Effects of floral sex allocation and phenology on the within-flower selfing rate and female reproductive success inPulsatilla alpina, a perennial herb with strong inbreeding depression

Within-flower self-pollination should be the major source of self-fertilization in mixed-mating species presenting single or few flowers simultaneously. It is also an often unmeasured source of selfing in species with many flowers open simultaneously. In general, the rate of within-flower selfing is expected to depend on the number of pistils and stamens, the timing of flowering, and subsidiary floral traits in self-compatible species in which pistil and stamen numbers vary. Accordingly, the intensity and direction of selection on these traits should depend on the level of inbreeding depression. Here, we measured the dependence of the within-flower selfing rate on floral sex allocation, phenology, petal length, and floral stalk height in a population of the perennial herb Pulsatilla alpina (Ranunculaceae) in which most individuals had single flowers. We estimated inbreeding depression in the population by comparing inbreeding coefficients between parents and seed progeny using microsatellite markers. We then estimated selection on the measured traits via female reproductive success at the flower level and compared our estimates with a hypothetical scenario in which inbreeding depression was assumed to be absent. We estimated inbreeding depression to be 0.93. The within-flower selfing rate varied widely among flowers and depended positively on stamen number and negatively on pistil number and flowering date, supporting the predictions of a mass-action model. The dependence of the selfing rate on the measured floral traits consistently predicted (non-linear) patterns of selection under high inbreeding depression that were distinct from those under a hypothetical scenario of no inbreeding depression. Synthesis: While previous research has emphasized the importance of mass-action mating on selfing among flowers of plants with large floral displays, our results demonstrate its importance for selfing within individual flowers. They also demonstrate the importance of accounting for both the selfing rate and inbreeding depression when inferring selection on floral and other traits via female fitness.