marker-assisted selection in wheat - ictsd

Chapter 19 – Technical, economic and policy considerations on marker-assisted selection in crops 383IntroductionDue to their usefulness in characterizingand manipulating genetic factors responsiblefor qualitative as well as quantitativetraits, molecular markers are considered tobe valuable tools for crop improvement.These uses of molecular markers have beeninvaluable in helping researchers understandcomplex traits, dissect them intosingle Mendelian genetic factors, and establishtheir chromosomal locations via the useof linkage maps and/or cytogenetic stocks.Availability of well characterized geneticlinkage maps is a prerequisite for taggingimportant agronomic or other traits withmolecular markers, enabling their use inMAS related activities. To date, however,few practical applications have been publishedfrom these studies. This paucity ofpublished studies may indicate the longtermnature of this research, or it mightsimply reflect the fact that marker technologyhas been applied to plant breedingefforts mostly by scientists working in theprivate sector (Hoisington and Melchinger,2004).Maize was one of the first crop speciesfor which molecular linkage maps weredeveloped, and Gardiner et al. (1993)consolidated several individual maps into aconsensus map. Rice is another species forwhich high-density linkage maps have beendeveloped (reviewed in Gowda et al., 2003)while, due to its high ploidy level and largegenome (21 linkage groups, as opposed to 10in maize and 12 in rice), efforts to developwell characterized, saturated linkage mapswith wheat have lagged behind. Otherimportant cereals and legumes are at variousstages of linkage map development. Theavailability of well-defined linkage mapsand the extent of genetic studies conductedon them therefore vary among differentcrops, and this influences the feasibility ofany MAS-related activity. Thus, while it ispossible to carry out MAS to some degreein cereals such as rice, maize and wheat, andin legumes such as soybean, for species suchas cassava and sweet potato, the so-called“orphan crops”, genetic improvement withMAS may not yet be feasible. These cropspecies may benefit more readily fromgenetic modification arising from directintroduction of genes isolated from otherspecies or organisms, which is not the focusof this chapter.Citing practical lessons learned atthe International Maize and WheatImprovement Center (CIMMYT) as well asfindings of studies conducted elsewhere, thischapter describes some actual and potentialapplications as well as the advantages anddisadvantages of MAS, and outlines possibleapplications of MAS in developingcountry plant breeding programmes.Lessons Learned from CropsNumerous scientific reports describemolecular mapping and analysis of quantitativetrait loci (QTL) for nearly everyagronomic trait in a diverse array of cropspecies. The traits covered include manyparameters associated with tolerance todrought and other abiotic stresses, maturity,plant height, quality parameters, qualitativeand quantitative factors of disease and pestresistance, and numerous seed traits andyield. Although these efforts have resultedin a vast amount of knowledge and betterunderstanding of the underlying geneticfactors that control these traits, applicationof this knowledge to manipulate genes in aneffective or simple manner for improvingcrop species has had limited success. Thescientific community is faced with thechallenges of accurate and precise QTLidentification and application of the informationderived to successful MAS efforts.