Tricky little orchids
October 4, 2010 3 Comments
Orchids attract collectors all over the world. One of the things that draws us to these unusual plants is their Machiavellian approach to life. They unfeelingly employ deception to their benefit, usually practicing their art on unsuspecting members of the insect community. Research has revealed that one species of orchid, Anacamptis morio (or Orchis morio), or the green-winged orchid, lays its bold insect trap in an attempt to avoid a trap itself.
Inbreeding avoidance: not just for royalty
Although plants can do many things that most members of the animal kingdom cannot—self-fertilize or increase chromosome numbers in a generation—they’re still better off when reproductive measures result in an increase in genetic variation. As with most organisms, inbreeding is not a healthy thing for a plant, and many plants have mechanisms to avoid it.
The idea of inbreeding avoidance led researchers to a theory to explain the remarkable behavior of many orchids. These beautiful, much-coveted flowers attract humans and insects with their alluring fragrances and colors. For insects, some orchids add to the attraction by mimicking the female of the insect species, or wafting the scent of eau d’ dung for insects that prefer laying their eggs in such places. But of the 30,000 known orchid species, about 10,000 have nothing to offer the hapless insect in return: their flowers have no nectar.
Why keep coming back for nothing?
Researchers have sought to explain why insects would continue to visit such a stingy plant, and why the plants continue to get away with and employ their nectar-free strategy. The strategy itself seems in violation of so much of our understanding of the natural world, a place typically characterized by tradeoffs. In fact, orchids without nectar are not wildly popular among insects—it is difficult in many cases to witness a bee pollinating a green-winged orchid in the wild—but they still do manage to get pollinated.
Scientists investigated wild-growing green-winged orchids on a Swedish island and figured out why this species cheats insects so mercilessly. It’s about genetic variation. The flowers attract the bugs, but offer the foraging insects nothing, driving them on to explore other plants. Although the orchids have not provided food, they have given the unsuspecting insect a payload of a different kind: pollen. The bug—still on a quest for nectar—forages in other plants, pollinating as it goes along. Voila! No self-pollination. Plants that result from self-pollination are usually weak and unhealthy, and self-pollinating can be a waste of precious pollen.
Scientists detected this self-pollination avoidance by interviewing bees. They queried specific bees with plants that had been artificially dosed with nectar or with plants in their natural nectar-free state. The researchers found that bees stayed around the nectar-ful plants twice as long and investigated twice as many flowers on the same plant, which would promote self-pollination. Bees that found no nectar moved along to other plants, promoting cross-pollination.
One thing that could confound the interpretation of these results is that bees can remember how a plant smells. If a bee strikes out with one orchid, it will remember that orchid’s smell and not waste its time foraging around in other flowers that smell the same.
In separate research performed by a team in Switzerland, scientists found that the flowers of a nectar-producing orchid species all smell very much the same. But flowers on different plants of the green-winged orchid all smell different. A bee might have failure at one green-winged orchid and remember the smell, but then fly straight into another green-winged orchid plant because its smell is different. The unhappy bee falls into the orchid’s trap and gets nothing, but the deceitful orchid itself has had a great success: avoiding the trap of self-pollination.