Relative energetic economy of cleistogamous selfing in three populations of the perennial Ruellia humilis

Premise of researchWhat maintains mixed selfing and outcrossing is an enduring mystery in evolutionary biology. Cleistogamy, where individuals produce both potentially outcrossing chasmogamous and obligately selfing cleistogamous flowers, provides an ideal framework in which to study the evolutionary forces maintaining mixed-mating. Despite their promise for providing insight into mating system evolution, few studies of cleistogamous species have comprehensively investigated the relative costs and benefits of cleistogamous selfing. MethodologyWe quantified the necessary components to calculate the relative energetic cost of reproduction of the different flower types in three natural populations of the perennial Ruellia humilis Nutt (Acanthaceae). These components include flower dry mass, fertility (fruit set), and seed mass and number per fruit. We also measured pollen-ovule ratios for both flower types as another measure of relative energetic investment. We additionally tracked phenology of the two flower types and used the proportion of chasmogamous flowers to estimate maximum potential outcrossing rates. Pivotal ResultsWe found that the energetic cost of reproduction via cleistogamous flowers was about 4-10 times less than that of reproduction via chasmogamous flowers, and this energetic economy was from both reduced mass and increased fertility of cleistogamous flowers. Pollen ovule ratios in cleistogamous flowers were one third to one half those of chasmogamous flowers, providing additional support for their energetic economy. Maximum potential outcrossing rates in these populations based on chasmogamous flower production were between 43-61%, but chasmogamous flowers can autogamously self at rates of 33-75%. ConclusionsThe relative energetic economy of cleistogamous flowers suggest that progeny from chasmogamous flowers would have to have 4-10 times greater relative fitness to explain their evolutionary maintenance. These values are likely even greater considering the reduced investment in pollen production in cleistogamous flowers. Ongoing work will quantify potential advantages of chasmogamous flowers due to inbreeding depression and heterosis.