Matt Walker, BBC News 20 Jul 09;
The origin of the voracious Venus flytrap has been uncovered.
The flytrap, and one other carnivorous snap-trap plant which grows underwater, evolved from a more conventional relative that had sticky leaves.
Over time, the plants added elaborate structures and weapons such as trigger hairs and teeth to trap and immobilise their meaty prey, botanists say.
Ultimately, the need to hunt and eat ever larger animals drove the plants' evolution, say the scientists.
Carnivorous plants come in many forms, and are known to have independently evolved at least six separate times.
For example, pitcher plants create tube-like leaf structures into which insects tumble and become trapped, while other plants have sticky surfaces that act like flypaper.
But among all these plants, two species stand out: the Venus flytrap ( Dionaea muscipula ) and the waterwheel plant ( Aldrovanda vesiculosa ).
Both are known as snap-traps because they actively hunt animals, snapping shut specially adapted leaves to trap any hapless creature that crosses them.
The Venus flytrap closes around an insect in just 0.3s or faster, while the waterwheel use thin translucent traps to snare copepods and other aquatic invertebrates.
Charles Darwin was so enamoured by this striking adaptation, and the speed with which it works, that he described the Venus flytrap as being "one of the most wonderful plants in the world."
"Darwin was fascinated by carnivorous plants in general and the Venus flytrap in particular, I think, partly because they go against type," says Don Waller, a botanist at the University of Wisconsin, in Madison, US.
"In his time and ours, most of us feel that plants are passive, harmless, and can't move. But the Venus flytrap acts like an animal, it moves fast and eats fresh meat."
Elusive origins
But until now, it has not been clear which plant the Venus flytrap and waterwheel evolved from.
Waller and his colleague Thomas Gibson now think they have the answer, which they have published in the journal New Phytologist.
What is more, they think its possible to trace a series of steps that the Venus flytrap and waterwheel must have undergone to become accomplished hunters.
They started by comparing snap-traps with other carnivorous plants known as sticky traps.
The two types look very different. While snap traps have gaping leaves, sticky traps have small leaves that are covered by simple stalks, which are often covered in sticky glue.
Many sticky trap plants belong to the genus Drosera , such as the circumboreal sundew ( Drosera rotundifolia ), and come in various forms.
A DNA analysis by Ken Cameron of the University of Wisconsin confirmed that the Venus flytrap and waterwheel are indeed related, and the closest relative of both turns out to be a species called Drosera regia .
Then Waller and Gibson mapped out the steps that would have been required to evolve from a sticky trap ancestor into a snap-trap.
First the ancestral plant must have adapted to move its tentacles and leaves in a particular direction, giving it a greater chance of sticking to and engulfing a passing insect.
Next it sped up how quickly it detected prey and tried to respond.
Then the plant would have had to find a way to become selective, so it only tried to trap live prey and not any detritus that landed upon it.
Finally, it must have evolved its tentacles into sensory hairs and teeth that detect and wrap around prey, respectively, while also losing its sticky glands and growing new digestive glands capable of digesting the victim's corpse.
But that raises the question of what strong selection pressure could have driven the evolution of such an extraordinary series of adaptations?
Bigger and better
The short answer, say Waller and Gibson, is the benefits of eating ever larger prey.
"Capturing big prey provides big rewards to any carnivorous plant, and snap-traps can immobilize and digest these bigger prey far more effectively than the sticky fly-paper type traps we see in sundews," says Waller.
"A bug twice as long as another has more like eight times the biomass and nutrients, so these rewards accelerate as prey get bigger. This is why flytraps build bigger traps as soon as they can, unlike almost all other plants where the size of leaves or traps remains the same as plants get bigger."
Waller also suspects that engulfing and sealing in prey may also provide other advantages to the Venus flytrap and waterwheel.
It stops other more sure-footed predators coming along and stealing a captured insect or spider, he says, and any nutrients being washed away by water or rain before the plant can digest them.
Answering such questions is particularly important because the Venus flytrap is becoming increasing rare in the wild, and should be listed as endangered in the US, where it only lives in North and South Carolina, says Waller.
Knowing its evolutionary history will help botanists work out, for example, if it is becoming dangerously inbred.
