Female compensation through the quantity and quality of progeny in a gynodioecious plant, Geranium maculatum (Geraniaceae)

One of the major evolutionary trends in flowering plants is the evolution of unisexual flowers (male or female) from perfect flowers. This transition has occurred repeatedly in many taxa and has generated a wonderful array of variation in sexual expression among species. Theoretical studies have proposed a number of mechanisms to explain how this level of variation could be maintained in natural systems. One possible mechanism is the female compensation hypothesis, which predicts that female mutants require an increase in their seed fitness in order to invade a hermaphroditic system. Using Geranium maculatum, I tested this hypothesis and showed that female mothers produced more and larger seeds than hermaphroditic mothers even though they were indistinguishable in their vegetative traits and the flower production. Seeds from females were also more likely to germinate and produced seedlings with larger above- and belowground biomass. These seedlings were more likely to flower than those from hermaphrodites in at least one of the two populations studied. Combined, these results indicated that females in G. maculatum did compensate for their loss of male function by producing more and better seeds than hermaphrodites. This provides a mechanism for the maintenance of female plants in this species.