Sex allocation in hermaphrodites with partial overlap in male/female resource inputs

Abstract

In most plants individuals function as both male and female parents (hermaphrodite, monoecious) rather than as separate sexes (dioecious). Charnov et al. (1976) suggested that hermaphroditism would be favored over dioecy when reproduction through pollen and seed occurred at different times during a season or required different resources, because an hermaphrodite could make use of resources not available to an individual of a single sex. Here we use resource allocation models and the tools of mathematical game theory to explore these ideas explicitly. Model 1 considers a plant in which pollen and ovule production and pollination take place early in the reproductive season, followed by a period of seed and fruit maturation. We show that when differential timing of sex function translates into differences in the resource pools available to pollen and seed, hermaphroditism is indeed the optimal breeding system in that it is evolutionarily stable to invasion by individuals of a single sex. We determine the optimal allocation of resources to the two sex functions, and show that when seeds are costly relative to ovules the ratio of investment to male vs female functions will be female biased. This is often the case in plants. We discuss the importance of differential timing of sex functions in annuals and perennials. In Model 2, we show that if the constituents of pollen and seed differ in two (or more) limiting resources, hermaphroditism is again favored. To date, there is little evidence that pollen and seeds differ in their biochemical composition but few studies have addressed this point.