A phylogenetic analysis of pollination mode and the evolution of dichogamy in angiosperms

Dichogamy, the temporal separation of male and female function, is widespread among angiosperms, yet its causes and consequences are not well understood. Two forms of dichogamy exist: protandry, in which pollen dispersal precedes stigma receptivity, and protogyny, in which the reverse occurs. Species-level comparative studies show that protandrous species tend to be pollinated by bees or flies, whereas protogynous species tend to be windor beetle-pollinated. This suggests a functional role for pollinators in the evolution of dichogamy. We mapped dichogamy and pollination characters onto a phylogeny of angiosperm species. Using the program Discrete, two models of evolutionary change (one allowing only for independent evolution and the other allowing correlated evolution of the two traits) were fit to the phylogeny to test for correlations. Log-likelihood ratio tests and Monte Carlo simulations support a correlated model for the evolution of the type of dichogamy and the form of pollination, demonstrating that the previously reported correlations are robust to phylogenetic correction. However, pollination mode was not found to affect transitions between protandry and protogyny. Rather, an examination of transition rates revealed that the rate of transitions between biotic and abiotic pollination depends on whether a species is protandrous or protogynous. Additionally, we found more support for a role of pollination in the evolution of protogyny from non-protogyny than in the evolution of protandry from non-protandry. This study calls into question some previous findings regarding the role of pollinators in the evolution of dichogamy.