Pollen loads deposited onto stigmas vary in size and genetic composition. When the amount of pollen is deposited exceeds the number of ovules, its arrival schedule, speed of germination, and pollen tube growth rate can determine which microgametophytes fertilize the ovules. When differences in pollen tube growth are genetically based, selection can operate during between the time of deposition and fertilization. Moreover, because of the common gene expression in the two phases of the plant life cycle, selection on the microgametophyte may alter the resulting sporophytic generation. The extent to which this occurs in nature has been unclear. Most tests of the predictions of the pollen competition hypothesis use cultivars and/or artificial growth conditions and hand pollination techniques. Our experiments employed a wild species, Cucurbita foetidissima, in its natural habitat. Rather than manipulating pollen load by hand pollinations, we varied pollen intensity by controlling access to flowers by natural pollinators. Single visits by pollinators deposited pollen well in excess of ovule number (653 pollen grains). Conditions for pollen competition (full seed set) were created by just a few visits: > 4000 grains arrived within the first two hours of anthesis. Fruits produced by multiple visits had greater seed numbers (214 vs. 151) than fruits produced by a single visit. Finally, the progeny produced by multiple pollinator visits were more vigorous than those produced by single visits with respect to five measures of plant vigor (MANOVA, Wilks' lambda = 0.97, F1,401 = 19.92, p < 0.0002. These data demonstrate that conditions for pollen competition exist in nature, and support the prediction that pollen competition enhances offspring vigor.

Key words: Cucurbita foetidissima, plant reproduction, pollen competition