Comparative triple-color FISH mapping and genome size advances understanding of the cytogenetic diversity in wild Solanum species

Solanum is the most representative and largest genus of the family Solanaceae with numerous important crop species including potato, tomato and eggplant, etc. Wild species are crucial resources for elucidating interspecific relationships and breeding in this genus. Nightshades are various wild plant species belonging to the Solanum, consisting of many morphological variants collectively known as the Solanum nigrum complex. They are exploited for food, medicines, animal feed, and spiritual uses. They have especially proven useful in Solanum crop breeding. However, only a limited number of molecular cytogenetic studies have been reported, which has impeded comparative analyses that would allow a better understanding of Solanum genome evolution and facilitate breeding improvements. In this study, chromosomal level genome constitution and genome size were analyzed in three wild species of Solanum through triple-color fluorescence in situ hybridization (FISH) and flow cytometry. The chromosome number was 2n = 2x = 24 in S. americanum, 2n = 4x = 48 in S. villosum, and 2n = 6x = 72 in S. nigrum. Each pair of 5S and 45S rDNA signals was observed in diploid species; however, they proportionally increased with the ploidy level with two and three pairs in tetraploid and hexaploid species, respectively. Telomeric signals were detected in all the chromosome terminal regions of the three species. The diploid genotypes have an average genome size of 2.65 pg/2C, whereas the tetraploid and hexaploid species have approximately two- and three-fold larger genome sizes with 5.06 and 7.70 pg/2C, respectively. Our results suggest that the diploid species S. americanum and the tetraploid species S. villosum may be potential parents of the hexaploid species S. nigrum. This data will be helpful for future taxonomic, cytogenetic, phylogenetic, and evolutionary studies in Solanum as well as breeding improvement of this genus.