What Factors Maintain the Mixed Mating System of a Local Annual Plant, Triodanis perfoliata?

Abstract

A plant capable of reproducing by cross- and self-fertilization is known to have a mixed mating system. This system is an evolutionary puzzle to many researchers because the factors maintaining both outcrossing and selfing are not fully understood. Cleistogamy, a form of mixed mating, describes the production of both open (chasmogamous, CH) flowers that can be cross-pollinated and closed (cleistogamous, CL) flowers that are obligately self-fertilizing. Studies show that inbreeding depression, a negative consequence of selfing, is present in low quantities in CL offspring resulting in slightly reduced fitness relative to CH offspring. The minor fitness benefit of CH offspring cannot negate the advantage of producing low cost CL flowers that have guaranteed seed production. This prompts the question: why do plants continue to make costly, larger CH flowers? Heterosis, the increased fitness of offspring from crosses between different populations or species, may be a key factor in maintaining CH flowers. Heterosis can occur when populations are fixed for different mildly deleterious recessive alleles. The offspring of crosses between them will be heterozygous and show higher fitness. Since heterosis occurs through outcrossing, only CH flowers can benefit from this effect. We test the hypothesis that heterosis provides an advantage to CH flowers in Triodanis perfoliata, a weedy cleistogamous annual plant. In a growth room experiment, we hand-pollinated plants of three populations of T. perfoliata to generate offspring from self-fertilized CH flowers, within-population crosses, and between population-crosses with two populations. We also collected seeds from CL flowers to compare CL and selfed CH offspring to determine if the allocation of resources by the maternal plant was different for each flower type. Lifetime fitness was quantified by measuring seed germination, seedling survivorship, and plant biomass. On average across the three populations the CL offspring had 21.9% lower cumulative fitness than selfed CH offspring indicating a slight flower effect. Cumulative fitness of selfed CH offspring were 23.7% lower than outcrossed CH offspring due to inbreeding depression. Finally, no evidence of heterosis was found as the between-population crossed offspring were 10% lower in fitness than the within-population crossed offspring. These results indicate that heterosis cannot explain the maintenance of CH flower in Triodanis perfoliata.