Understanding intraspecific variation of floral scent in light of evolutionary ecology

Abstract

Background and aims: Among the various floral traits involved in pollinator attraction and potentially under selection mediated by pollinators, floral scent/fragrance has been less investigated than other components of floral phenotype. Whether or not pollinator-mediated selection impacts floral scents depends on the heritability of scent/fragrance and the occurrence of some variation within species. Although most studies have investigated how scent varies among species, growing amounts of data are available on variation at the intraspecific level.

Methods: The results of 81 studies investigating intraspecific variation of floral scents in 132 taxa were reviewed. For each study, whether variation was found in either identity, proportion or absolute quantities of volatile organic compounds (VOCs) was recorded, as well as information with the potential to explain variation, such as methodology, plant origin or pollination biology.

Key results: Variation was found for almost all investigated species, both among individuals (among and sometimes within populations) and within individuals across different temporal scales. Cases in which such variation is a possible result of pollinator-mediated selection were analysed, by discussing separately selection related to variation in pollinator identity/behaviour among populations or across time, deceit pollination and sex-specific selection. Not surprisingly, in many cases, pollinator-mediated selection alone does not explain the observed variation in floral scent. This led us to review current knowledge on less investigated factors, such as selection mediated by natural enemies, genetic drift and gene flow, environmental constraints, phylogenetic inertia, or biochemical constraints that could be invoked to explain scent variation.

Conclusions: This review highlights the great potential of analysing floral scent variation and including it in integrated studies of floral phenotypes. We also have identified the current gaps in our understanding of this complex signal and we propose several methodological and conceptual future directions in this research area.

Keywords: Pollination; chemical ecology; evolution; intraspecific variation; odour; reproductive strategies.

Fig. 1.

Schematic representation of the various scales of floral scent variation and the main ecological and evolutionary processes associated. (A) Variation in floral scents among individuals can occur both within and among populations (solid circles). Within populations, variation can occur between groups of plants (e.g. sexual types, colour phenotypes, dashed circles) or among equivalent individuals. (B) Variation in floral scents within individuals occurs at several time scales: across flower ontogeny, between day and night and along a diel period. In the box are listed the main processes involved in variation of floral scents: (1) Pollinator-mediated selection can generate variation in floral scents (1a) if divergent selection occurs, either because the cost–benefit balance of the interaction with the same pollinator species varies (between plant genders ot between plant phenological stages) or because they interact with different pollinator species (that can vary both among populations and along with time); (1b) if pollination is achieved by deceit, since pollinators can cause variation through balanced selection. (2) If the identity, occurrence or effect of herbivores varies among populations or across time, herbivore-mediated selection can generate variation in floral scent. (3) The balance between the effects of genetic drift and gene flow impact variation among populations, particularly for biologically inactive volatile compounds. (4) The variation in some environmental components that affect the functioning of metabolic pathways or compound release (e.g. temperature, humidity or soil) should cause some variation in floral scents. (5) Finally, biochemical processes may explain some of the observed variations both within individuals (depending on the metabolic rhythms) and between individuals (of different colour).