The Role of Geitonogamy in the Gradual Evolution to Dioecy in Cosexual Plants

We present a model for the gradual evolution towards dioecy in cosexual plants with geitonogamous selfing. We show how geitonogamous selfing (i.e., transfer of pollen between flowers on the same plant) can facilitate the evolution of dioecy (i.e., separate male and female individuals) in cosexual plants (i.e., both sexual functions on the same plant). We study the effect of parameters such as inbreeding depression, the attraction costs per flower, and the total amount of resources available per plant. We also consider different flower architectures (limited versus unlimited potential number of seeds per flower) and pollination biologies (biotic versus abiotic). We find that (1) if there is no maximum to the number of seeds per flower, then cosexuality is evolutionarily stable whenever the inbreeding depression is less than one-half. With abiotic pollination and an inbreeding depression greater than one-half, dioecy evolves via evolutionary branching, that is, through the gradual differentiation towards male and female plants, but only after the population has first evolved to a cosexual strategy with an intermediate sex ratio. The evolution of dioecy requires higher levels of inbreeding depression if pollination is by insects, but is facilitated by increasing the total amount of resources available per plant. (2) If the potential number of seeds per flower is limited, we get basically the same results as with an unlimited seed number per flower, but the outcome now also depends on the attraction costs per flower. With high attraction costs, the population can evolve to gynodioecy (females and cosexuals in the same population). Further increasing inbreeding depression leads to dioecy. Our results give a possible explanation of Darwin’s observation that dioecy is more common in plant species with abiotic pollination and in large species with many flowers such as trees.