1. Unreliable pollinator service is thought to promote the evolution of self-compatible plant breeding systems, because selfing may provide reproductive assurance when outcrossing opportunity is limited. The recurrent evolution of self-compatible homostyly from obligately outcrossing heterostylous species has been regarded as a classic example of evolutionary response to lack of pollinators or mates, as homostyly frequently occurs in pollinator-limited or marginal environments. However, male and female sexual organs of homostylous species may display spatial separation (herkogamy), an arrangement presumed to promote outcrossing. It is largely unknown to what extent variation in herkogamy affects opportunities for autonomous selfing and reproductive assurance in self-compatible, homostylous species.
2. Using the homostylous Primula halleri, restricted to alpine environments, we investigated whether herkogamy occurs and varies during anthesis, among individuals, and populations, and compared the effects of herkogamy on seed set among three experimental treatments, to elucidate how herkogamy affects reproductive strategies in a homostylous species.
3. Herkogamy decreases during anthesis, but the ultimate expression of herkogamy in mature flowers differs among individuals and populations. Caging caging experiments indicate that herkogamy reduces a plant's potential for autonomous selfing, and emasculation and open-pollination treatments demonstrate that herkogamy markedly decreases total seed set and the potential for reproductive assurance.
4. Herkogamy early in anthesis may enhance outcrossing potential, while its decrease later could enable reproductive assurance via delayed autonomous selfing in some, but not all plants. Conversely, pronounced herkogamy in older flowers comes at the cost of reduced total reproductive output and imposes pollinator dependence for reproduction, but may promote the genetic diversity of populations.
5. Our study suggests that even small amounts of herkogamy can have large effects on the reproductive strategy of homostylous species, by enabling more outcrossing than generally thought to be typical of homostyly.