Beyond Simple Reproductive Assurance: Cleistogamy Allows Adaptive Plastic Responses to Pollen Limitation

Historically, the persistence of mixed mating (reproduction via both self- and cross-fertilization) has presented a puzzle because classic theory predicts that mixed mating should be evolutionarily unstable. One mechanism that could contribute to the maintenance of mixed mating in cleistogamous species is the ability to invest in the appropriate type of reproduction (outcrossing vs. selfing) for a given pollination environment. We tested whether Collomia grandiflora plants responded to pollen limitation by plastically shifting relative investment in cleistogamous (obligately self-fertilizing) versus chasmogamous (potentially outcrossing) reproduction. We estimated reproductive effort (seed production) for chasmogamous and cleistogamous reproductive modes in hand-pollinated and unpollinated plants. We also investigated whether plastic responses to pollen limitation could be constrained by water availability or early nutrient fertilization. Pollen limitation significantly increased cleistogamous reproductive effort, and responses were consistent across gradients of water availability and soil fertility. Our results suggest that dimorphic cleistogamy can provide more than a fixed level of reproductive assurance, as plasticity in allocation to cleistogamy versus chasmogamy can simultaneously enable outcrossing when pollinators are abundant and increase selfing when pollinators are scarce. We argue that such plasticity could be a factor in the evolutionary maintenance of mixed mating in cleistogamous plants.