The role of predispersal seed predators and their parasitoids for mating system expression in Ruellia humilis
Plants interact with a multitude of species. However, we have a poor understanding of how multiple ecological interactions jointly influence the expression and evolution of plant mating systems. I examined how predispersal seed predators and their parasitoids jointly influence the mating system and reproductive success of Ruellia humilis. Ruellia humilis has an obligate mixed mating system by producing both cleistogamous (obligately selfing) and chasmogamous (facultatively outcrossing) flowers on an individual plant. Fruits of R. humilis are attacked by Tripudia rectangula larvae that feed on seeds before fruits are dispersed. Adult moths oviposit on recently formed fruits and larvae develop inside the fruit, consuming all of the seeds. In addition, T. rectangula is attacked by parasitic wasps (Bracon sp.). I conducted observational experiments in natural populations of R. humilis in Oklahoma to address the following questions: (1) Is there spatial variation in plant-seed predator-parasitoid interactions? (2) Does the frequency of seed predation and parasitism differ between cleistogamous and chasmogamous fruits? (3) How do plant mating system and reproductive traits influence seed predation rates?
I found substantial spatial variation in T. rectangula attack rate. Across multiple R. humilis populations in Oklahoma, seed predator attack rate varied 10-67%. Approximately 10% of fruits attacked by seed predators contained parasitic wasps (Bracon sp.) that kill the seed predator. T. rectangula preferentially fed on chasmogamous fruits. Ruellia humilis plants that reproduced proportionally more through chasmogamy had higher rates of seed predation than plants that reproduced proportionally more through cleistogamy. In total, these findings demonstrate that seed predators and their parasitoids affect plant mating system expression and thus should be considered a factor shaping evolution of this trait.