One Orchid Two Colors

Bumblebees are no dummies. Far from being mindless drones whose sole purpose it to benefit the colony, these industrious insects are quite capable of learning and memory. They are constantly evaluating their foraging strategies and are quick to abandon a food source that doesn't deliver. For plants that rely on bumblebees, this presents a particular challenge. 

Of course, plants want to maximize their reproductive effort while at the same time minimizing their energy investments. For this reason, some plant species have foregone any sort of reward. Nectar is costly to produce after all. This non-rewarding strategy is particularly widespread among the orchids. Take for instance the case of the elder-flowered orchid (Dactylorhiza sambucina) of Europe. A species of meadows and alpine grasslands, it prefers calcarious conditions. What is most stunning about this species are its floral displays. 

Its inflorescence is made up of a dense cluster of flowers. Unlike what we are used to with most flowering plants, the flowers of the elder-flowered orchid come in two distinct color morphs - purple and yellow. They are so drastically different that one could be excused for thinking they were two different species. What's more, the different color morphs cooccur throughout the species' range. What could be causing this dimorphism? The answer lies in the flowers themselves. 

The edler-flowered orchid is one of those non-rewarding species. It has no nectar and its pollen is bunched up in sacs called pollinia that bees can't really harvest. The main pollinators of this species are bumblebees. As I have hinted, bumblebees are all about optimizing their foraging efforts. They quickly learn which plants are worth visiting and which plants are not. They do this via a highly tuned search image. Any plant that doesn't give them what they want will soon be shunned. 

This is where having different colored flowers comes in handy. Researchers have discovered that the color ratios of any given orchid population are under what is referred to as "negative frequency-dependent selection." Here's how it works: naive bumblebees that visit a non-rewarding flower of one color (purple in this example) are then much more likely to visit a flower of a different color (yellow). It just so happens that the plant with a different flower color (yellow) often turns out to be the same species of orchid. 

The result of this behavior is that in any given population, the plants with the rarer flower color (yellow) get visited more often. Because flower color is under genetic control, that particular morph (yellow) will gradually rise in frequency. Once it becomes the dominant flower color, the reverse happens and the first color (purple) is then visited more often. 

Over time this causes back and forth shifts in flower color that eventually settles on some sort of stable ratio of purple to yellow flowers. Thus anyone botanizing a high-elevation meadow in Europe can find purple and yellow flowered orchids in the same population. By tapping into the bees' natural foraging tendencies, this non-rewarding orchid species is able to maintain its presence in the landscape without having to invest valuable energy into floral rewards. 

Photo Credit: Emilio (http://bit.ly/22CHigV)

Further Reading:
http://www.pnas.org/content/98/11/6253.full.pdf