Systematic Increase in Pollen Carryover and Its Consequences for Geitonogamy in Plant Populations

The fraction of pollen that is carried over from one flower to the next as a pollinator visits a sequence of flowers has a strong effect on patterns of mating in plant populations. Although most studies have used simple exponential or geometric functions to represent carryover, pollen deposition patterns often show longer-than-geometric tails, suggesting a systematic increase in the carryover fraction. We introduce a changing carryover model that allows the carryover fraction to increase or decrease during a sequence of visits. Maximum Likelihood methods were used to fit constant and changing carryover models to twelve previously published data sets. A model with an increasing carryover fraction provided a better fit than a constant carryover model in 75% of the cases examined. We found only partial evidence for one proposed cause of increasing carryover, the burial of donor pollen beneath pollen collected from recipient flowers. Finally, we use the results for Ipomopsis aggregata, a hummingbird-pollinated plant that presents many flowers simultaneously, to show that the changing carryover model predicts less pollen will be transferred among flowers on the same plant (geitonogamy) than does a constant carryover model fit to the same experimental data.