Environmentally induced variation in floral traits affects the mating system in Datura wrightii

1. If pollination is unpredictable, selection may favour the production of selfed seeds in the absence of pollen vectors, even in plant species with obvious adaptations for outcrossing. Pollination may be less predictable for plants growing in certain environments if environmental factors affect the floral phenotype. Through effects on flower morphology and the floral display, the environment may affect the outcrossing rate. 2. We manipulated two environmental factors, water availability and exposure to insect herbivores, in a common-garden experiment using a perennial herb, Datura wrightii. We measured herkogamy (the separation of anthers and stigmas within flowers), total flower length, and flower number, and used a single-gene trichome dimorphism as a marker to determine per-plant outcrossing rates. 3. The large amount of variation in herkogamy was affected by trichome type, irrigation and herbivory. In addition, watered plants had longer corollas, and plants attacked by herbivores had fewer open flowers. Thus environmental factors affect floral phenotype. 4. However, irrigation and herbivory did not directly affect outcrossing rate. There were indirect effects of these treatments on outcrossing because plants with increased herkogamy and fewer open flowers had higher outcrossing rates. 5. A greenhouse experiment showed that autonomous selfing is more likely when herkogamy is reduced, and can occur both as the flower opens and when the corolla is shed. 6. These experiments are among the first to show that within-population variation in the mating system can be due to environmentally induced variation in floral traits.