Teams of scientists in the US and Mexico have independently sequenced the most complex plant genome to date. Comparison of two varieties offers insight into the early domestication of corn.
Jason Williams: This week, scientists in the United States and Mexico announced the complete genetic sequence, or genome, of the corn plant. Corn, or maize as it is known by scientists or much of the world, is the most important crop in the Americas, and accounts for 47 billion dollars in revenue in the US alone. According to Rob Martienssen of Cold Spring Harbor Laboratory, the multi-million dollar effort was the culmination of a century of intense genetic research.
Rob Martienssen: Maize of course is well known by the general public because of its use as food and actually as feed as well, for animal feed and so on, so it has huge agricultural value, but actually that is not the only reason to sequence the maize genome. Maize has been a very important model in genetics for more than a century now. It was actually used by some of the earliest pioneers of genetics at the beginning of the 20th century to discover the rules around heredity. The fact that genes mapped to chromosomes were first discovered in maize, and so it’s very exciting now to see the maize genome come to fruition with all of that wonderful rich history of knowledge behind it.
Jason Williams: Over a four year period, researchers at Washington University in Saint Louis, Cold Spring Harbor Laboratory in New York, the Arizona Genomics Institute, and Iowa State University dissected the corn genome into millions of pieces, determined the genetic sequence of each piece, then reassembled them into the 10 chromosomes of maize. Richard Wilson of Washington University Genome Center explains the challenge of assembling the maize genome, which has thousands more genes than the human genome, and contains vast regions of repetitive DNA.
Richard Wilson: The maize genome is about 80 percent repetitive, so imagine if you will a large jigsaw puzzle cut into 1000 pieces, and it’s a picture of a boat on blue water on a beautiful cloudless day, so you’ve got a lot of blue there and it’s hard to figure out if this piece goes next to this piece or someplace entirely different. And so that’s kind of how the genome is to put together. So we’ve had with some methods that would allow us to simplify that problem, so that we know that we’re just coming from one corner of the jigsaw puzzle if you will, and that’s helped out a lot.
Jason Williams: It is believed corn was domesticated in Mexico, about 10,000 years ago. So, in a larger sense, the maize genome sequence is the end point of thousands of years of agricultural breeding by Meso-American farmers. Luis Herrera-Estrella of the Mexican National Laboratory of Genomics for Biodiversity explains the cultural and scientific importance of the maize genome to Mexico.
Luis Herrera-Estrella: Well, maize is probably the most important crop in Mexico. If you go back to the Indian culture, the Mayans believe that men 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.
Jason Williams: While the U.S. team sequenced a modern variety of corn, called B73, the Mexican team worked with an ancient variety, called Palomero, which is used for popcorn. Palomero has much less repetitive DNA than B73, which simplified sequencing. Jean-Philippe Vielle-Calzada of the National Laboratory of Genomics, talks about Palomero and how comparing the two genomes provides new insight into the domestication of corn.
Jean-Philippe: B73 is an inbred line that is very much used as a parental line in many varieties that are grown in the midwest in the U.S. The ability to sequence that genome offers tremendous opportunities for conducting what we call genomic comparisons. What we did was basically to try to identify genomic regions that were fully conserved between Palomero and B73.
From an evolutionary point of view, those regions are interesting because they are candidates to represent segments that were very early selected in maize evolution.
What is very interesting from more study of Palomero is that the domestication genes we found encode for genes that confer heavy metal tolerance and these are hard to associate with morphological changes. The question is: why genes that confer heavy metal tolerance would have been selected so early on in maize domestication? We believe it might be linked to volcanic activity.
Jason Williams: The side-by-side publication of two maize genomes by groups in the United States and Mexico highlights centuries of collaboration between farmers and geneticists that have made corn the most important crop in the new world. The comparison of these two sequences may help scientists understand the key events that shaped this uniquely American plant.