Pollen carryover: experimental comparisons between morphs of Palicourea lasiorrachis (Rubiaceae), a distylous,...

In the Monteverde (Costa Rica) cloud forest, the distylous treelet Palicourea lasiorrachis (Rubiaceae) is pollinated by the hummingbird Lampornis calolaema. Only intermorph pollinations result in seed set. In a series of 47 experimental trials, we compared pollen carryover from thrum (short-styled) flowers to pin (long-styled) flowers with carryover from pin to thrum flowers. In one experiment, captive L. calolaema first probed 2 flowers of one style morph, then a series of 20 flowers of the other style morph. In the second experiment, 2 flowers of a frequent associate of P. lasiorrachis, the treelet Cephaelis elata (Rubiaceae), intervened between donor and recipient Palicourea flowers. We assessed intermorph pollinations by counting pollen tubes in recipients' styles. Without Cephaelis intervening, thrum donors transferred many fewer total grains to pins than did pin donors to thrums. However, thrum grains were dispersed more evenly among recipient pin flowers than were pin grains among thrum recipients. As a result, the potential for paternal reproductive success was at least as great for thrum flowers as for pins. With insertion of Cephaelis flowers into the foraging sequence, the magnitude of differences between carryover patterns declined.