Mechanisms of floral scent production and hawkmoth pollination in Clarkia breweri (Onagraceae).

Abstract

Floral scent is important to plant-pollinator interactions, functioning as an attractant for diverse pollinator classes. The mechanisms of floral scent inheritance and emissions are poorly understood, as are the responses of pollinators to floral scent variation. These questions were examined in Clarkia breweri (Onagraceae), C. concinna (a closely-related species) and their hybrid plants. The flowers of C. breweri produce a strong, sweet floral scent and are pollinated mainly by hawkmoths, which are novel traits in Clarkia. The "scentless" flowers of C. concinna are pollinated by bees, butterflies and flies. The floral scent of C. breweri was found to comprise 8 to 12 terpenoid and aromatic compounds. Linalool and the aromatic compounds are emitted by the petals, whereas the linalool oxides are emitted by the style tissues. Flowers of C. concinna, the putative ancestor of C. breweri, emitted 90-300 fold lower amounts of linalool and its oxides from style tissues. Hawkmoths (Hyles lineata; Sphingidae) were found to be the most effective pollinators of C. breweri in natural populations in central California, depositing three times more pollen per visit than hummingbirds or noctuid moths. H. lineata's electroantennogram responses to floral scent compounds from C. breweri revealed that they detected most of these compounds at nanomolar concentrations and were most responsive to benzyl acetate, linalool, the linalool oxides and methyl salicylate. Genetic studies of C. breweri $\times$ C. concinna hybrids demonstrated that linalool and its oxides are inherited as simple, dominant mendelian traits, but the amount of compound produced is under polygenic control. The emissions of linalool oxides and trans-B-ocimene were significantly correlated with length of the style, their site of synthesis. The inheritance of aromatic scent compounds was complex and likely due to epistatic interactions. It is concluded that high levels of monoterpenoid scent production in C. breweri probably evolved gradually via the regulatory modification of a biochemical pathway present in C. concinna, while the origin of aromatic volatile emissions in C. breweri is not yet clear. The results of this study support the hypothesis that floral scent production could have evolved in C. breweri as part of an evolutionary shift to hawkmoth pollination.