Rice Genetics IV - IRRI books - International Rice Research Institute

Molecular mapping and markerassistedselection for major-genetraits in riceD.J. Mackill and Junjian NiSince the late 1980s, many loci controlling both qualitative and quantitativetraits of rice have been mapped using DNA markers. This chapter focuses onthe mapping of major genes in rice. The distinction between major and minorgenes has become unclear because of advances in molecular-marker mapping.A major gene can be involved in determining both qualitative and quantitativetraits. Here we define a major gene as a locus that results in discretephenotypes in a segregating population or one that controls more than 50%of the phenotypic variation for a continuously distributed trait. The latter iscommon for major genes controlling traits that are highly influenced by environment.Resistances to diseases and insects have received the most attentionfor mapping because of their importance as breeding objectives. Molecularmapping of major genes is important in determining the allelism ofgenes conferring identical phenotypes, use as a selectable marker in a breedingprogram, and positional cloning of genes. Molecular markers will be mostuseful for selection when (1) the phenotype is difficult or expensive to measuredirectly, (2) genes of similar phenotype are being pyramided into a singleline, or (3) markers are being used to select against the donor genome in abackcrossing program.The use of genetic markers for identifying and manipulating plant genes is not a newconcept (Tanksley 1983). Morphological and isozyme variants have been used on alimited scale to map genes of economic importance. However, the insufficient numberand variability of such markers prevented their wide-scale adoption for breedingpurposes. This situation changed dramatically with the discovery that differences inendonuclease restriction sites or insertions/deletions between adjacent sites could beused as markers to construct genetic maps (Botstein et al 1980). The first restrictionfragment length polymorphism (RFLP) map of rice was constructed in the 1980s atCornell University (McCouch et al 1988) and high-density maps were subsequentlydeveloped (Causse et al 1994, Kurata et al 1994). Rice geneticists have now widelyapplied these maps to map genes controlling qualitatively and quantitatively inheritedMolecular mapping and marker-assisted selection . . . 137