Reproductive assurance for a rewardless epiphytic orchid in Puerto Rico:Pleurothallis ruscifolia(Orchidaceae, Pleurothallidinae)

Abstract

Most angiosperms sustain pollinator visits by offering a reward, such as nectar or pollen, yet there are plants that do not offer rewards and instead depend on deception for successful pollen transfer. Unless rewardless plants have an extremely efficient means of deceit or are autogamous, they tend to experience less fruit set than those that offer pollinator rewards. In Puerto Rico, we studied the reproductive biology of a rewardless epiphytic orchid, Pleurothallis ruscifolia (Pleurothallidinae), a widespread Neotropical species. The Pleurothallidinae are primarily myophilous, self-incompatible, and pollinator dependent for fruit set and seed production. However, because our preliminary observations indicated that populations of P. ruscifolia on the island of Puerto Rico exhibited an unusually high fruit set, we hypothesized that our population is autogamous and that reproductive effort and fruiting success are resource limited. We monitored 168 plants in a single population during the 2009 reproductive season to determine the mating system of P. ruscifolia while looking for evidence of reproductive constraints. Fruit set in our population was high (66%) and no floral visitors were seen. Fruit set for our pollinator exclusion experiment was no different than fruit set for open pollinated plants. Most plants produced both chasmogamous and cleistogamous flowers, the majority of fruits produced by the latter. Furthermore, reproductive effort and fruiting success were limited by plant size, which is likely the result of resource constraints. We expect that autogamy is prevalent in P. ruscifolia of the West Indies, but outcrossing should be dominant in continental populations. Selection for reproductive assurance in the absence or rarity of an effective pollinator is the likely process leading to this autogamous system.