Quantitative Trait Loci Mapping in Plants: Concepts and Approaches

Abstract

The narrow genetic base of modern crop cultivars is a serious obstacle to sustain and improve crop productivity due to rapidly occurring vulnerability of genetically uniform cultivars to potentially new biotic and abiotic stresses. Plant germplasm resources, originated from a number of historical genetic events as a response to environmental stresses and selection, are the important reservoirs of natural genetic variations that can be exploited to increase the genetic base of the cultivars. However, many agriculturally important traits such as productivity and quality, tolerance to environmental stresses, and some of forms of disease resistance are quantitative (also called polygenic, continuous, multifactorial, or complex traits) in nature. The genetic variation of a quantitative trait is controlled by the collective effects of numerous genes, known as quantitative trait loci (QTLs). Identification of QTLs of agronomic importance and its utilization in a crop improvement requires mapping of these QTLs in the genome of crop species using molecular markers. This review will focus on the basic concepts and a brief description of existing methodologies for QTL mapping and their merits and demerits including traditional biparental mapping and the advanced linkage disequilibrium (LD)-based association mapping. Examples of some of the recent studies on association mapping in various crop species are provided to demonstrate the merits of high-resolution association mapping approach over traditional mapping methods. This review thus will provide non-expert readers of crop breeding community an opportunity to develop a basic understanding of dissecting and exploiting natural variations for crop improvement.