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 infloresc ence (bottom, middle and top) on the frequency of self-fertilization u sing genetic markers and experimental manipulation of garden populatio ns of Eichhornia paniculata, a self-compatible bee-pollinated plant. 2 . Based on the observed tendency for bees to forage upwards on inflore scences and a model of the relation between pollen carry-over and the number of flowers visited per inflorescence, we predicted that the fre quency of self-fertilization should increase from bottom to top flower s 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 b ottom to top flowers within an inflorescence and there was a significa nt increase in the frequency of self-fertilization with daily inflores cence 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 a nd model expectations emphasizes the strong influence of pollinator be haviour on mating patterns in self-compatible plants. Such effects hav e the potential to act as strong selective forces maintaining both ant i-selfing mechanisms in mass-flowering species and protandry in specie s with vertical inflorescences visited by negatively geotactic pollina tors.