Chromosome numbers, nuclear DNA content, and polyploidy in Consolea (Cactaceae), an endemic cactus of the Caribbean...

Polyploidy, an important mechanism of plant evolution, was investigated in Consolea, an endemic Caribbean opuntioid genus represented by nine subdioecious species with very narrow distributions, including species classified as rare or threatened. Standard chromosome counting and flow cytometric analyses were used to determine chromosome numbers and ploidy of each taxon. Compared to the base number (x = 11), the mitotic and meiotic counts indicated that there are seven hexaploid (2n = 66) and two octoploid species (2n = 88); no diploids were found. Histograms of intact nuclei confirmed that all species are polyploid, with C-DNA values ranging from 4.88-9.50 pg. The variation of DNA content was significantly higher for the octoploids than for the hexaploids. Male and female sexual morphs had similar DNA content, suggesting that there are no sex chromosomes. Cytomixis between cells and microsporocytes with no chromatin were observed. This provides a mechanism whereby gametes with variable chromosome numbers are produced, influencing reproduction and promoting speciation. In conclusion, C-DNA content and chromosome number separated Consolea species into two groups, which may correspond to two phylogenetic lineages or indicate that polyploidization occurred independently, with comparable effects on C-DNA content.