Evolution and diversity of floral scent chemistry in the euglossine bee-pollinated orchid genus Gongora

Abstract

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Background and aims: Animal-pollinated angiosperms have evolved a variety of signalling mechanisms to attract pollinators. Floral scent is a key component of pollinator attraction, and its chemistry modulates both pollinator behaviour and the formation of plant-pollinator networks. The neotropical orchid genus Gongora exhibits specialized pollinator associations with male orchid bees (Euglossini). Male bees visit orchid flowers to collect volatile chemical compounds that they store in hind-leg pouches to use subsequently during courtship display. Hence, Gongora floral scent compounds simultaneously serve as signalling molecules and pollinator rewards. Furthermore, because floral scent acts as the predominant reproductive isolating barrier among lineages, it has been hypothesized that chemical traits are highly species specific. A comparative analysis of intra- and inter-specific variation of floral scent chemistry was conducted to investigate the evolutionary patterns across the genus. •

Methods: Gas chromatography-mass spectrometry (GC-MS) was used to analyse the floral scent of 78 individuals belonging to 28 different species of Gongora from two of the three major lineages sampled across the neotropical region. Multidimensional scaling and indicator value analyses were implemented to investigate the patterns of chemical diversity within and among taxonomic groups at various geographic scales. Additionally, pollinator observations were conducted on a sympatric community of Gongora orchids exhibiting distinct floral scent phenotypes. •

Key results: A total of 83 floral volatiles, mainly terpenes and aromatic compounds, were detected. Many of the identified compounds are common across diverse angiosperm families (e.g. cineole, eugenol, β-ocimene, β-pinene and terpinen-4-ol), while others are relatively rare outside euglossine bee-pollinated orchid lineages. Additionally, 29 volatiles were identified that are known to attract and elicit collection behaviour in male bees. Floral scent traits were less variable within species than between species, and the analysis revealed exceptional levels of cryptic diversity. Gongora species were divided into 15 fragrance groups based on shared compounds. Fragrance groups indicate that floral scent variation is not predicted by taxonomic rank or biogeographic region. •

Conclusions: Gongora orchids emit a diverse array of scent molecules that are largely species specific, and closely related taxa exhibit qualitatively and quantitatively divergent chemical profiles. It is shown that within a community, Gongora scent chemotypes are correlated with near non-overlapping bee pollinator assemblies. The results lend support to the hypothesis that floral scent traits regulate the architecture of bee pollinator associations. Thus, Gongora provides unique opportunities to examine the interplay between floral traits and pollinator specialization in plant-pollinator mutualisms.

Keywords: Euglossa; Euglossine bees; floral scent; orchid genus Gongora; plant–pollinator mutualism.

Fig. 1.

(A) Gongora sp. chemotype M (subgenus Gongora, section Gongora) being visited by Euglossa dodsoni. (B) Male euglossine bees collect perfume compounds and store them in hind-leg pockets (arrow) subsequently to expose to females during courtship display (photo: B. Jacobi). (C) Putative phylogenetic relationships of Gongora lineages sampled in the present study (as summarized by Hetherington-Rauth and Ramírez, 2015).

Fig. 2.

Floral volatile diversity across sampled species. (A) Bar graph depicting compound prevalence. Compound prevalence refers to the number of sampled species that produced a given compound. Compounds are arranged along the x-axis in order of decreasing prevalence. Colours represent the chemical class of the compound, including unidentified compounds. Asterisks above the bars indicate floral volatiles that are known to attract and elicit collection behaviour in male euglossine bees. (B) Bar graph indicating the average relative abundance for each floral volatile. Lines laid on top of the bars indicate the maximum and minimum relative abundance values. Compounds along the x-axis are arranged in the same order as in (A). Colours correspond to the colours used in (A) Compound abbreviations are as follows: DMB, dimethoxybenzene; MMC, methyl methoxycinnamate; PAME, phenylacetic methyl ester; BAPEEE, benzonic acid p-ethoxy ethylester.

Fig. 3.

Non-metric multidemsional scaling (nMDS) of floral scent projected in two-dimensional space using the Bray–Curtis dissimilarity metric implemented using the R software package ‘ecodist’. The stress value reported was calculated using the nMDS algorithm in ‘ecodist’. (A) Plotted results of nMDS analysis using a sub-set of species (n = 12). Coloured symbols represent separate species. (B) Plotted results of nMDS analysis of Gongora pleiochroma individuals. Ellipses correspond to the three chemical phenotypes that differ in the relative contribution of cis-β-ocimene, linalool and β-bisabolene to the overall floral scent bouquet. (C) Plotted results of nMDS analysis of La Gamba Gongora. Groupings correspond to three identified chemotypes – A, M and S – characterized by cis- and trans-methyl methoxycinnamate (MMC), terpinen-4-ol and unidentified compound #51, respectively. (D) Bipartite network showing bee–orchid associations between the three chemotypes of La Gamba Gongora and nine species of euglossine bees observed actively collecting floral scent from the flowers. Bee species marked with an asterisk indicate species observed carrying Gongora pollinaria at chemical baits (S. R. Ramírez unpubl. res.).

Fig. 4.

Depiction of the eight biogeographic regions as described by Gentry (1982). Points indicate the collection localities of all sampled (79) individuals for which we obtained scent profile data. Circles represent the true collection localities, whereas triangles represent inferred collection localities based on species distributions and type localities (Jenny, 1993). Squares represent the mid-point of the country of collection and are used for two individuals for which precise locality data were not available; these two individuals could not be unambiguously assigned to a single biogeographic region. Table 2 provides the distribution of fragrance groups and species within each biogeographic region.