A CYTOGENETICAL AND MOLECULAR ANALYSIS OF THE RIBOSOMAL CISTRONS OF ALLIUM-SPHAEROCEPHALON L (LILIACEAE)

Allium sphaerocephalon is a species with a high number of secondary constrictions and wit intra-and interindividual variation in their number. We analysed whether all the secondary constrictions are involved in nucleolus formation, whether the same variability exists in the number of NOR, and the mechanism(s) responsible for this variability. Our cytological data have shown that (i) all the secondary constrictions may be involved in the nucleolus organization, (ii) the variability in number is in accordance with the numbers of NOR, and (iii) the variability could be due to unequal exchanges among ribosomal genes on homologous and non-homologous chromosomes. Ribosomal DNA sequence homogeneity detected in this species supports this conclusion.