Chemical communication is ubiquitous. The identification of conserved structural elements in visual and acoustic communication is well established, but comparable information on chemical communication displays (CCDs) is lacking. We assessed the phenotypic integration of CCDs in a meta-analysis to characterize patterns of covariation in CCDs and identified functional or biosynthetically constrained modules. Poorly integrated plant CCDs (i.e. low covariation between scent compounds) support the notion that plants often utilize one or few key compounds to repel antagonists or to attract pollinators and enemies of herbivores. Animal CCDs (mostly insect pheromones) were usually more integrated than those of plants (i.e. stronger covariation), suggesting that animals communicate via fixed proportions among compounds. Both plant and animal CCDs were composed of modules, which are groups of strongly covarying compounds. Biosynthetic similarity of compounds revealed biosynthetic constraints in the covariation patterns of plant CCDs. We provide a novel perspective on chemical communication and a basis for future investigations on structural properties of CCDs. This will facilitate identifying modules and biosynthetic constraints that may affect the outcome of selection and thus provide a predictive framework for evolutionary trajectories of CCDs in plants and animals.

Summary 1.Patterns of floral scent are generally assumed to have been shaped by pollinator-mediated natural selection. However, while many studies document behavioural responses of pollinators to floral scent, few document the relationship between floral scent and fitness. 2. In this study, we explore the effect of variation in floral scent emission in colour polymorphic Hesperis matronalis on both pollinator visitation and seed fitness. 3. Using target inflorescences augmented with colour-specific floral scent extracts, we find that diurnal floral visitors significantly prefer night-scent extracts to non-augmented controls; inflorescences augmented with day-scent extracts receive an intermediate level of visits. Colour did not have a significant effect on visitation. 4. We characterized the relationship between natural variation in floral scent emission rate and seed production for plants under two settings: in small experimental arrays exposed to either dayor night-flying pollinators, and in wild populations exposed to all pollinators. In arrays, we found greater emission rate led to higher seed production, but only in plants exposed to day-flying pollinators. In contrast, we found a significant positive relationship between night-time floral emission rate and seed fitness in wild populations. In neither setting did floral anthocycanin concentration (colour) affect fitness. 5. This study reinforces the idea that scent-mediated pollinator visitation is an important component of plant fitness. Moreover, our results suggest that plants emitting more scent have higher fitness, although there is variation as to when this positive relationship occurs (i.e., at day or night). Research connecting floral scent and fitness is a necessary first step in understanding the evolution of floral scent.

Shared biochemistry alone cannot explain the variation in floral scent phenotype found for H. matronalis. Such a result may suggest that the biochemical association between floral scent and floral colour is complex or dependent on genetic background. Floral scent does vary significantly with population membership; several factors, including environmental conditions, founder effects and genetics, may account for this differentiation and should be considered in future studies.

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