Discovering Genes Underlying QTL

摘要

Rice is the most important cereal feeding developing countries for centuries and continue to be more important as fertile land has now become limited. New rice varieties that can combat well with stresses, responsive to production inputs and highly nutritious are the most desirable for the next century. In the case of rice, conventional breeding still play important roles in generating new types of rice cultivars. Therefore, technology that can improve breeding efficiency will have great impact on making new strains of rice. In the past few years, molecular markers and copies of expressed genes have been identified and mapped to segregating populations. With those technologies, genes responsible for biotic and abiotic stress resistance, yield components and cooking quality have been mapped in several chromosomes. Innovative breeding programs that utilize tightly linked markers to genes of interest are collectively called marker-assisted selection. However, the new breeding technologies have not been implemented successfully in every case. One of the most limiting factor was the use of molecular markers that were not physically linked to the responsible genes. Secondly, cost of detection is too expensive for conventional breeders. If genes controlling the traits were used directly, marker-assisted selection would have been the most effective. A map-based approach has allowed scientists to discover few genes at a time. In addition, the reproductive barrier between cultivated rice and wild relatives has prevented us from utilizing the valuable germplasm by a map-based approach. Most genetic traits important to agriculture or human diseases are manifested as observable, quantitative phenotypes called Quantitative Trait Loci (QTL). In many instances, the complexity of the phenotype/genotype interaction and the general lack of clearly identifiable gene products render the direct molecular cloning approach ineffective, thus additional strategies like genome mapping are required to identify the QTL in question. Genome mapping requires no prior knowledge of the gene function, but utilizes statistical methods to identify the most likely gene location. To completely characterize genes of interest, the initially mapped region of a gene location will have to be narrowed down to a size that is suitable for cloning and sequencing. Strategies for gene identification within the critical region have to be applied after the sequencing of a potentially large clone or set of clones that contains this gene(s). Tremendous success of positional cloning has been shown for cloning many genes responsible for human diseases, including cystic fibrosis and muscular dystrophy as well as plan t disease resistance genes (Martin et al., 1993; Bent et al., 1994; Grant et al., 1995; Song et al., 1995).