Amount and organization of the heterochromatin in Olea europaea and related species

The amount and spatial organization of the heterochromatin in nuclei of the shoot meristem and the frequency in the nuclear DNA of sequences belonging to a family of tandem repeats were investigated in cultivars of Olea europ aea and related species. Significant differences between Olea species and b etween cultivars of O. europaea were observed: (i) in the spatial organizat ion of the heterochromatin in interphase nuclei as determined by the number and surface area of the chromocentres; (ii) in genome size; and (iii) in t he amount of condensed chromatin as measured by cytophotometry carried out at different thresholds of optical density. DNA elements belonging to a fam ily of tandem repeats about 80 bp in length (OeTaq80 repeats) were isolated from the genomic DNA of an olive cultivar. It was shown: (i) by nucleotide sequence comparisons, that these repeats display variability in structure even within the same array, where different elements may share no more than 74% homology; (ii) by in situ hybridization, that OeTaq80-related DNA sequ ences are mainly localized in the heterochromatin at the chromosome ends; ( iii) by dot-blot hybridization experiments, that these sequences are highly represented in the genome of all the olive cultivars and the majority of O lea species studied, and that their frequency may differ significantly even between olive cultivars; and (iv) by calculating the copy number of OeTaq8 0-related sequences per haploid (1C) genome, that the redundancy of these D NA elements may differ significantly between the genomes tested. It is sugg ested that the inter- and intraspecific changes in the nuclear and genomic traits observed can contribute to the understanding of the phylogenetic rel ationships between Olea species and in defining parameters to be exploited in varietal identification within cultivated olives.