Explosive Origins of Corn

Travel to Mexico to see the site of the oldest evidence of corn cultivation and learn why comparing two maize genomes implicates a volcano in the early evolution of corn.

Research teams from the United States and Mexico have just published the complete maize genome -- the entire set of genetic information belonging to the plant better known to us as corn. The American sequence, from a modern variety called B73, provided a baseline of genetic information, against which to compare the Mexican race, Palomero. I went to Mexico to get the story behind the research and put the genome sequence into the context of the history of maize cultivation. (MAP ANIMATION). The story began just southwest of Mexico City at Nevado de Toluca. The Aztecs called this volcano, Lord of the Cornstalks because it looks down on cornfields that have been cultivated for millennia . In the end, a 15-mile trip on unsurfaced road took me most of the way up. Then a lung-aching hike took me up the final pitch – to a pass at 14,000 feet that looks down on the lake at the center of the vast caldera. The Toluca volcano erupted most recently 10 thousand years ago. Carpeting the region with ash and debris. Now a comparison of genome sequences of 2 races of Maize shows this eruption may have been critical to the evolution of modern corn. Both races have conserved genes from the decontamination of heavy medals, such as cadmium and zinc. Prior to industrialization volcanic eruptions would have been the major source of heavy metals in the soil and it appears that the cultivation of maize selected for plants that were tolerant. To get the details on this story I traveled 200 miles northwest to the National Laboratory of Genomics for Biodiversity in Irapuato. This immaculately modern facility was designed to bring Mexico into the international genome sequencing club, and the Palomero genome was its first big project. Luis Herrera-Estrella, the Laboratory's Director, and Jean-Philippe Vielle-Calzada, who headed the genome sequencing effort, told me about the cultural significance of the project. Well, maize is probably the most important crop in Mexico. If you go back to the Indian culture, the Mayas believe that man was made out of maize and since then maize has taken an integral part of the development of Mexico culturally in terms of religion, economically, and also is a major component of the Mexican diet, so we consider that it was an absolute requirement of Mexico to participate in the sequences of the maize genome, and that we could select a maize genotype that was ancient and it could be used also to compare with other sequencing projects that were done in the U.S. Palomero has been found by Mexican archaeologists dating back to 1900 before Christ, so this is what we call the classic period of Mexican Meso-American cultures, and in Teotihuacan, there were offerings, burial offerings, where you could find the cobs of Palomero that had their seeds still attached and popped, so it is very interesting. It is only one of four landraces that have been found in archaeological remains, so therefore it is believed to be one of the most ancestral landraces. Contrary to the expected sequence differences that increased as maize diversified into different varieties, Jean-Phillipe's comparison of Palomero and B73 turned up long stretches of identical sequences shared by both genomes. Interestingly enough, among the collection of genes that are present within those conserved regions, the most highly represented ones encode for genes that confer responses to abiotic stress, in other words to environmental stresses. and among those, the most highly conserved and that really behaved as what we call domestication genes encode for heavy metal tolerance response. That was really intriguing for us. We found that at least three of them, which are non-linked meaning separated in the genome quite distantly, are clearly behaving as domestication genes, meaning that these genes were probably selected very early on during maize evolution. And then the question comes: Why would genes that encode for heavy metal tolerance be selected very early on? Could these native Mexicans have the ability to distinguish responses to heavy metal? That’s clearly not the case. What is most probable is that in these environmental conditions that were present at that time, that were dominating at that time influenced maize domestication. We start inquiring about the volcanic activity in the region that is considered the cradle of maize domestication, and amazingly enough we found out that there is a period of time, in which we call the trans-Mexican volcanic belt was very active. We believe it might be linked to volcanic activity. My exploration came full circle when I traveled 50 miles south of Nevado de Tuluca, to the agricultural region near Igaula. I went looking for a rock shelter where the story of maize begins. Biologists believe maize was first domesticated in this area, along the tributaries of the Balsas River, and this is where the most closely related wild relatives of corn are found. After a jarring ride up a rock-strewn road, I followed an old man up an obscure path to find a small shelter beneath a giant boulder. It wasn't much to look at, and it was filled with threatening bees, but it was thrilling all the same. I've just hiked up to the shelter here which takes us back to the very origins of maize cultivation. Microscopic analysis of grinding stones found here carry traces of maize that date back 8700 years ago. That’s as ancient as the oldest evidence we have for cultivation of wheat in the fertile crescent. Ancienc Meso-American farmers would likely have grown and harvested maize in the fields of the tributaries the Balsas River valley just below us. The the would have brought the dried maize here to this shelter and ground it into meal. The grinding stones were really nothing fancy, they were just like the river rocks I have here. They would have been effective enough though in breaking up the maize kernels into a rough cornmeal that could have been combined with water and heated to make a simple porridge or made into a stiffer dough and fashioned into that quintessential Mexican staple, the tortilla. In Mexico, I learned that the story of maize is so wrapped up in human history that it is difficult to draw a line between the plant and the people who brought it under domestication. The line also blurs between science and culture.

genomes, maize, volcano, corn, origins, evolution, mexico

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