Opuntia series Armatae is evaluated considering morphological (vegetative, floral, and carpological) and cytogenetical (diploid number, presence of heterochromatin, and physical localization of ribosomal genes) features to shed light on their systematics and evolution. Three complexes (named O. elata, O. megapotamica, and O. monacantha) are proposed to accommodate seven species according to the tepal, fruit, stigma, and seed aril traits. Additional systematic conclusions include the following: (i) O. stenarthra, O. assumptionis, O. cognata, and O. subsphaerocarpa are synonyms; (ii) O. elata is a variable species with two varieties (var. cardiosperma and var. obovata); (iii) O. rioplatense is a synonym of O. elata var. obovata; and (iv) O. megapotamica is a polymorphic species with two varieties (one newly described and a new combination). All species had small similar-sized symmetrical chromosomes and were tetraploid (2n = 44), except for a population of O. arechavaletae and O. monacantha (2n = 22). All showed one or two pairs with CMA+/DAPI− NOR associated bands. The 18–5.8–26S rDNA loci seem to coincide with CMA+/DAPI−/NOR blocks. The number of 5S signals detected was proportional to the ploidy level of the species. A combination of cytogenetic and morphological features helped to differentiate the complexes, species, and varieties of Opuntia ser. Armatae.

The somatic chromosomes of Cestrum bigibbosum Pittier, C. buxifolium Kunth, Sessea corymbosa Goudot ex Taylor and Phillips and Vestia foetida (Ruiz et Pav.) Hoffmanns. were examined by classical staining. The number 2n = 2x = 16 was found for all species. The total genome length ranged from 57.70 to 92.09 µm and the average chromosome length from 7.21 to 11.51 µm. Karyotypes were notably symmetrical. Haploid karyotype formulae were 8 m pairs (C. bigibbosum, V. foetida) or 7 m + 1 sm pairs (S. corymbiflora, C. buxifolium). Except Sessea corymbiflora in which satellites were not visualized, the remaining species showed one satellited pair. The species can be distinguished by a combination of karyotype formula, karyotype length, and the position of satellites in a particular chromosome pair. Karyotype diversification in tribe Cestreae is suggested to have been accompanied by a tendency towards slight asymmetry and increased genome length.