High autonomous selfing capacity and low flower visitation rates in a subalpine population of Prunella vulgaris (Lamiaceae)

Background and aims – Autonomous selfing is a potential strategy to overcome pollinator scarcity or unreliability, but the specific mechanisms of autonomous selfing need further study to clarify our understanding of this phenomenon. This study investigated the mechanism of autonomous selfing in a subalpine population of Prunella vulgaris (Lamiaceae), a species with didynamous stamens.Methods – We studied the floral biology, pollinator activity, and breeding system of Prunella vulgaris in a natural population. The short stamens and long stamens were removed separately to examine their contribution to self-pollinated seed production. In addition, the stamens were removed at different developmental stages to determine the timing of autonomous selfing.Key results – The short stamens were closer to the stigma than the long stamens. Seed production after removal of the long stamens was higher than following removal of the short stamens, suggesting that spatial separation of the anthers from the stigma is crucial and pollen from the short stamens contributes most frequently to self-pollination. Self-fertilization started at the final bud stage of flower development, which may be consistent with the prior selfing mode, but seed production gradually increased during the flower life-span, suggesting that competing selfing and delayed selfing may also contribute to autonomous selfing. Bagged flowers set as many seeds as flowers hand-pollinated with outcross pollen or self-pollen, and thus the population shows a high capacity for autonomous selfing. Emasculated flowers open to visitation by pollinators produced almost no seed, showing that the pollinator-visitation rate was low, and that the majority of the seeds in intact flowers resulted from self-fertilization.Conclusions – The findings show that under low pollinator availability, preanthesis autogamy is likely to be selectively advantageous as a reproductive assurance mechanism in a subalpine population of Prunella vulgaris