Milestone in maize genomics

Rutgers researchers, with the support of the U.S. National Science Foundation, have pushed back the frontiers on the genetic nature and history one of the world’s most important crops – corn. This crop dominates agriculture in the United States, where approximately 9 billion bushels are produced annually at a value of $30 billion. Maize (or corn) is also an important dietary staple in much of the third world. Rutgers University’ Joachim Messing and his colleagues announced this month discoveries about the inner workings of corn, its origins and evolution, with implications for breeding, genetic engineering and future genomic studies.

"This latest research, conducted with worldwide collaborations, led us to a new understanding of maize that will help enable scientists and farmers to make major improvements in one of the world’s most significant crops and gain new and important insights in plant genomic studies," said Messing, director of the Waksman Institute of Microbiology at Rutgers, The State University of New Jersey. The findings are presented in three papers in the journal Genome Research and one in the Proceedings of the National Academy of Sciences.

The scientists conducted the most comprehensive survey of the maize genome ever performed and established for the first time the genome’s magnitude – approximately 59,000 genes – and the relative position of the genes. This is twice as many as the human genome and the highest number of genes of any genome sequenced to date. Messing emphasized that this survey is only a first step and conducting a whole genome sequence is a priority dictated by nutritional, economic and societal needs.

The research further established that in addition to its immense size, the corn genome is extremely complex due, in part, to positional instability as well as its genetic history. Messing and his colleagues concluded that maize genes are scrambled, having moved around to different locations throughout the genome – an occurrence unheard of in other species, including the human genome. This has important implications for genetic engineering.

"An argument often cited against the introduction of external genes, a common practice in genetic engineering, suggests that it would create an unnatural instability in the genome," said Messing. "With all the maize genes moving around by themselves in nature, perhaps conveying some selective advantage in doing so, this argument is unfounded."

Through sophisticated computational analysis, the researchers concluded that today’s corn is the product of two very closely related ancestral species that no longer exist. About 5 million years ago the species crossed and, in doing so, doubled the number of genes. Through mechanisms not yet revealed, many of these genes were shed and then others duplicated through gene amplification as this process is termed.

When compared to closely related species today, the researchers found that as much as 22 percent of the maize genes could be identified as being different. This was surprising, considering that other close relatives – such as chimpanzees and humans – differ in less than one percent of their genes.

"It looks like significant evolutionary change happened in a relatively short time," said Messing. "Because they are immobile, plants have to adapt to changes more rapidly than animals that can move to escape environmental impacts. Plants are continually faced with a variety of seasonal challenges and assaults by a series of different pests which may well lead to evolution on a fast track."

While the findings offered in the four newly published papers provide exciting, new glimpses into the nature of maize, Messing stressed the need for the completion of a whole genome sequence, a more detailed analysis of gene expression in maize, and a better understanding of its genetic and cellular mechanisms.