Floral scent may vary across reproductively isolated populations, between sexes, and over the course of the day. Schiedea globosa sequentially colonized the Hawaiian Islands and exhibits wind pollination and low gene flow between islands. We hypothesize that scent divergence is proportional to genetic and geographic distance. Sexual dimorphism in floral scent may exist if the sexes experience differential selection and sex expression is linked directly or indirectly to scent production. Species that recently evolved wind pollination from biotic pollination may either retain changes between day and night in floral scent used to attract nighttime pollinators, or the pattern may be lost in the absence of biotic selection.
We tested these hypotheses in twelve populations (four islands) of the subdioecious subshrub Schiedea globosa. Floral volatiles were collected using a dynamic headspace technique from inflorescences of greenhouse-grown plants (females, males and hermaphrodites) during the day and subsequent night and analyzed by GC-MS. Differences in volatiles among groups were examined with canonical analysis of principal coordinates. Genetic distance among populations was calculated using public sequences for two nuclear and one plastid locus. Positive correlations of scent dissimilarity among populations and geographic or genetic distance were tested with Mantel tests.
Results/Conclusions
Scent differed quantitatively among populations, but there were mixed results with how this variation corresponded with genetic and geographic distance. Divergence in scent composition increased with genetic distance for males for both day and night periods, but this pattern did not hold for females. Divergence in scent composition increased with geographic distance only for females during the day. Scent composition and total emission rates varied between sexes. Females emitted less total scent per flower but more 4-methylpentan-1-ol than males, and males emitted more ketones and aldoximes than females.
Despite morphological adaptations for wind pollination in S. globosa and presumed loss of biotic selection, scent composition (relative emission rates of all compounds) and total emissions varied consistently from day to night among plants and populations. At night flowers increased total emission rates through enhanced emissions of several ketones, nitrogen-bearing oximes, and phenylacetaldehyde. Surprisingly, few of these compounds that increase during the night were present in two close relatives that are moth-pollinated, and so the function of these novel volatiles is unknown.