Evidence for Short-Distance Dispersal of Pollinia in Asclepias syriaca L.

Abstract

In milkweeds (Asclepiadaceae) pollen is produced in paired packages (pollinia) and pollination occurs when a pollinium is inserted by a pollinator into a stigmatic chamber. Previous studies have suggested that this mode of pollen delivery results in predominantly long-distance pollen dispersal. This assertion was tested for Asclepias syriaca L. A plant whose pollinia had been labelled with 14C was placed within a large population of milkweeds and a single subject umbel was exposed to honey-bee pollinators for 5 days. Hand pollinations involving another umbel on this plant verified that labelled pollinia dispersed to flowers within the umbel (geitonogamy), or to flowers on other plants, could be detected by scintillation counts. The labelled plant was at the centre of a 6 X 6m study plot. Of the 38 insertions on the labelled umbel, 14 (37%) were from the labelled umbel itself (geitonogamy). Thirty labelled pollinia were detected in umbels on neighbouring plants within the study plot. It is estimated that few labelled pollinia were inserted into flowers outside of the study plot, although the possibility of some long-distance dispersal cannot be excluded. The results indicate that much of the pollen dispersal was short distance. This makes pollen dispersal characteristics in this species similar to those of non-milkweed species. Shortdistance pollen dispersal can have important implications for A. syriaca which reproduces vegetatively and for which geitonogamous pollination does not result in fruit set. Key-words: Asclepias, 14C, milkweeds, pollen dispersal, pollinia