Benefits of autonomous selfing under unpredictable pollinator environments

Pollinator unpredictability favors evolutionary shifts from outcrossing to autonomous selfing, which provides reproductive assurance. Our goal was to quantify the reproductive assurance benefit of autonomous selfing and the stochastic nature of pollinator-mediated pollen receipt using three wild populations of the annual species Collinsia verna (Scrophulariaceae) over three years. The timing of autonomous self-pollination in C. verna ranges from competing to delayed, with more than half of the autonomous pollen arriving late in floral life. Floral density, identity of insect floral visitors, and visitation rate varied within and among years, with significant variation in pollinator community between years within populations. Pollinator failure reduced fruit set within and among populations across years. However, reproductive assurance selfing increased fruit set on average by 8% overall and up to 30% in one population. Despite this fitness benefit, fruit set (= Sigmafruits/Sigmaflowers) estimated in pollinator-exclusion cages was significantly less than the fruit set in paired open-pollinated plots. In a series of experiments, we found no difference in fruit set from self- vs. outcross pollen, ruling out early inbreeding depression. However, flowers, hand-outcrossed at the same time that autonomous selfing naturally occurs, produced significantly more fruits relative to flowers that received pollen only through autonomous selfing. These results suggest that pollen type (self/outcross) is less important to fruiting success than is the timing or efficiency of autonomous pollination relative to vector-mediated pollination. Our results support the reproductive assurance hypothesis and suggest that autonomous selfing in C. verna is adaptive under variable pollinator conditions. Fruit set results from both pollinator-delivered and autonomous self-pollen, suggesting a "best of both worlds" scenario, with mixed mating a likely outcome.