Cytogenetic Characterization and Ploidy Diversity of Seven Opuntia Species (Cactaceae) from Southeastern Coahuila, Mexico

Abstract

Species of the genus Opuntia exhibit considerable variation in chromosome number and ploidy level, making cytogenetic information essential for studies on evolution, taxonomy, and genetic improvement. However, cytogenetic information on several species of this genus remains limited. Therefore, the objectives of this study were to determine chromosome numbers and ploidy levels in seven Opuntia species and to characterize the karyotype of Opuntia microdasys as a representative diploid species. A total of 69 accessions representing seven species were collected from wild and cultivated populations in southeastern Coahuila, Mexico. Somatic cells obtained from root apices developed from cladodes were used to analyze chromosome numbers. Cytological preparations included determination of mitotic activity, pretreatment with p-dichlorobenzene, enzymatic digestion with pectolyase and cellulase, and acetocarmine staining, followed by chromosome counting and karyotype analysis of O. microdasys. The highest mitotic activity was recorded between 10:00 and 11:00 h. Chromosome counts revealed diploid (2n = 22), tetraploid (2n = 44), hexaploid (2n = 66), and octoploid (2n = 88) cytotypes. Notably, O. engelmannii exhibited intraspecific cytotypic variation, with both hexaploid and octoploid populations. The karyotype of O. microdasys was highly symmetrical, with a karyotype formula of 10m + 1sm, small chromosomes, and low asymmetry indices (A1 = 0.15; A2 = 0.106). These findings reveal substantial ploidy diversity among the seven Opuntia species analyzed and provide cytogenetic information relevant to the characterization, management, and conservation of Opuntia genetic resources, including their potential use in ex situ conservation and botanical collections.