The effect of population sex ratio on male and female fitness in the gynodioecious geranium, Geranium maculatum

Background/Question/Methods

Although most plants are hermaphroditic, with both male and female function within the same flower, there are also many species that partition male and female function differently either within or among individuals. A major transition of sexual function in angiosperms is the evolution from hermaphroditism to dioecy, a system with separate male and female individuals. One possible intermediate between these extremes is gynodioecy, a breeding system in which female and hermaphroditic individuals coexist within populations. We tested several expectations arising from this predicted pathway using experimental populations. First, we expected that selection would favor “more male” hermaphrodites because they can gain more total fitness by increasing male fitness rather than female fitness. Second, we expected that this selection will be stronger as the population female frequency increases, which may lead to the gradual evolution of dioecy. To evaluate these expectations, we determined the male and female fitness for plants placed in experimental populations of varying sex ratios. Selection on several floral traits was compared within and among experimental populations to determine whether selection differed between hermaphrodites and females and among populations of different sex ratios. 
Results/Conclusions

Preliminary results suggest that there may be selection for more flowers but lower seed production in hermaphrodites. This may support the expectation that selection favors “more male” hermaphrodites leading to the evolution of separate sexes.