Mesoamerican masters of biotechnology
Maize is undeniably humanity's greatest, and the first, feat of genetic engineering. But the success story cannot be attributed to modern humans. A recent research shows that it is only due to the selective breeding efforts of ancient Americans that maize today has its huge ears, each packed with firmly attached kernels sumptuously filled with starch, protein and oil. It is these characteristics that made a useless grassy weed, called teosinte, into an edible species.
The origin of maize has long intrigued geneticists. Molecular methods, evolved in the beginning of the 19th century, enabled the evolutionary sleuths to pinpoint its genesis, but not with certainty. In the 1920s, a group of researchers examined the chromosomes in teosinte-maize hybrids and concluded that the two plants belonged to the same species. That should have resolved the question of maize's origins, but it didn't. In 1938, eminent agricultural geneticist Mangelsdorf proposed that the modern crop evolved from an extinct South American maize species, and teosinte had originated from a cross between maize and another grass Tripsacum . Although cumbersome, this hypothesis was widely accepted. But by the mid-1990s, molecular evidence overwhelmingly favoured the notion that teosinte was the ancestor of maize.
The new study nailed down the genetic changes that enabled the radical transformation of teosinte into contemporary maize. It was conducted by researchers from Germany-based Max Planck-Institute for Evolutionary Anthropology, uk-based University of Oxford and us-based North Carolina State University, University of Wisconsin and the Smithsonian Institution.
To arrive at their conclusion, the researchers studied fossil cobs from the Ocampo caves in northeastern Mexico that are 2,300 to 4,400 years old, and those from the Tularosa cave in New Mexico which are 650 to 1,900 years old. They extracted the cobs' dna and then amplified, cloned and sequenced its fragment. After extensive analysis, they found that ancient Americans systematically enhanced only a few genetic qualities of teosinte to develop maize. Their technique was not as sophisticated as the methods used for biotechnology, but experts say that the effect was the same: plants with improved traits and greater yield.
So when and where was teosinte transformed into maize? To get an answer the researchers constructed family trees of maize-teosinte, using similar dna sequences from different varieties of the two. The results were unequivocal: all contemporary maize varieties belong to a single family, pointing to a single domestication event. Further investigation showed that they arose roughly 9,000 years ago from teosinte of the subspecies parviglumis found in the Balsas river basin of southern Mexico.
Teosinte didn't all of a sudden metamorphose into contemporary maize, the researchers found. Naturally, it was a grassy-like plant with many stems bearing small cobs with kernels sheathed in hard shells. With the help of careful nurturing, farmers caused teosinte to morph into a crop with larger kernels by 5,500 years ago. It took another thousand years to incorporate all of the gene variants found in the modern maize. Analysing backcrossed maize-teosinte hybrids, the researchers came to a startling conclusion: the difference between maize and teosinte could be traced to just three genes. One gene changed the architecture of the crop