Floral traits and autonomous seed set across populations of a mixed-mating species

1. Background & Objectives: Floral traits impact plant mating systems. When traits are correlated, they may facilitate or constrain the evolution of floral morphology and mating systems. In mixed-mating systems, evolution is difficult to project due to self-fertilization, which leads to conflicting outcomes of reproductive assurance and inbreeding depression. Because self-fertilizing reproductive strategies are typically accompanied by a characteristic suite of floral traits, studying the floral morphology of mixed-mating species can be a useful way of predicting whether a population is maintaining outcrossing habits, or whether it may be evolving selfing habits. The two questions I address in this study are: 1) Are floral traits and their correlations conserved across populations of a mixed mating species? and 2) Is herkogamy correlated with autonomous seed set?
2. Methods: Seeds of the mixed-mating plant, Mimulus ringens, were collected from five natural populations around Virginia. Plants came from populations of different sizes and habitats. Seeds were planted and phenotypes were measured in the greenhouse to minimize environmental effects. Floral characteristics and autonomous seed set were measured and compared across populations.
3. Results: Floral traits were significantly correlated within populations, and the strength of correlations differed among populations. Overall flower size and shape varied among populations, but most traits maintained patterns of association. Neither flower size nor herkogamy explained variation in autonomous seed set among individuals.
4. Synthesis: Despite divergence in floral traits among populations, most traits retained their associations, suggesting that evolution of individual traits may be constrained by genetic relationships. The lack of relationship between herkogamy and autonomous seed set suggests a high cost to self-fertilization. The evolution of M. ringens mating systems should therefore depend more on the relationship between floral traits and facilitated self-fertilization rather than that between floral traits and autonomous selfing. While pollinators are active and mating opportunities are available, M. ringens should maintain a mixed-mating system, as would be predicted with a need for reproductive assurance but a cost to self-fertilization.