Reduced pollination in bilateral flowers could reflect selfing avoidance

Abstract

The multiple evolutionary transitions from radial to bilateral flower symmetry recorded across the angiosperms have been related to pollinator specialization and the promotion of outcross pollen transfer. In particular, the limited number of physical orientations associated with bilaterality results in a tighter morphological match between a flower and its specialized pollinators, which can enhance the transfer of incoming pollen while preventing autonomous, or even pollinator-mediated self-pollination when associated with dichogamy. In contrast, radial flowers have various planes of symmetry, which may allow pollination by a wider range of pollinators at the cost of more promiscuous pollen transfer. Here we used data from the literature and our own on pollen production, ovule production, and stigmatic pollen load compiled for 171 angiosperm species distributed worldwide to assess differences in pollen transfer in relation to flower symmetry within a phylogenetic comparative framework. We did not find evidence that bilateral and radial flowers differ in average pollen and ovule production. However, bilateral flowers tended to receive less pollen on their stigmas. This decrease in pollen receipt was most apparent among self-compatible species after accounting for the covarying effects of flower size and ovule production. Therefore, our findings indicate that the evolution of flower bilaterality, a trait we found is strongly associated with self-compatibility, comes at a quantitative pollination cost. However, they support the view that a bilateral flower morphology may provide the benefit of limiting self-pollen deposition, particularly in the absence of an alternative mechanism of selfing avoidance.