marker-assisted selection in wheat

Chapter 8 – Marker-assisted selection in maize 119IntroductionThe ability to identify genetic componentsof traits, particularly quantitative traits, inMendelian factors, and to monitor or directtheir changes during breeding through theuse of DNA-based markers has createdmuch enthusiasm. Claims were sometimesmade that marker-assisted selection(MAS) would rapidly replace phenotypicselection and dramatically reduce the timerequired to develop commercial varieties(Mazur, 1995). At the turn of this century,phenotypic selection was still theapproach on which maize breeding programmesmostly relied to develop new andimproved cultivars while MAS had contributedto advances in introgression, orbackcross breeding (Ragot et al., 1995; Ho,McCouch and Smith, 2002; Ribaut, Jiangand Hoisington, 2002; Morris et al., 2003).Overly optimistic statements and exaggeratedpromises about the power of MAS toimprove complex traits created excessivelyhigh and largely unfulfilled hopes andprompted a wave of cautious and sometimespessimistic views (Melchinger, Utzand Schön, 1998; Young, 1999; Goodmanand Carson, 2000; Bernardo, 2001).Recently, multinational corporationswith large maize breeding programmesreported the routine and successful use ofMAS (Johnson, 2004; Niebur et al., 2004;Eathington, 2005; Crosbie et al., 2006).Rates of genetic gain twice as high as thoseachieved through conventional breedingwere reported for MAS in maize. Accountswere also given of a number of MAS-derivedsingle-cross (i.e. simple) hybrids being currentlyon the market. Although too littleis known about the methods (e.g. breedingschemes, mathematical algorithms) andtools (e.g. marker technologies, computerprograms, databases) used to develop thesehybrids, these results have raised confidencein the ability of MAS to increasethe rate of genetic gain over what can beachieved through conventional breeding.As technologies evolve and marker genotypesbecome less expensive, MAS becomesincreasingly within the reach of developingcountries. Whenever necessary, transfer ofmethods or tools from private companiesto developing countries should be madepossible while preserving the commercialinterests of the companies concerned,thereby contributing to increasing the rateof genetic gain where it is most needed.Much has happened in maize breedingsince Stuber and Moll (1972) first reportedthat selection for grain yield in maize hadresulted in changes in allele frequenciesat several isozyme loci throughout thegenome. In so doing, they essentially laidthe grounds for MAS in maize. Indeed,if phenotypic selection could produce achange in marker allele frequencies, thenwhy could deliberately altering markerallele frequencies at specific loci not producepredictable phenotypic changes forone or several traits?The objectives of this chapter are toprovide the scientific community anddecision-makers with information on thecurrent status of MAS in maize breedingprogrammes, including the major stepsthat led to it, and to provide suggestionsto developing countries for deploying thetechnology and methods involved in an efficient,cost-effective and realistic manner.How has MAS been used by theprivate sector to improve themaize crop?Applications of DNA markers in privatemaize breeding programmes startedin the 1980s with the identification ofDNA clones used to detect restrictionfragment length polymorphisms (RFLPs)