EFFECTS OF FLOWER NUMBER AND POSITION ON SELF-FERTILIZATION IN EXPERIMENTAL POPULATIONS OF EICHHORNIA-PANICULATA (PONTEDERIACEAE)

1. We examined the effects of daily inflorescence size (three-, six-, nine- and 12-flowered) and the position of flowers within an inflorescence (bottom, middle and top) on the frequency of self-fertilization using genetic markers and experimental manipulation of garden populations of Eichhornia paniculata, a self-compatible bee-pollinated plant. 2. Based on the observed tendency for bees to forage upwards on inflorescences and a model of the relation between pollen carry-over and the number of flowers visited per inflorescence, we predicted that the frequency of self-fertilization should increase from bottom to top flowers and with increasing inflorescence sizes. 3. Electrophoretic analysis of open-pollinated progeny arrays supported both of these predictions. The fraction of self-fertilized seeds increased progressively from bottom to top flowers within an inflorescence and there was a significant increase in the frequency of self-fertilization with daily inflorescence size. Inflorescences of all sizes exhibited equivalent increases in the frequency of self-fertilization of flowers from bottom to top positions. 4. The general agreement between our experimental results and model expectations emphasizes the strong influence of pollinator behaviour on mating patterns in self-compatible plants. Such effects have the potential to act as strong selective forces maintaining both anti-selfing mechanisms in mass-flowering species and protandry in species with vertical inflorescences visited by negatively geotactic pollinators.