Recurrent Events of Polyploidy in Ficus carica L. (Moraceae)

Premise of research. The fig, Ficus carica (2n=26), is a widely cultivated tree of the Mediterranean region. Because of the scarcity of cytogenetic studies, intraspecific polyploidy in the fig has been overlooked until some years ago, when a few 2n=3x=39 genotypes were identified. This finding stimulated further explorations to learn more about the real extent of cytotypic diversity in this species and verify the presence of higher ploidy levels.Methodology. A total of 84 wild fig plants sampled from two different geographic areas was examined for the determination of chromosome numbers and karyotype analyses. The fluorescence in situ hybridization technique using rDNA sequences as probes was applied for a further characterization of the populations and polyploid genotypes discovered. The analysis of the nucleotide composition of 5S rDNA repeats was included in this study to acquire information on the intraspecific variability of F. carica and its genome constitution.Pivotal results. Chromosome counts led to the discovery of additional triploid plants and a new variant with 2n=52, which is the first case of tetraploidy so far reported in F. carica. In contrast to the ploidy-level variations, the karyotype structure and the distribution pattern of rRNA genes (rDNA) were stable. The number of rDNA loci detected in polyploid plants appeared consistent with the number of their respective genomes. The limited heterogeneity of 5S rDNA repeats renders these sequences of little use in discriminating fig genotypes or populations; however, they could be advantageously used for establishing phylogenetic relationships within the genus and resolving taxonomic questions.Conclusions. The results of this study demonstrate that events of polyploidization in fig are quite a widespread phenomenon, which seems connected to the formation of a triploid cytotype. However, the presence of a tetraploid plant suggests a more complex situation that would be interesting to study further. Large-scale chromosome investigations would be very helpful in understanding the role of polyploidy in wild and cultivated fig populations.