Intra-umbel geitonogamy and umbel size in a natural population of Common Milkweek (Asclepias syriaca).

Abstract

Geitonogamy may have important consequences for the reproductive success of a self-incompatible, clonal species with a large floral display, such as Common milkweed (Asclepias syriaca). It is suggested that there is a high frequency of self-pollinia deposition, and this may be a partial explanation for high levels of abscission of pollinated flowers, high frequency of pod abortion, and small fruit set. Further, frequency of geitonogamy may increase with increasing floral display size due to longer duration of pollinator visits. Actual levels of geitonogamy occurs within an umbel and whether umbel size influences rates of geitonogamous pollination. Umbels with self-pollinia present received 1.7 times as much pollinia as emasculated umbels, indicating that geitonogamy occurs within an umbel. Large umbels received 2.2 times as much pollinia as small umbels, although there was no difference in the propotion of self-pollinia received (42-43%). My results demonstrate that geitonogamy occurs within an umbel at a high rate, but they suggest that geitonogamy does not increase with umbel size. I suggest that frequency of pollinator visitation increases with umbel size, although duration of visitation does not. My estimation of geitonogamy is an underestimate because flowering of umbels overlaps within a ramet, and several ramets may belong to one clone. It is likely, then, that geitonogamy accounts for a large percentage of post-fertilization abortions of fruits.