Rice Genetics IV - IRRI books - International Rice Research Institute

Application of Mendelian geneticsin rice breedingJ.N. Rutger and D.J. MackillThe application of Mendelian genetics has clearly led to many breeding advancesin rice as well as in other crops. In this chapter, we emphasize economicallyimportant traits for which segregation ratios can be distinguishedwithout elaborate progeny testing or molecular markers. Four general groupsof traits are reviewed: agronomic and physiological traits, grain quality, pestresistance, and resistance to abiotic stresses. The single most importanttrait has been semidwarfism, conferred by the sd1 gene. Other importantagronomic and physiological traits are photoperiod sensitivity, glabrous hulls,gold hull color, and purple leaf. Among grain quality characters, amylose contentis the most important, ranging from waxy types with essentially no amyloseto temperate japonica short- and medium-grain types with 16–18% amylose,to tropical japonica long grains with 21–24% amylose, and to manyindica types with up to 28% amylose. Another significant grain quality trait isaroma, which often appears to be under simple genetic control but which isdifficult to recover in high-yielding backgrounds. Breeding for pest resistance,including both diseases and insects, has been one of the most successfulexamples of the use of major genes in crops, and yet it is a recurring challenge.The most important disease example is blast resistance, which hasbeen a focus of breeding efforts for decades. The exploitation of major genesfor bacterial blight resistance has been more successful than for blast. Manysuccessful cases of major gene resistance for brown planthopper, green leafhopper,and gall midge have been reported in the past three decades. Ingeneral, resistance to abiotic stresses has been polygenic. Exceptions havebeen low-temperature-induced chlorosis at the seedling stage and submergencetolerance.King and Stansfield (1985) define Mendelian genetics as referring to the inheritanceof chromosomal genes following the laws governing the transmission of chromosomesto subsequent generations. This is also called Mendelism. For the purposes ofthis chapter, this all-inclusive definition will be modified to emphasize economicallyApplication of Mendelian genetics in rice breeding 27