P1. Syst Evol. 159, 85-94 lant tematics and Evolution © by Springer-Verlag 1988 Pollination biology of Opuntia polyacantha and Opuntia phaeacantha ( Cactaceae) in southern Colorado MARTHA M. OSBORN, PETER G. KEVAN and MEREDITH A. LANE Received September 10, 1986 Key words: Angiosperms, Cactaceae, Opuntiapolyacantha, O. phaeacantha.- Pollination biology, phenology, compatibility, pollinator fauna. Abstract: The phenology, compatibility system, and pollinator fauna of Opuntiapolyacantha HAW. and O. phaeacantha ENGELM. in southern Colorado were studied and compared. The total blooming periods overlap, but the peak of blooming differs between the species with O. polyaeantha blooming first and for longer. Neither species is apomictic, O. poly- acantha is largely self-incompatible and O. phaeacantha is self-compatible but may not automatically fully self-pollinate. Examination of flowering structures showed that the former has larger flowers with more perianth parts and heavier (but fewer) anthers. Further the flowers of the allogamous species are visited by greater diversity (but similar abundance) of pollinators. Medium to large bees of the genera Diadasia, Lithurge, Melissodes,Bombus, Agapostemon and Megachile were found to be effective pollinators of the Opuntias studied. The two species were compared as to their floral attributes, breeding systems, and polli- nators, and the possible role of competition for pollinators is discussed with respect to its role in their evolutionary paths. Little is known about the pollinators and breeding systems of the genus Opuntia (Cactaceae) (RowLEY 1980). TOUMEY (1899) briefly referred to bee visitors of Opuntia spp. and PORSCH (1938) mentions bees of the genera Lithurge, Megachile (Megachilidae) and Diadasia (Anthophoridae) as pollinators of flowers of cacti. GRANT & HURD (1979), GRANT & GRANT (1979), and GRANT & al. (1979), doc- umented pollination of a few Opuntia spp. by specific taxa of bees. PARFITT d~; PICKETT (1980) observed several genera of bees visiting the flowers of two Opuntia species that they studied for pollination. Opuntia stricta HAYWORTH, is self-com- patible but sets more seeds when visited by pollinators (SPEARS 1987). Although most of the literature published before 1976 indicates that apomixis and self- compatibility were common in Cactaceae (FRYXELL 1957, EAST 1940), GANDERS (1976) in his review of the subject found that documentation of the breeding systems of Cactaceae is lacking. He reported that in 15 of 17 genera self-compatibility was absent and that there was no evidence for apomixis. MCFARLAND (1983) investi- gated the breeding system of Opuntia irnbricata, and found it to be an obligate entomophile. O. polyacantha is found in arid regions of the western plains states into Alberta and O. phaeacantha is found in in southeastern Colorado southward