Macroevolution of floral scent chemistry across radiations of male euglossine bee-pollinated plants

Floral volatiles play key roles as signaling agents that mediate interactions between plants and animals. Despite their importance, few studies have investigated broad patterns of volatile variation across groups of plants that share pollinators, particularly in a phylogenetic context. The “perfume flowers”, Neotropical plant species exhibiting exclusive pollination by male euglossine bees in search of chemical rewards, present an intriguing system to investigate these patterns due to the unique function of their chemical phenotypes as both signaling agents and rewards. We leverage recently-developed phylogenies and knowledge of biosynthesis along with decades of chemical ecology research to characterize axes of variation in the chemistry of perfume flowers, as well as understand their evolution at finer taxonomic scales. We detect pervasive chemical convergence, with many species across families exhibiting similar volatile phenotypes. Scent profiles of most species are dominated by compounds of either the phenylpropanoid or terpenoid biosynthesis pathways, while terpenoid compounds drive more subtle axes of variation. We find recapitulation of these patterns within two independent radiations of perfume flower orchids, in which we further detect evidence for rapid evolution of divergent floral chemistries, consistent with the putative importance of scent in the process of adaptation and speciation.

Methods
We built a database of floral perfume chemical composition, as well as pollinator identity, for any angiosperm pollinated by perfume-collecting male euglossine bees. Both published and unpublished data from the literature and from collaborators were used to build this database. For the published data, a literature search in ISI Web of Science and Scopus using the following search terms was conducted: (“scent plant” OR “perfume plant” OR “VOC” OR “volatile” OR “scent reward* plant” OR “perfume reward* flower” OR “scent reward* flower” OR “perfume reward* flower) AND (eugloss* OR “orchid bee”). In addition to this search, we screened the reference list of all obtained articles to check for works not obtained from the literature search. Only studies using headspace analyses were used. Following compilation of perfume data in a single database, we searched for the CAS number of individual floral perfume compounds in the online database (https://webbook.nist.gov/chemistry/cas-ser.html). Based on the number, we checked for possible synonyms. Compounds included more than once were then merged.

Following data curation, we excluded compounds present in relative proportions below 1% of each species’ perfume to avoid biased sampling of rare compounds with more sensitive technology in recent years. This also allowed us to compare studies that elected to characterize compounds below 1% as “trace” without providing further quantitative information. Species with less than 70% of their total perfume blends resolved were then excluded from the dataset. The resulting chemical matrix was then re-standardized such that the sum of relative proportions within each species was 1.