Opuntia spinosissima (Martyn) Mill. (Cactaceae) is an extremely rare taxon with a single remaining wild population of 13 plants located on Little Torch Key, Florida. The plants rarely set viable seeds and in most cases the ovary abscises before maturation. Pollination and breeding system were examined in Little Torch Key and with plant accessions located at Fairchild Tropical Garden. Four pollination treatments were carried out to test for apomixis, self-pollination, outcrossing and autogamy; a control treatment was also monitored. Additionally, pollen viability, pollinators, and seed set and viability were investigated. On selfed, crossed, and open (control) stigmas pollen grains germinated, but the tubes usually did not reach the base of the style. This suggests that O. spinosissima has gametophytic self-incompatibility (SI), but since none of the pairwise crosses set seeds, the extant plants were apparently not inter-compatible. Out of 173 manipulated and control flowers, only one set fruit. Although this flower was outcrossed, its style had no pollen tubes that reached the ovary. This indicates apomixis, a process common in the Cactaceae. In those flowers where pollen tubes did reach the ovary they failed to penetrate ovules, suggesting this taxon lost the ability to be fertilized. Most field-collected seeds were viable, but there is no seedling recruitment under natural conditions, and vegetative reproduction is common. Based on these findings, I hypothesize that O. spinosissima is a sterile polyploid and that the 13 extant plants are asexually derived from a single lineage.

Key words: apomixis, asexual reproduction, Opuntia spinosissima>, polyploid