Why Sequence Maize Genome?
Recent completion of human, plant and animal genomes have demonstrated that genomic sequence is the most comprehensive way toward gene discovery - a first step toward identifying the role of each gene. The completion of the Arabidopsis genome sequence has been very informative as to the gene content of this organisms; and, this project has strengthened research programs throughout plant biology. Comprehensive understanding of gene function in crop plants such as maize and rice will require thorough investigations of their own genomes to identify all of their genes and to determine the function of those genes.
The maize genome is large. It is estimated that up to 50,000 genes are distributed among the ten chromosomes of maize and contribute to an estimated haploid genome size of ~2.5 billion nucleotides. The vast majority of which is made up of a small number of retrotransposons that are highly repeated in the genome (Bennetzen, 1996; SanMiguel et al., 1996). Maize and rice share a common ancestor, thus the recently sequenced rice genome (Feng et al., 2002; Sasaki et al., 2002) improves our understanding of maize.
The rice genome sequence is essentially complete; and, given that rice and maize are related, it is likely that a great deal about the maize genome can be learned by studying rice. For example, comparative genomics has demonstrated that closely related species have similar gene content, and that the order of genes along discrete segments of the genome of one organism are preserved to greater or lesser extents in the genomes of other related species. This phenomena, termed synteny, is well documented among the vertebrates where the existence of genome co-linearity between mouse and human was leveraged while assembling the genome sequence of mouse. Comparative analysis of several grass genomes, including maize, rice, sorghum, barley and wheat, have revealed extensive conservation of gene content and order at the gross level of the overall genetic map. Llaca and Messing and Song and Messing have recently investigated synteny in cereal genomes.
However, 60 million years and as many as 15000 local rearrangements may differentiate the maize and rice genomes. Thus, rice may often be a too-distant model to facilitate map based cloning in maize. Therefore, A broad understanding of the genes present in maize and their relationship within the genome will provide a foundation for understanding and improving maize, as well as other cereal crops. For any crop plant, high quality, integrated, genetic and physical maps serve as the foundation for numerous studies, especially those aimed at improving the agronomic characteristics of the plant.
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