A ghost of dioecy past and the legacy of sexual dimorphism: low siring success of hermaphrodites after the breakdown of dioecy

Abstract

Evolutionary transitions from dioecy to hermaphroditism must overcome the inertia of sexual dimorphism because modified males or females will express the opposite sexual function for which their phenotypes have been optimized. We tested this prediction by comparing the siring success of female-derived hermaphrodites of the plant Mercurialis annua with males and hermaphrodites that present a male-like inflorescence. We found that pollen dispersed by female-derived hermaphrodites was about a third poorer at siring outcross offspring than that from hermaphrodites with male-like inflorescences, illustrating the notion that a ghost of dioecy past compromises the fitness of derived hermaphrodites in outcrossing populations. We conclude that whereas dioecy might evolve from hermaphroditism by conferring upon individuals certain benefits of sexual specialization, reversals from dioecy to hermaphroditism must often be limited to situations in which outcrossing cannot be maintained and inbreeding is favored. Our study provides novel empirical support for evolutionary models for the breakdown of dioecy.