XVI International Botanical Congess

Chromosome numbers have been published for 537 species (650 total taxa) in 78 genera of the Cactaceae. Base number is x=11. Chromosomes are small, one pair usually noticably smaller than the others, metacentric to submetacentric. Synapsis in triploids yields 11 trivalents (not 11 bivalents + 11 univalents), even in an intergeneric hybrid. Natural interspecific hybridization, polyploidization, vegetative propagation and apomixis play a role in the evolution of cacti, particularly in Opuntioideae. Polyploidy is common, aneuploidy is rare. Polyploidy occurs in 0% of taxa in Pereskioideae, 64.3% of 199 taxa in Opuntioideae, 12.9% of 435 taxa in Cactoideae, or 28.3% of all Cactaceae. Triploids are not sterile, produce stainable pollen, and hybridize with other species. Polyploidy arised via restitution gametes. Meiotic counts are more reliable than mitotic ones because a single root tip can yield multiple polyploid counts.