Sequencing the Maize Genome

拟南芥（Arabidopsis）基因组已经完成测序，水稻（rice）基因组也即将完成。这些植物物种代表了植物界的主要分类，包括第一种主要作物。水稻也是包括小麦和玉米在内的谷物中的一员，这两种作物的基因组大小要大得多，分别是35倍和5倍，因此更难测序。然而，由于玉米是一种主要的农业作物，也是美国经济的一个重要因素，对其基因组进行测序将为提高食品供应、质量和安全提供关键的基础设施，同时减少化学污染对健康的风险。为了能够对更大的玉米基因组进行测序，该项目寻求提供一组重叠的克隆DNA片段，大小约为150 kb，放置在玉米遗传图谱上。这些克隆DNA片段的类似邮政编码的序列将极大地促进这种安置。这些邮政编码特别有价值，因为在玉米和密切相关的水稻基因组（synteny）之间，一定比例的序列和顺序是保守的。由于水稻基因组序列与水稻遗传图谱是一致的，因此重叠玉米DNA片段的排序将在合成区域大大增强。得到的DNA片段图谱将用于生成连续的基因组序列。这将表明，玉米基因组的较大长度（主要是由于非基因重复序列）是否需要一种不同的方法来获取玉米的基因含量。几个玉米基因组DNA长片段的例子首次为生物学家提供了关于基因密度变异性的信息，以及几个厘米器官的遗传和物理图谱的相关性。这样的遗传距离已经足够向育种者展示基因是如何根据性状与DNA的联系发现的。

One plant genome, Arabidopsis, has been sequenced, and a second one, rice, is nearing completion. These plant species represent the major taxonomic division of the plant kingdom and include the first major crop. Rice is also a member of the cereals that include crops like wheat and maize, which are much larger in genome size, 35-times and 5-times, respectively, and, therefore, more difficult to sequence. However, since maize is a major crop in agriculture and a significant factor to the economy of the United States, sequencing its genome will provide a critical infrastructure to advance food supply, quality, and safety, with simultaneous reduction of risk to health by chemical pollution. To enable sequencing the larger maize genome, this project seeks to provide sets of overlapping cloned DNA fragments, about 150 kb in size that are placed on the genetic map of maize. This placement will be greatly facilitated by zip code-like sequences of these cloned DNA fragments. These zip codes are in particular valuable since a certain percentage is conserved in sequence and order between maize and the closely related rice genome (synteny). Since the rice genome sequence is aligned to the rice genetic map, ordering the sets of overlapping maize DNA fragments will be greatly enhanced in regions of synteny. The resulting map of DNA fragments will be used to generate contiguous genomic sequence. This will show whether the larger size of the maize genome, largely due to non-genic, repeat sequences, will require a different method for obtaining the gene content of maize. A few examples of long stretches of maize genomic DNA provide biologists for the first time information about the variability in gene density and a correlation of the genetic and physical map over several centiMorgans. Such genetic distances are already long enough to demonstrate to breeders how genes are discovered based on linkage of traits to DNA.