Tanksley S D, Ganal M W, Martin G B
Department of Plant Breeding and Biometry, Cornell University, Ithaca, NY 14853.
Trends Genet. 1995 Feb;11(2):63-8. doi: 10.1016/s0168-9525(00)88999-4.
The original concept behind map-based or positional cloning was to find a DNA marker linked to a gene of interest, and then to 'walk' to the gene via overlapping clones (e.g. cosmids or YACs). While chromosome walking is straightforward in organisms with small genomes, it is difficult to apply in most plant species, which typically have large, complex genomes. The strategy of chromosome walking is based on the assumption that it is difficult and time consuming to find DNA markers that are physically close to a gene of interest. Recent technological developments invalidate this assumption for many species. As a result, the mapping paradigm has now changed such that one first isolates one or more DNA marker(s) at a physical distance from the targeted gene that is less than the average insert size of the genomic library being used for clone isolation. The DNA marker is then used to screen the library and isolate (or 'land' on) the clone containing the gene, without any need for chromosome walking and its associated problems. Chromosome landing, together with the technology that has made it possible, is likely to become the main strategy by which map-based cloning is applied to isolate both major genes and genes underlying quantitative traits in plant species.
基于图谱或定位克隆背后的最初概念是找到与目标基因连锁的DNA标记，然后通过重叠克隆（如黏粒或酵母人工染色体）“走向”该基因。虽然染色体步移在基因组较小的生物体中很简单，但在大多数植物物种中很难应用，因为这些植物通常具有庞大而复杂的基因组。染色体步移策略基于这样一种假设，即找到与目标基因物理距离很近的DNA标记既困难又耗时。最近的技术发展使许多物种的这一假设不再成立。因此，现在的定位模式已经改变，即首先在与目标基因的物理距离小于用于克隆分离的基因组文库平均插入片段大小的位置分离一个或多个DNA标记。然后使用该DNA标记筛选文库并分离（或“着陆”于）包含该基因的克隆，而无需进行染色体步移及其相关问题。染色体着陆及其得以实现的技术，可能会成为在植物物种中应用基于图谱的克隆来分离主基因和数量性状相关基因的主要策略。
