Molecular-cytogenetic studies of ribosomal genes and heterochromatin reveal conserved genome organization among 11 Quercus species

Genomes of 11 Quercus species were characterized using cytogenetic (Giemsa C-banding, fluorochrome banding), molecular-cytogenetic (fluorescence in si tu hybridization, FISH, to ribosomal genes) and molecular (dot-blot for rib osomal gene-copy number assessment) techniques. Ribosomal genes are the fir st DNA sequences to be physically mapped in oaks, and the copy number of th e 18S-5.8S-26 S rRNA genes is estimated for the first time. Oak karyotypes were analysed on the basis of DAPI banding and FISH patterns; five marker c hromosomes were found. In addition, chromosomal organization of ribosomal g enes with respect to AT- and GC-differentiated heterochromatin was studied. Fluorochrome staining produced very similar CMA/DAPI banding patterns, and the position and number of ribosomal loci were identical for all the speci es studied. The 18S-5.8S-26 S rRNA genes in oak complements were represente d by a major locus at the subterminal secondary constriction (SC) of the on ly subtelocentric chromosome pair and a minor locus at paracentromeric SC o f one metacentric pair. The only 5 S rDNA locus was revealed at the paracen tromeric region of the second largest metacentric pair. A striking karyotyp ic similarity, shown by both fluorochrome banding and FISH patterns, implie s close genome relationships among oak species no matter their geographic o rigin (European or American) or their ecophysiology (deciduous or evergreen s). Dot-blot analysis gave preliminary evidence for different copy numbers of 18S-5.8S-26 S rRNA genes in diploid genomes of Q. cerris, Q. ilex, Q. pe traea, Q. pubescens and Q. robur (2700, 1300, 2200, 4000 and 2200 copies, r espectively) that was correlated with the size polymorphism of the major lo cus.