How to seduce a bee (as an orchid)

Figure 1: Plant and pollinator. Insects spread plants' genetic material (pollen) from flower to flower in exchange for a sugar (nectar) reward.

Flowing plant reproduction - Imagine that you are a plant, rooted in the ground, and it's that time of year when you want to spread your genetic material to other plants to reproduce. You can't walk around to find a mate, so what do you do? Some plants solve this problem by allowing wind to carry their pollen in vast plumes across the landscape, hoping that it will reach the flowers of other members of the same species. However, many other plant species have stumbled upon a fortuitous partnership with bees, butterflies, and other insects that directly carry plant pollen from one flower to another.

Flowering plants work with pollinators to reproduce - Pollen-spreading insects (pollinators) make plant reproduction very efficient. Plants that recruit pollinators have been very successful - they are now the most common plants on earth. Some relationships between flowering plants and pollinators are specialized, so that only certain pollinators may spread the pollen of a certain plant species. Other relationships are general, so a single plant species is compatible with many pollinator species. In either case, plants often provide a sugary reward (nectar) for insects transferring pollen from one plant to another, which many insects and even some birds use as a primary resource (Figure 1).

Figure 2: Some flowers attract male bees to be pollinators by looking and smelling like female bees. Instead of providing a nectar reward to attract pollinators, some flowers develop to look like female bees. These flowers also release bee sex pheromones into the air to attract nearby male bees. By moving from one deceptive flower to another, male bees transfer the plants' pollen. Photo used with the permission of Carlos Enrique Hermosilla (https://www.flickr.com/photos/26925527@N07/)

Some flowers attract pollinators with pheromones - Some plant species are able to get bees to collect and spread their pollen without providing a reward. In the orchid family, a plant family with a tremendous number of species, some species produce flowers that look like female bees. These flowers entice male bees to land, pick up pollen while they wiggle around, and then carry it away while in search of a mate with a heartbeat (Figure 2). In doing this, male bees transfer pollen from one flower to another. To encourage male pollinators to land on these deceptive flowers, these plants scent the surrounding air with aphrodisiacs - bee sex pheromones that draw the male bees in with great effect. This is where chemistry comes into play!

Figure 3: Scents produced by flowers can attract specific pollinator species. The flowers of different plant species can produce different mixtures of scents, in some cases insect sex pheromones, to attract pollinators (top panels). Depending on the specific pheromones produced, different insect species may be attracted (middle and lower panels).

Deceptive pheromones are species-specific - There is a great variety of chemical compounds that plants use as deceptive pheromones with which to attract bees, and in many cases these chemical compounds are species specific [1]. For example, the orchid Ophrys exaltata attracts the bee Colletes cunicularius by producing a specific type of chemical called a 7-alkene. In contrast, Ophrys sphegodes attracts Andrena nigroaenea with 9-, 11-, and 12-alkenes (Figure 3). Even though these pheromones are extremely similar (they only differ in the position of their carbon-carbon double bond), adding the pheromones of one orchid species to the flower of the other creates a mixture that neither bee species is attracted to [2]. Overall, the exact identity and proportions of the different chemical compounds that flowers produce to attract pollinators is important [4].

Deceiving pollinators may encourage speciation- Let's perform a thought experiment - if an O. sphegodes plant acquired a mutation that caused it to only produce two of its three alkenes, we know it would probably not attract either C. cunicularius or A. nigroaenea. But, if there were another insect species nearby that was attracted by the two alkenes alone, then the mutant plant could still survive. Over time, the offspring of the mutant plant could become a population of individuals all carrying the mutation and producing only two alkenes. Perhaps the flowers of these mutants would also change shape or color over generations so as to more effectively attract the new insect pollinators. Eventually, this could lead these mutants to become an entirely different species!

So, in addition to being crucial for the reproduction of some flowering plant species, scientists think that specificity in plant-pollinator relationships is a substantial driving force in speciation, perhaps occurring via a process similar to that described above [5, 6]. Flowers make fragrances to do more than just smell nice!

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[1] Bjorn Bohman, Gavin R Flematti, Russell A Barrow, Eran Pichersky and Rod Peakall. Pollination by sexual deception — it takes chemistry to work. Current Opinion in Plant Biology 2016, 32:37–46
[2] Xu S, Schluter PM, Grossniklaus U, Schiestl FP: The genetic basis of pollinator adaptation in a sexually deceptive orchid. PLOS Genet 2012:8.

[3] Bohman B, Philips RD, Menz MHM, Berntsson BW, Flematti GR, Barrow RA, Dixon KW, Peakall R. Discovery of pyrazines as pollinator sex pheromones and orchid semiochemicals: implications for the evolution of sexual deception. New Phytol 2014, 203:939-952.
[4] Bohman B, Karton A, Dixon RCM, Barrow RA, Peakall R. Parapheromones for thynnine wasps. J Chem Ecol 2016, 42:17-23.
[5] Peakall R, Whitehead MR: Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids. Ann Bot 2014, 113:341-355.
[6] Xu S, Schluter PM, Grossniklaus U, Schiestl FP: The genetic basis of pollinator adaptation in a sexually deceptive orchid. PLOS Genet 2012:8.