Monday, June 27, 2011
DNA tells the Tale of Two Branches of the Coconut Palm
(This fascinating article is a republish courtesy of the Washington University St. Louis. Original article published on June 23 online issue of the journal PLoS One.)
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
“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 PLoS One.
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
“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
“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.
The project got started when Ms. Gunn, who had long been interested in palm evolution, and who was then at the Missouri Botanical Garden, contacted Olsen, who had the laboratory facilities needed to study palm DNA.
Together they won a National Geographic Society grant that allowed Ms. Gunn to collect coconut DNA in regions of the western Indian Ocean for which there were no data. The snippets of leaf tissue from the center of the coconut tree’s crown she sent home in zip-lock bags to be analyzed.
“We had reason to suspect that coconuts from these regions —especially Madagascar and the Comoros Islands — might show evidence of ancient gene flow events brought about by ancient Austronesians setting up migration routes and trade routes across the southern Indian Ocean,” Olsen says.
Olsen’s lab genotyped 10 microsatellite regions in each palm sample.
The new collections were combined with a vast dataset that had been established by the French agricultural research center, using the same genetic markers. “These data were being used for things like breeding, but no one had gone through and systematically examined the genetic variation in the context of the history of the plant,” Olsen says.
The most striking finding of the new DNA analysis is that the Pacific and Indian Ocean coconuts are quite distinct genetically. “About a third of the total genetic diversity can be partitioned between two groups that correspond to the Indian Ocean and the Pacific Ocean,” says Olsen.
In the Pacific, coconuts were likely first cultivated in island Southeast Asia, meaning the Philippines, Malaysia, Indonesia, and perhaps the continent as well. In the Indian Ocean the likely center of cultivation was the southern periphery of India, including Sri Lanka, the Maldives, and the Laccadives.
The definitive domestication traits — the dwarf habit, self-pollination and niu vai fruits — arose only in the Pacific, however, and then only in a small subset of Pacific coconuts, which is why Olsen speaks of origins of cultivation rather than of domestication.
“At least we have it easier than scientists who study animal domestication,” he says. “So much of being a domesticated animal is being tame, and behavioral traits aren’t preserved in the archeological record.”
One exception to the general Pacific/Indian Ocean split is the western Indian Ocean, specifically Madagascar and the Comoros Islands, where Ms. Gunn had collected. The coconuts there are a genetic mixture of the Indian Ocean type and the Pacific type.
Olsen and his colleagues believe the Pacific coconuts were introduced to Austronesians establishing trade routes connecting Southeast Asia to Madagascar and coastal east Africa.
Olsen points out that no genetic admixture is found in the more northerly Seychelles, which fall outside the trade route. He adds that a recent study of rice varieties found in Madagascar shows there is a similar mixing of the japonica and indica rice varieties from Southeast Asia and India.
To add to the historical shiver, the descendants of the people who brought the coconuts and rice are still living in Madagascar. The present-day inhabitants of the Madagascar highlands are descendants of the ancient Austronesians, Olsen says.
Much later the Indian Ocean coconut was transported to the New World by Europeans. The Portuguese carried coconuts from the Indian Ocean to the West Coast of Africa, Olsen says, and the plantations established there were a source of material that made it into the Caribbean and also to coastal Brazil.
So the coconuts that found today in Florida are largely the Indian Ocean type, Olsen says, which is why they tend to have the niu kafa form.
pre-Columbian times by ancient Austronesians moving east rather than west.
During the colonial period, the Spanish brought coconuts to the Pacific coast of Mexico from the Philippines, which was for a time governed on behalf of the King of Spain from Mexico.
This is why, Olsen says, you find Pacific type coconuts on the Pacific coast of Central America and Indian type coconuts on the Atlantic coast.
“The big surprise was that there was so much genetic differentiation clearly correlated with geography, even though humans have been moving coconut around for so long.”
Far from being a mish-mash, coconut DNA preserves a record of human cultivation, voyages of exploration, trade and colonization.
With the above information, we now know more about the colonization around the world by just following the leads of the coconut DNA! So if you
Much more about coconuts in future blog posts!! It’s the fruit that keeps on giving!!
Kimberly Barron, originally from Malibu, California has lived in Parismina and Manuel Antonio, Costa Rica for 20 years. Starting as a certified tour guide, she spent 15 years managing fishing lodges on the Caribbean Coast and later 4* & 5* Hotels on the Pacific Coast of Costa Rica. Currently semi-retired, Kimberly still works as the Marketing Director for Byblos Resort & Casino and Hotel Makanda by the Sea.
Bee F. Gunn1, Luc Baudouin2, Kenneth M. Olsen3*
1 Division of Evolution, Ecology and Genetics, Research School of Biology, The Australian National University, Canberra, Australia, 2 Centre International de Recherches en Agronomie pour le Développement (CIRAD), Montpellier, France, 3 Biology Department, Washington University, St. Louis, Missouri, United States of America