevented identification of the number of rDNA transcriptionally active regions. Data formerly obtained using silver impregnation (Ag-NOR), C- G- and DAPI banding are here improved and completed by application of CMA3 staining and rDNA FISH with the aim to investigate heterochromatin base composition and locate rDNA regions with respect to NOR-associated heterochromatin. Our results show that B. bison has a high amount of heterochromatin (almost 50%) and that - as revealed by rDNA FISH - major rRNA genes are spread over the heterochromatic telomeric regions of eight chromosomes, thus suggesting that only a portion, although consistent, of total heterochromatin is associated with ribosomal clusters. Moreover, DAPI-positive (AT-specific) and CMA3-negative (GC-specific) reactions of heterochromatic DNA confirm its AT-rich composition. Finally, possible explanations for the bright DAPI-fluorescence of both heterochromatin and rDNA sequences are discussed.