The completion of a high-quality sequence of the maize (corn) genome was announced in the cover story of the 20 November issue of Science. This new maize sequence provides significant refinements over the draft sequence that was announced in February 2008, including the elimination of redundancy and improvements in the ordering and orientation of chromosomal segments.
This new genome sequence reports the sequence of genes in maize and provides a detailed physical map of the maize genome. This map identifies the order in which genes are located along each of maize’s 10 chromosomes and the physical distances between those genes.
Additional information provided by the new maize genome sequence includes the locations on chromosomes of interesting, repeated sections of DNA (called centromeres) that are responsible for the faithful inheritance of those chromosomes by daughter cells during cell division.
This new genome sequence promises to:
Advance basic research of maize and other grains; and
Help scientists and breeders improve maize crops, which are economically important and serve as globally important sources of food, fuel and fiber.
Resulting improved strains of maize may, for example, produce larger yields, show resistance to disease, offer efficiencies in nitrogen use that would enable farmers to reduce applications of costly, polluting fertilizers, and tolerate changes in rainfall or temperature accompanying climate change.
The new maize sequence was produced by a consortium of researchers that was led by the Genome Sequencing Center (GSC) at Washington University in St. Louis, Mo., and included the University of Arizona, Iowa State University and Cold Spring Harbor Laboratory in New York. This sequencing project was part of a joint Department of Energy/Department of Agriculture (USDA)/ National Science Foundation (NSF) effort that was funded by NSF under the auspices of the National Plant Genome Initiative (NPGI).
The NPGI, which began in 1998, is an ongoing effort to understand the structure and function of all plant genes at levels from the molecular and organismal to interactions within ecosystems. The NPGI focuses on plants of economic importance and plant processes of potential economic value.
The same Science issue also announces the results of two other NPGI-funded studies that were enabled by the new maize sequence. One of these studies produced a so-called HapMap of the maize genome, which describes the genetic differences between various strains of maize that are currently bred around the world. This resource will help researchers identify the genes that control various maize traits. The HapMap was produced by a team led by Edward S. Buckler of the USDA and Cornell University and Doreen H. Ware of the USDA and Cold Spring Harbor Laboratory.
The other NPGI-funded study builds on the new maize genome sequence by identifying a surprisingly widespread biological process that determines the level of expression of certain genes present in hybrid strains of maize. This study was produced by a team led by Patrick S. Schnable of Iowa State University.
The 20 November issue of Science also reports on the sequencing by a Mexican consortium led by Luis Herrera-Estella of CINVESTAV, Irapuato, Mexico of the popcorn variety Palomero toluqueño, which is bred in central Mexico. Comparisons between Palomero toluqueño and the NSF-funded genome sequence, which is from a maize strain that is inbred in mid-western regions of the US, reveals important clues about how maize has been domesticated over the last 10,000 years and highlights the importance of biodiversity.
In addition, the 20 November issue of PLoS Genetics features an editorial on the new maize sequence and ten more companion studies—each of which either provides background information on the development of the maize sequence or uses the new maize sequence to produce additional insights into maize genetics. In addition to advancing research on maize, the maize genome sequence is also expected to advance other cereal genome sequencing projects, such as those for wheat and barley.
The maize sequencing project, which was initiated in 2005, is a notable achievement because it was completely quickly and because the maize genome is among the most challenging genomes sequenced to date. The complexity of the maize genome is partly due to its size: with 2.5 billion base pairs covering ten chromosomes, the maize genome is almost as big as the human genome.
The complexity of the maize genome is also partly due to the fact that about 85% of its DNA is composed of transposable elements—segments of DNA that can move between locations.
Patrick S. Schnable et al. (2009) The B73 Maize Genome: Complexity, Diversity, and Dynamics. Science 326 (5956), pp. 1112-1115 doi: 10.1126/science.1178534
Catherine Feuillet and Kellye Eversole (2009) Perspective: Solving the Maze. Science 326 (5956), pp. 1071 - 1072 doi: 10.1126/science.1183463