Genome size and karyotype diversity in Solanum sect. Acanthophora (Solanaceae)

In order to study the evolution of different cytogenetic characters in species of Solanum sect. Acanthophora in relationship to the known phylogeny for this group, the following techniques were used: CMA/DAPI chromosome banding; fluorescent in situ hybridization with probes for the 18-5.8-26S and the 5S rDNA genes in mitotic chromosomes; nuclear DNA quantification by flow cytometry. Depending on the species, 2-6 of the 12 basic chromosome pairs were identified. The heterochromatic banding patterns were shown to be species-specific. All species presented one chromosome pair bearing a 18-5.8-26S signal and one pair (rarely two) with a 5S signal, the two rDNA sites being non-syntenic. The techniques employed allowed us to establish two species groups within sect. Acanthophora: one with small, symmetric chromosomes, little heterochromatin and lower DNA content, and the other one with larger and more asymmetric chromosomes, more heterochromatin CMA(+)/DAPI(-) (associated with NOR or not) and a higher DNA content. An elevated karyotype asymmetry would be associated with a high amount of heterochromatin and a high DNA content. The trend within sect. Acanthophora would be towards a loss of heterochromatin, a reduction of chromosome size, and an increase in symmetry.