25 January 2011

Finding the middle road: Flowers evolve to work with multiple pollinators

"I’ve had a lot of jobs in my life: boxer, mascot, astronaut, baby proofer, imitation Krusty, truck driver, hippie, plow driver, food critic, conceptual artist, grease salesman, carny, mayor, grifter, body guard for the mayor, country western manager, garbage commissioner, mountain climber, farmer, inventor, Smithers, Poochie, celebrity assistant, power plant worker, fortune cookie writer, beer baron, Kwik-E-Mart clerk, homophobe, and missionary, but protecting people, that gives me the best feeling of all."
—Homer Simpson
In twenty-two seasons of The Simpsons, the eponymous family's bumbling father Homer has tried his hand at dozens of different jobs, and failed hilariously at most of them. Homer is a one-man illustration of "Jack of all trades, master of none," the idea that it's hard to do many different things well. This principle applies more broadly than the curriculum vitae; in biology, it means that living things face trade-offs between different ways of making a living.

A wild radish (Raphanus raphaistrum) flower. Photo by Valter Jacinto.
For instance, a plant whose pollen is carried from flower to flower by just one pollinating animal only needs to match that one pollinator very well. But most plants' flowers are visited by many different potential pollinators, and matching all of them probably means finding a middle ground among the best ways to match each individual pollinator. A study of one such "generalist" flower, the wild radish, has found exactly this: working with multiple partners takes evolutionary compromise [$a].

Wild radishes are visited by a wide variety of different insects, including honeybees, bumblebees, syrphid flies, and cabbage butterflies, among others. Each of these pollinators comes to a radish flower with a slightly different agenda. Butterflies are there for nectar, but bees like to eat pollen as well—and bumblebees will sometimes bite into the base of a flower and "steal" nectar without ever coming into contact with pollen. Figuring out how natural selection from each of these different pollinators adds up required some clever experimental design.

The study's authors arrayed potted radish flowers inside a big mesh flight cage, and then introduced either bumblebees, honeybees, cabbage butterflies, or all three pollinators to visit the plants and circulate pollen from flower to flower. They measured the plants' flowers before putting them in the flight cage, then let the pollinators do their thing. Afterward, the authors collected seeds resulting from the pollinators' activity, grew them up, and measured the offspring to see whether their traits differed. The procedure was essentially one generation of experimental evolution.

A cabbage white butterfly (Pieris rapae), one of many pollinator species exerting natural selection on wild radishes. Photo by ComputerHotline.
By taking DNA fingerprints of both the parents and the offspring, the authors could also estimate the relationship between each parental plant's floral measurements and the number of offspring it produced, either from its own seeds or by pollinating another plant.

The results are complex. Depending on the floral measurement under consideration, different pollinators selected in different directions, or the same direction, or not at all. One particularly interesting result, though, was in the effects each pollinator had on the "dimorphism" of the radish flowers' stamens—the difference between the length of the shortest, and longest, of the male parts of the flower. Flowers only visited by honeybees evolved less dimorphic stamens, while flowers visited by either bumblebees or cabbage butterflies evolved more dimorphic stamens. Flowers in the treatment visited by all three pollinators, however, evolved to find a happy medium, an evolutionary compromise to work with the different partners.

The way these interactions played out in a flight cage probably don't reflect exactly how they operate in the wild, but this is a pretty cool result all the same. I've written in the past about how incorporating multiple interactions can alter the way coevolution works. Gerbils under attack by fleas are less careful about watching for predators; but for the protists living inside pitcher plants, competitors can help distract predators. Here we have an example of multiple similar interactions pulling a generalized plant in different evolutionary directions.


Sahli, H., & Conner, J. (2011). Testing for conflicting and non-additive selection: Floral adaptation to multiple pollinators through male and female fitness. Evolution DOI: 10.1111/j.1558-5646.2011.01229.x

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