Evolution of hierarchical floral resource allocation associated with mating system in an animal-pollinated hermaphroditic herb, Trillium camschatcense (Trilliaceae)

Flowers of selfing plants generally have smaller attractive structures than those of outcrossers. This phenomenon has been explained by sex-allocation theory, which considers allocation of resources to different floral functions, yet its hierarchical structures have mostly been ignored in previous studies. By assuming a three-level hierarchy, we investigate the relationships between floral resource allocation and mating system (partial selfing or obligate outcrossing) in 13 Trillium camschatcense populations from northern Japan. In general, selfing populations had smaller petals than outcrossing populations, and multiple levels in the hierarchy were essential to explain these differences in petal size. Within flowers, selfing plants allocated fewer resources to petals and more to pistils, supporting the prediction of sex-allocation theory. Moreover, selfing plants tended to increase their flower number and decrease investment per flower under the constraints of nonlinear trade-offs between these traits. Selfers may produce more flowers to enhance their female reproductive success under the nonlinear trade-offs in which more resources can be used by increasing flower number. Although our results suggest that the evolution of resource allocation associated with selfing can occur at several hierarchical levels, these allocation patterns varied widely among selfing populations and recent evolution of self-compatibility may explain this variation.