RDNA SITE NUMBER POLYMORPHISM AND NOR INACTIVATION IN NATURAL-POPULATIONS OF ALLIUM-SCHOENOPRASUM

Nucleolar-organiser activity has been studied by silver staining and b y in situ hybridization with an rDNA probe in two populations of Alliu m schoenoprasum. One population is monomorphic with NORs and rDNA site s terminal on the short arm of pair 8 in all individuals. The other po pulations is monomorphic for pair 8 NORs but is also polymorphic for N ORs on the long arm of pair 7. All plants in this population carry rib osomal cistrons on both chromosomes of pair 7 but 0, 1 or 2 of these s ites can be active in rRNA synthesis. Cis-acting nucleolar-suppression affects the pair 7 locus. We suggest that there has been progressive reduction in the number of NORs during the evolution of A. schoenopras um.