by Tony Fitzpatrick
As the 20th century draws to a close, still little is known about the origins of a staple subsistence crop that feeds an estimated 600 million Third World people.
The plant is cassava (Manihot esculenta), a bushy plant producing tubers -- the starchy underground stem of the plant -- that have fed the indigenous people of the Americas for millennia and much of Africa since the 17th century.
But now biologists at Washington University in St. Louis have written the ultimate roots story for this plant in a paper published in the May 11, 1999, Proceedings of the National Academy of Sciences.
Barbara A. Schaal, Ph.D., Washington University professor of biology in Arts and Sciences, and her graduate student Kenneth M. Olsen, have pinpointed cassava's origins to the southern border of the Amazon River basin in Brazil. The Explorer's Club, National Science Foundation, the Rockefeller Foundation and the Guggenheim Foundation supported the research.
Using sophisticated DNA sequencing techniques that traced variation in a single gene found in cultivated and wild cassava, Schaal and Olsen have identified a cassava subspecies, still present in the diminishing wilds of the Amazon basin, as the plant's progenitor.
The find provides important insights into cassava's evolutionary origin. Their work also reveals a wealth of genetic diversity in wild and domesticated cassava strains, information that plant breeders can use to create hardier plants that are more resistant to disease. With the newly found genetic information, there are greater possibilities now to transform cassava from an "orphan" crop to a more commercially viable one that could be used as a fiber source.
Cassava ranks sixth among crops in global production. Denizens of the developed world have sampled cassava in tapioca and perhaps some flours at specialty food stores or specialty dishes in the Miami area. But the vast majority of consumers are the world's poor who grow their own in patches of poor soil.
The Washington University findings also settle disputes among plant scientists, a group of whom have believed that the plant's origins were in Mexico. They also show that the original cassava is not a derivative of several different progenitor species, nor is it a hybrid between a closely related species, Manihot pruinosa. Both possibilities have been hypothesized by some evolutionary and population plant biologists.
Schaal and Olsen looked at variation in the nuclear gene G3pdh in cassava (Manihot esculenta) and in a wild subspecies, flabellifolia, to develop a "phylogeographic" family tree. This is a kind of branching map that yields both evolutionary and geographic information. This technique has been used successfully by only a few plant scientists worldwide. They focused on the G3pdh gene because it is common to domesticated and wild strains and carries identifiable genetic variation.
Earlier results clearly ruled out a Mexican origin. In fine-tuning their sampling of wild populations, they found that cassava is derived from wild flabellifolia populations along the southern border of the Amazon basin.
"When we compared the groups from Mexico to those from Brazil, we found the Mexican ones were clearly separated on a different branch of the phylogenetic tree," says Schaal. "Then we sampled the Brazilian species and differentiated even more between the various groups of plants. The argument for this kind of work is to pinpoint where domestication occurred and to save these wild populations for breeding improved varieties of cassava that can better feed the developing world. Today, this region of Brazil where the wild cassava grows is being turned into massive soybean farms, so we may have found these populations just in time."
Olsen collected wild specimens of flabellifolia Manihot pruinosa, another close relative of M. esculenta, on two different trips to Brazil in 1996 and 1997. He collaborated with scientists from the Brazilian agency CENARGEN, roughly the equivalent of the U.S. Department of Agriculture. He sequenced the DNA for nearly two years and then, with Schaal, developed a phylogenetic tree that ultimately linked the subspecies flabellifolia with the domesticated crop.
"The overall picture of cassava's origin represents a fundamental departure from the traditional view that envisioned this crop as a derivative from one or more complexes of interbreeding wild species, from one or more regions in the Neotropics," says Olsen. "This new insight will be useful for targeting wild germplasm for crop improvement and for understanding the genetic consequences of domestication of the species."
Primary carbohydrate sourceCassava was introduced to Africa by the Portuguese more than 300 years ago and today is the primary carbohydrate source in sub-Sahara Africa. The plant grows as a bush or little tree and is harvested for its tubers, which are grated and pounded into a spread or flour. Currently, the African crop is threatened by African cassava mosaic virus, which can reduce yields by 75 percent. Over the past decade, the virus has caused famines in several local regions of central Africa. The disease is prevalent because the African cassava varieties, having been introduced, have no natural defense against the virus.
It is best to think of cassava as tapioca pudding because it's hard to crave the food once it's known how most consumers prepare it. All cassava contains varying amounts of cyanide, a toxin that protects the plant from insects. Indigenous people have learned to avoid poisoning themselves by spitting into batches of the ground tubers; the saliva introduces bacteria and fungi, which activate an enzyme that breaks down the cyanide. Villagers accomplish the same thing by depositing freshly dug cassava tubers into a community pond; microorganisms in the water degrade the cyanide.
It is speculated that starvation indirectly led to these culinary rituals; starving people may have found that, having once spit out the bitter cassava tubers, then trying them again, the bitter taste was lessened.
Schaal says the diversity of the plant is remarkable. "There must be thousands of different varieties. In Brazil, every little village has its own varieties. There are bitter ones, sweet ones, even ones used as baby food."
Schaal says the results of the work will be of use immediately.
"This provides the basis for a better understanding of all Manihot species and how they relate to each other," she says. "There has been a growing interest in a broader commercialization of cassava, and now that it is known where the key wild relatives are, breeders can look for DNA no longer in the domesticated crop that might increase yield, protein content and provide other useful traits."
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