Losing a Scent of One’s Self: Is There a Reduction in Floral Scent Emission in Self-Pollinating Phlox cuspidata versus Outcrossing Phlox drummondii?

Abstract

Premise of research. Studies of the evolution of self-pollination have generally predicted reductions in pollinator-attractive morphological floral traits over time as a result of relaxed pollinator-mediated selection and/or positive selection associated with improved self-pollination. Nonmorphological traits, such as floral scent, may also become reduced in self-pollinating taxa due to relaxed pollinator-mediated selection. However, evidence of such reductions in floral scent of selfing versus outcrossing taxa is limited. We explored this possibility in the scent of the predominantly selfing taxa Phlox cuspidata and its close outcrossing relative Phlox drummondii.Methodology. Scent was collected and analyzed from four populations of each plant species using dynamic headspace extraction and gas chromatography-mass spectroscopy. Scent emission rates were compared between species and among populations nested within species via MANOVA.Pivotal results. Of the 37 scent variables examined, only three showed significant differences between species. However, all three were emitted in decreased quantities from selfing P. cuspidata.Conclusions. While our findings provide some support for reductions of scent emission in selfing versus outcrossing annual Phlox, the retention of substantial scent production by P. cuspidata suggests continued dependence on pollinators (for either outcrossing or facultative selfing). This is further supported by maintenance of a pollen-ovule ratio similar to outcrossing P. drummondii. The retention of significant terpenoid emission alternatively indicates potential defensive functions for floral scent, as these compounds are known to have antiherbivore and antipathogen properties. Future research should examine both possibilities.