Plants Develop Horn Like Structures To Ward Of Intrasexual Competition

By Shweta Iyer on March 20, 2014 12:44 PM EDT

.plant horns
A team of researchers has uncovered that plants are evolving weaponry in order to spread their pollen (Photo: Scott Parker, CC BY ND 2.0)

Somewhere in South America, a milkweed plant is developing horns to compete with other milkweed plants to spread its seed and reproduce. A team of researchers has uncovered that plants are evolving weaponry in order to spread their pollen, according to a press release Thursday. This article is based on the research headed by Dr. Andrea Cocucci from the Instituto Multidisciplinario de Biologia Vegetal of Argentina, and has been published in the New Phytologist.

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In animals, competition between individuals of the same species for a mate is common. Although, plants can reproduce sexually they are immobile and cannot seek out sexual partners for reproduction. So, they depend on external agents like birds or insects that carry their pollen to other plants. These agents are called pollinators. The researchers studied a species of milkweed (Apocynaceae), found in tropical climates and found that they use pollinators to reproduce.

Pollination in milkweed is achieved in an unusual manner. Pollen is grouped into structures called pollinia or pollen sacs. When flower visiting insects or birds hover around the flower of the milkweed, bases of the pollinia get mechanically attached to the feet or mouth-parts of the pollinator, pulling a pair of pollen sacs free when the pollinator flies off. Pollination occurs when the pollinator visits another milkweed plant and drops of the pollen into its flower.

But pollinarium from different plants may get entangled together since pollinators visit several flowers and they have limited number of attachment points on their bodies. According to the research team, competition between plants to exclusively make use of the pollinators can result in confrontation.

The researchers studied the milkweed genus Oxypetalum, which grows in South America. They found horn-like structures on the pollinia sacs. These horns have no apparent biological use, which led the team to believe that maybe the horns are used to prevent the sacs from being hooked together with pollinia from other parent plants.

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

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