Effects of Pollen-Load Size and Number of Donors on Sporophyte Fitness in Wild Radish (Raphanus raphanistrum)

Competition among pollen tubes within styles sometimes results in progeny with enhanced growth rates, presumably resulting from selection for superior pollen genotypes. Although such gametophytic selection is often invoked as an agent of evolution, its importance in existing natural populations is unknown. I tested for the effects of past and current pollen competition on sporophytic fitness in wild radish (Raphanus raphanistrum). The intensity of previous pollen competition had no overall effect on progeny characteristics such as seed size, germination rate, plant size, or fecundity, as measured in both field and greenhouse environments. Using pollen from "intense" previous competition, I also tested for separate effects of pollen-load size (60 vs. 300 grains) and number of pollen donors per stigma (one vs. three). Again, these pollination treatments had no consistent effects of progeny fitness. In a few cases, plants resulting from three pollen donors were smaller than those from one. My results do not support the prevailing view that differences in the size or genetic diversity of pollen loads on stigmas allow selection for superior offspring. These negative results should be interpreted cautiously, however, because other explanations could account for a lack of treatment effects.