REPRODUCTIVE-BIOLOGY OF A RARE CACTUS, OPUNTIA-SPINOSISSIMA (CACTACEAE), IN THE FLORIDA KEYS - WHY IS SEED SET VERY-LOW

Opuntia spinosissima (Martyn) Mill. (Cactaceae) is an extremely rare t axon with a single remaining wild population of 13 plants located on L ittle Torch Key, Florida. The plants rarely set viable seeds and most ovaries abscise without initiation of fruits. Pollination and breeding system were examined in Little Torch Key and in plant accessions loca ted at Fairchild Tropical Garden. Four pollination treatments were car ried out to test for apomixis, self-pollination, outcrossing, and auto gamy; a control treatment was also monitored. Additionally, pollen via bility, 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 sugges ts that O. spinosissima has pre-zygotic self-incompatibility. None of the pairwise crosses set seed, so the extant plants were apparently no t inter-compatible. Out of 173 manipulated and control flowers, only o ne set fruit. Although this flower was outcrossed, no pollen tubes ger minated on the stigma; this suggests agamospermy, a process common in the Cactaceae. In those flowers where pollen tubes did reach the ovary they failed to penetrate ovules. suggesting ovarian inhibition or tha t this taxon has lost the ability to be fertilized. Most field-collect ed seeds were viable, but there is no seedling recruitment under natur al conditions, and vegetative reproduction is common. Based on these f indings, I hypothesize that O. spin osissima is a sterile polyploid an d that the 13 extant plants are asexually derived from a single lineag e.