The role of multitrophic interactions for mating system expression in Ruellia humilis

Background/Question/Methods

Plants interact with a multitude of species.  However, we lack understanding of how multiple ecological interactions jointly influence the expression and evolution of plant mating systems.  We examined how two important guilds of plant-insect interactions, predispersal seed predators and their parasitoids, jointly influence the mating system and reproductive success of Ruellia humilis (fringeleaf wild petunia).  Ruellia humilis has an obligate mixed mating system by producing both cleistogamous (obligately selfing) and chasmogamous (facultatively outcrossing) flowers on an individual plant.  Over two years, we conducted observational experiments in a natural population of R. humilis to address the following questions: (1) Is there temporal variation in plant-seed predator-parasitoid interactions? (2) Does the frequency of seed predation and parasitism differ between cleistogamous and chasmogamous fruits? (3) What are the fitness consequences of seed predation and parasitism? (4) How do plant mating system and reproductive traits influence seed predation rates? 

Results/Conclusions

Fruits of R. humilis are attacked by larvae of a tortricid moth that feeds on seeds before fruits are dispersed.  Adult moths oviposit on recently formed fruits and larvae develop inside the fruit, consuming all of the seeds.  Across two years of study in one R. humilis population, this seed predator attacked 47-67% of fruits. On average, seed predation reduced total seed production per plant by 50%.  Approximately 10% of fruits attacked by seed predators contained parasitic wasps (Bracon sp.) that kill the seed predator.  Parasitism resulted in one seed per fruit being saved from predation.  Additionally, we found that the seed predators preferentially feed on chasmogamous fruits.  Ruellia humilis plants that reproduce proportionally more through chasmogamy have higher rates of seed predation than plants that reproduce proportionally more through cleistogamy. In total, our findings demonstrate that seed predators and their parasitoids affect plant mating system expression and thus should be considered a factor shaping mating system evolution.