Copenhagen, Denmark
November 17, 2016
Source: University of Copenhagen
ANCIENT MAIZE Genome sequence of a 5,310-year-old maize cob provides new insights into the early stages of maize domestication. The specimen is important because it dates to a point in time approximately halfway between the beginning of maize domestication and today's corn. In a gene-by-gene analysis, the ancient sample shows that many key genes had already been affected by human selection, including the lack of a hard seed coat and changes in flowering time. Other traits were not yet under selection, including sugar content of the kernels and, surprisingly, a gene that is related to dispersal of kernels from the plant. These details are the result of a study conducted by postdoc Nathan Wales and PhD student Jazmín Ramos Madrigal from the Natural History Museum of Denmark. The results have just been published in the scientific journal Current Biology.
Researchers have long debated how and why ancient people domesticated maize, in large part because the wild ancestor of maize--known as teosinte--provides surprisingly little nutrition. Each teosinte ear produces only 5 to 12 kernels, and each kernel is surrounded by a hard casing that must be removed prior to eating.
Now, the impressive study of a 5,310-year-old maize cob from the Tehuacan Valley of Mexico brings us closer to answering these questions.
- Our study is like a genomic window into the past, says postdoc Nathan Wales, who has conducted the research together with his colleague PhD student Jazmín Ramos Madrigal, both from the Natural History Museum of Denmark.
Jazmín Ramos adds:
- By exploring the genetic history of this ancient maize it is now possible for us to infer how people used the crop in the past, selecting for certain traits at different points in time.
The study first of all shows that the ancient cob is genetically intermediate between teosinte and modern maize.
A half modern maize
In a gene-by-gene analysis, the ancient sample shows that many key genes had already been modified through human selection, including the lack of a hard seed coat and changes in flowering time.
Other traits were not yet under selection, including sugar content of the kernels and, surprisingly, a gene that is related to dispersal of kernels from the plant.
Nathan Wales explains:
- Wild plants naturally release their seeds at the appropriate time, but humans have modified domesticated cereals so they retain their seeds so they can be easily collected from fields. The finding that the ancient maize cob has the ancestral version of the gene is unexpected and encourages further research.
From a snack to a proper meal
Archaeological evidence suggests the people who planted and consumed maize 5000 years ago likely lived in small groups of several dozen people from extended families.
This could be an explanation for why the ancient Tehuacan Valley maize is morphologically and genetically so distinct from modern corn says Jazmín Ramos Madrigal and continues:
- Unlike most modern farmers, these ancient people moved seasonally and mostly consumed wild plants and animals, but supplemented their diets with some domesticated plants. It was only during later periods with higher populations and socially stratified societies that maize became the food staple. For example, the Olmecs (~1200 BC) and the Maya (200BC - 1000 AD) required reliable and predictable food sources to support their cities, and it was at that point that maize would have undergone further selection for important traits.
DNA evidence from 5,310-year-old corn cob fills gaps in history
Source: Cell Press
This photograph shows a 5,310-year-old corn cob - Credit: Bruce Smith
Researchers who have sequenced the genome of a 5,310-year-old corn cob have discovered that the maize grown in central Mexico all those years ago was genetically more similar to modern maize than to its wild ancestor. For example, the ancient maize already carried genetic variants responsible for making kernels soft, a common feature of modern corn. The findings are reported in Current Biology on November 17.
"Around 9,000 years ago in modern-day Mexico, people started collecting and consuming teosinte, a wild grass," says Nathan Wales of the Natural History Museum of Denmark. "Over the course of several thousand years, human-driven selection caused major physical changes, turning the unproductive plant into modern maize, commonly known as corn. Maize as we know it looks so different from its wild ancestor that a couple of decades ago scientists had not reached a consensus regarding the true ancestor of maize."
To better understand the domestication history of the world's most produced crop, Wales and his colleagues, including Jazmín Ramos-Madrigal, sequenced the genome of a 5,310-year-old maize cob from central Mexico. The cob, known as Tehuacan162, was excavated from a cave in the Tehuacan Valley in the 1960s, during a major archaeological expedition lead by Richard MacNeish.
Fortunately, the Robert S. Peabody Museum in Andover, MA, took excellent care of the ancient maize specimen--one of the five oldest known in the world--for decades. Wales explains that this particular cob and the DNA within it had been unusually well preserved.
"Archaeological specimens frequently have high levels of bacterial DNA due to decomposition and soil contaminants," he says. "However, during genetic testing of ancient cobs, we were astonished to find that 70 percent of the DNA from the Tehuacan162 cob was from the plant!" Most other ancient samples contain less than 10 percent plant DNA.
Tehuacan162 didn't have hard seed coats like its wild ancestor would have. But, the ancient cob is less than a tenth of the size of modern cobs, at less than two centimeters long. In addition, the ancient cob produced only eight rows of kernels, about half that of modern maize. That led the researchers to suspect that its genes would offer clues on the early stages of maize domestication.
To make the most of the small sample, Wales and Ramos-Madrigal used cutting-edge paleogenomic techniques. They extracted DNA with a method designed to recover ultra-short DNA, taking special care to avoid losing any genetic material. As a result, the researchers were able to prepare sufficient DNA for sequencing while still preserving enough of the sample to determine the cob's precise age via radiocarbon dating.
The new findings offer an informative snapshot in the 10,000-year evolutionary history of maize and its domestication, the researchers say. In addition to elucidating how maize provided a dietary foundation for ancient civilizations like the Maya, such studies can also aid in understanding and improving commercially important lines of modern maize, the researchers say.
"This is only the beginning of the story," Ramos-Madrigal says. "Humans dispersed maize across the Americas very quickly and very successfully. We want to know how humans dispersed it, which routes they took, and how maize adapted to such diverse environments."
This research was supported by the Lundbeck Foundation, the Danish Council for Independent Research, and the Danish National Research Foundation .
Current Biology, Ramos-Madrigal et al.; "Genome Sequence of a 5,310-Year-Old Maize Cob Provides Insights into the Early Stages of Maize Domestication" http://www.cell.com/current-biology/fulltext/S0960-9822(16)31120-4
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.cell.com/current-biology. To receive Cell Press media alerts, contact press@cell.com.
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