Cytogenetics of tuna in Argentina (two forms of Opuntia ficus - indica (L.) Mill. and O. robusta J. C. Wendl., Cactaceae)

Opuntia ficus - indica (L.) Mill. f. ficus - indica (yellow tuna, cordobesa orange tuna, italiana orange tuna, salteña tuna) and f. amyclaea (white tuna, reddish tuna) and O. robusta J. C. Wendl. (cuaresma tuna) are extensively cultivated cacti in Argentina, mainly used as fodder and human food. Despite the importance of these resources, no studies of molecular cytogenetics considering the morphological characteristics of the varieties have been carried out. The main goal of this contribution is to determine the chromosome number, physical localization of ribosomal genes and amount of DNA in these taxa, including several populations with different fruit colors. The f. ficus - indica samples were octoploid, f. amyclaea hexaploid, and O. rubusta tetraploid. All taxa exhibited small similar-sized symmetrical chromosomes. Karyotypes were symmetrical with slight variations within each species. The 18S–5.8S–26S sites in all taxa were located on the secondary constrictions and the adjacent satellites at telomeric positions. The f. ficus - indica presented four 18S–5.8S–26S rDNA sites and the f. amyclaea three, whereas O. robusta had two. There were eight and six 5S rDNA sites in f. ficus - indica and f. amyclaea , respectively, and four in O. robusta . They were always proximally located on short arms of different chromosome pairs, always in a pericentromeric position. Both ribosomal genes were always asynthenic. The nuclear DNA in the octoploid f. ficus - indica presented a lower value (1Cx = 0.91 pg) than the hexaploid f. amyclaea (1Cx = 1.06 pg), whereas the tetraploid O. robusta had 1Cx = 0.87 pg. This cytogenetic evaluation is important for developing more efficient and faster selection strategies, contributing to obtaining new and improved varieties.