Mating consequences of contrasting hermaphroditic plant sexual systems

For hermaphroditic angiosperms with multiple flowers, the sex roles can be exclusively combined in bisexual flowers (monocliny), strictly separated among different flowers (monoecy), or arrayed in mixtures of bisexual flowers with female flowers (gynomonoecy) or male flowers (andromonoecy). The hypothesized benefits favoring the evolution of these contrasting hermaphroditic sexual systems are typically examined individually, usually by assessing success through only one sex role. We tested predictions of most hypotheses experimentally with an andromonoecious species, Anticlea occidentalis (Melanthiaceae), based on the performance of intact plants (andromonoecy) and those with emasculated bisexual flowers (functionally monoecious) or emasculated male flowers (functionally monoclinous with sterile peripheral flowers). Andromonoecy in this species enables efficient, size-dependent resource allocation, emphasizing female function in large plants. Emasculation revealed that anthers in male flowers promote female mating quality (outcrossing rate and mate diversity), whereas anthers in bisexual flowers promote male mating quantity (pollen dispersal distance and probability of any siring success). Thus, different hermaphroditic sexual systems likely evolve to capitalize on suites of benefits, rather than just one, and provide compromises between quantitative and qualitative reproductive components. These compromises apparently maximize an individual's combined genetic contributions through female and male functions, rather than separate contributions through each sex role.