THE EFFECT OF INVESTMENT IN ATTRACTIVE STRUCTURES ON ALLOCATION TO MALE AND FEMALE FUNCTIONS IN PLANTS

Expressions for male and female fitnesses of partially self-fertilizing cosexual plants are derived, assuming that allocation to pollinator attraction at the time of flowering may decrease resources available for male and female primary structures. The total female fertility is assumed to be controlled by factors at two stages, flowering-time and fruiting-time, with resources for fruit maturation being limited so that maximum seed production may be limited by the availability of these resources. The fitness formulas are used to calculate ESS (evolutionarily stable strategy) allocations at flowering time to primary male and female sex functions and to attractive structures. These are compared with some data that are available for dry weights of different flower parts. The fitnesses of unisexual mutant forms are calculated, assuming that they are introduced into a population consisting mostly of the initial cosexual form and that they obey the same gain curves as that form. When compared with the fitness of the ESS cosexual form, this enables one to ask whether unisexual forms will be favored. We show that the spread of females is unlikely, unless there is high inbreeding depression and a rather high selfing rate, and that in some circumstances a linear relation between number of fertilized ovules and number of seeds matured can be less favorable for the invasion of females than is a highly concave relation. With a nearly linear relation between numbers of fertilized ovules and mature seeds, invasion by females is more likely when investment in attraction is low than when it is high. These effects are discussed in relation to the distribution of dioecy. The spread of male mutants is never likely in these models.