Sequencing the Maize Genome

一种植物基因组拟南芥已经完成测序，另一种植物水稻即将完成测序。这些植物物种代表了植物界的主要分类部门，并包括第一种主要作物。水稻也是包括小麦和玉米等作物在内的谷物的一员，这些作物的基因组大小分别是小麦的35倍和玉米的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，该项目试图提供几组重叠的克隆DNA片段，大小约为150kb，放置在玉米的遗传图谱上。这些克隆的DNA片段的邮政编码序列将极大地方便这种放置。这些邮政编码特别有价值，因为玉米和密切相关的水稻基因组(Synteny)之间的序列和顺序保留了一定的百分比。由于水稻基因组序列与水稻遗传图谱相一致，重叠的玉米DNA片段集在共性区域的排序将大大增强。所得到的DNA片段图谱将用于生成连续的基因组序列。这将表明，玉米基因组的更大规模(主要是由于非基因的重复序列)是否需要不同的方法来获得玉米的基因含量。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的可变性以及几个厘米器官上的遗传和物理图谱的相关性的信息。这样的遗传距离已经足够长，足以向育种者展示基因是如何根据性状与DNA的联系来发现的。

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.