Plants Evolving Weaponry to Compete in Species Selection

Botanists have pinpointed a piece of evolutionary weaponry in a South American milkweed plant that gives it an edge in propagating its own species.

The discovery has drawn comparisons to the species competition found in land animals; males of a variety of species will butt heads, bear claws, lock antlers and otherwise confront other males to assert their dominance and win the honor of mating - and this plant, while less dangerous, is essentially doing the same thing.

While plants do not mate like animals, they still face competition. Many plants, including milkweeds (Apocynaceae), rely on other species such as birds and pollinating insects to carry their seed to new locations.

Milkweed reproduces by way of tiny hooked sacs known as pollinia, which are stuffed with pollen grains. Birds and insects pick these up as they forage the milkweed plant and unwittingly disperse them into another flower to complete the pollination process.

Studying milkweeds in tropical climates, Andrea Cocucci from the Multidisciplinary Institute of Plant Biology (IMBV) in Argentina reports that the plants enter their own sort of confrontation among males.

Among the South American milkweed genus Oxypetalum, Cocucci and her colleagues found horn-like structures on the pollina sacs. These horns appear to not have any biological use, which the researchers suggest is evidence that they are used in "confrontations" with other plants. The horns prevent the pollen sacs from getting hooked together with pollina from other plants.

"Our results suggest that neither self-propulsion nor well-developed sensory perception are required for sexual selection to take place through intrasexual struggles," Cocucci said. "Apparently, only physical contact is enough to influence the mating success of competitors and to promote the evolution of defensive and attack weaponry."

Cocucci and her colleagues published their work in the journal New Phytologist.