By Diana Lutz, Washington University in St. Louis
The coconut (the fruit of the palm Cocos nucifera) is the Swiss Army knife of the plant kingdom; in one neat package it provides a high-calorie food, potable water, fiber that can be spun into rope, and a hard shell that can be turned into charcoal. What’s more, until it is needed for some other purpose it serves as a handy flotation device.
No wonder people from ancient Austronesians to Captain Bligh pitched a few coconuts aboard before setting sail. (The mutiny of the Bounty is supposed to have been triggered by Bligh’s harsh punishment of the theft of coconuts from the ship’s store.)
So extensively is the history of the coconut interwoven with the history of people traveling that Kenneth Olsen, a plant evolutionary biologist, didn’t expect to find much geographical structure to coconut genetics when he and his colleagues set out to examine the DNA of more than 1300 coconuts from all over the world.
“I thought it would be mostly a mish-mash,” he says, thoroughly homogenized by humans schlepping coconuts with them on their travels.
He was in for a surprise. It turned out that there are two clearly differentiated populations of coconuts, a finding that strongly suggests the coconut was brought under cultivation in two separate locations, one in the Pacific basin and the other in the Indian Ocean basin. What’s more, coconut genetics also preserve a record of prehistoric trade routes and of the colonization of the Americas.
The discoveries of the team, which included Bee Gunn, now of the Australian National University in Australia, and Luc Baudouin of the Centre International de Recherches en Agronomie pour le Développement (CIRAD) in Montpellier, France, as well as Olsen, associate professor of biology at Washington University in St. Louis, are described in the June 23 online issue of the journal PLoS One.
Morphology a red herring
Before the DNA era, biologists recognized a domesticated plant by its morphology. In the case of grains, for example, one of the most important traits in domestication is the loss of shattering, or the tendency of seeds to break off the central grain stalk once mature. The trouble was it was hard to translate coconut morphology into a plausible evolutionary history.
There are two distinctively different forms of the coconut fruit, known as niu kafa and niu vai, Samoan names for traditional Polynesian varieties. The niu kafa form is triangular and oblong with a large fibrous husk. The niu vai form is rounded and contains abundant sweet coconut “water” when unripe.
“Quite often the niu vai fruit are brightly colored when they’re unripe, either bright green, or bright yellow. Sometimes they’re a beautiful gold with reddish tones,” says Olsen.
Coconuts have also been traditionally classified into tall and dwarf varieties based on the tree “habit,” or shape. Most coconuts are talls, but there are also dwarfs that are only several feet tall when they begin reproducing. The dwarfs account for only 5 percent of coconuts.
Dwarfs tend to be used for “eating fresh,” and the tall forms for coconut oil and for fiber.
“Almost all the dwarfs are self fertilizing and those three traits—being dwarf, having the rounded sweet fruit, and being self-pollinating—are thought to be the definitive domestication traits,” says Olsen.
“The traditional argument was that the niu kafa form was the wild, ancestral form that didn’t reflect human selection, in part because it was better adapted to ocean dispersal,” says Olsen. Dwarf trees with niu vai fruits were thought to be the domesticated form.
The trouble is it’s messier than that. “You almost always find coconuts near human habitations,” says Olsen, and “while the niu vai is an obvious domestication form, the niu kafa form is also heavily exploited for copra (the dried meat ground and pressed to make oil) and coir (fiber woven into rope).”
“The lack of universal domestication traits together with the long history of human interaction with coconuts, made it difficult to trace the coconut’s cultivation origins strictly by morphology,” Olsen says.