Nonrandom mating and sexual selection in a desert mustard: an experimental approach.

We report on an experiment to explore the importance of pollinator behavior and postpollination events within flowers in generating nonrandom mating in the desert perennial Lesquerella fendleri (Brassicaceae). In this experiment, we crossed four plants with each other in all combinations. We performed these crosses in three different ways, which varied the opportunity for nonrandom mating: single-donor hand pollinations, mixed hand pollinations, and field pollinations. Number of seeds sired by each donor following single-donor pollinations differed only slightly (though significantly) from random, indicating little variation in siring ability. However, we found more dramatic (and significant) departures from random mating for mixed hand pollinations. In the field we found even more nonrandomness, with some donors siring >71% of seeds on some maternal plants, despite equal opportunity to father seeds. The rank ordering of donors was consistent across the four females and across treatments, indicating that there was concordant nonrandom mating, a requirement for sexual selection. The most successful donor in all treatments also had the greatest pollen production per flower, and this trait may therefore be an important cause of the observed patterns. We infer that pollinator behavior and postpollination processes both contribute to the nonrandom mating observed in the field, and discuss the advantages of this new experimental approach.