MECHANISMS OF POLLEN DEPOSITION BY INSECT POLLINATORS

Studies of pollen dispersal in insect-pollinated plants have often doc umented highly leptokurtic patterns of pollen deposition that can incr ease the likelihood of long-distance mating. To examine potential caus es of highly leptokurtic deposition, we introduce four functions that arise when (1) the duration of pollinator visits to pollen sources is limited, (2) the rate of pollen deposition varies randomly among polli nators and/or among visits, (3) the rate of pollen deposition changes monotonically over time or (4) pollen is carried in layers or compartm ents on the pollinator's body that differ in deposition rate. Maximum likelihood techniques were used to fit deposition functions to data ob tained from honey bees (Apis mellifera L.) visiting mustard plants (Br assica campestris L.) that contained a marker gene. Each of the altern ative leptokurtic functions fit the experimental data better than a si mple exponential function and the best-fit function predicted a mean p ollen dispersal distance more than three times greater than the expone ntial. We argue that studies of pollen deposition need to test a broad er range of deposition models to assess outcrossing distance in plant populations accurately.