Know thy neighbour: sagebrush plants warn clones of impending danger
Plants may be able to recognise themselves.
Experiments show that a sagebrush plant can recognise a genetically identical cutting growing nearby.
What's more, the two clones communicate and cooperate with one another, to avoid being eaten by herbivores.
The findings, published in Ecology Letters, raise the tantalising possibility that plants, just like animals, often prefer to help their relatives over unrelated individuals.
The ability to distinguish self from non-self is a vital one in nature.
It allows many animals to act preferentially towards others that are genetically related to themselves; for example, a female lion raising her young, or protecting other more distantly related cubs in her pride.
But the evidence that plants can do the same is limited and controversial.
Some experiments have shown that if a plant's roots grow near to those of another unrelated plant, the two will try to compete for nutrients and water. But if a root grows close to another from the same parent plant, the two do not try to compete with one another.
However, in these experiments, when two cuttings of the same plant are then grown alongside each other, their roots still compete for resources. That implies that two separate plants cannot recognise that they are genetic kin.
Now research by Richard Karban of the University of California, in Davis, US and Kaori Shiojiri of Kyoto University in Otsu, Japan has revealed that some plants are capable of doing just that.
Keep it in the family
They took cuttings of Artemisia tridentata, a species of sagebrush that does not normally reproduce by cloning itself.
They placed each cutting either near its genetic parent, essentially its clone, or near an unrelated sagebrush, and let the plants grow in the wild in the University of California Sagehen Creek Natural Reserve. The researchers clipped each clone they planted, feigning damage that might be caused by natural herbivores such as grasshoppers.
After one year, they found that plants growing alongside their damaged clones suffered 42% less herbivore damage than those growing alongside damaged plants that were unrelated.
Somehow, the clipped plants appeared to be warning their genetically identical neighbours that an attack was imminent, and the neighbour should somehow try to protect itself. But clipped plants didn't warn unrelated neighbours.
Karban says he was "pretty surprised" at the results. "It implies that plants are capable of more sophisticated behaviour than we imagined."
Karban suspects the plants are communicating using volatile chemicals. When one plant is clipped, or comes under attack from herbivores, it emits these chemicals into the air, warning those around it to put up a defence, either by filling their leaves with noxious chemicals, or by physically moving their stems or leaves in some way to make themselves less palatable.
Because his team doesn't yet know exactly how the plants are communicating, others remain sceptical of the research, Karban admits.
"It's controversial," he says. "But through this communication process, sagebrush appears able to distinguish self from non self. And that opens up a lot of other possibilities."
Not least is that wild plants may preferentially be cooperating with their relatives.
There is no hard evidence yet to show this is true, says Karban.
But he hopes others will now do more research to investigate the possibility. In animals, cooperation between related individuals is recognised to be a powerful evolutionary force, one that has been given its own name: kin selection.