marker-assisted selection in wheat - ictsd

122Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishother and with breeding activities, whichmeant that scientists needed to learn howand when MAS provided a comparativeadvantage over other methods.MAS: enabling methods, tools andinfrastructurePerhaps the component of infrastructure ingreatest need of development was related tothe acquisition of genotypic data (i.e. DNAmarkers). Although the concept of associatingmarkers with quantitative traits wasnot new (Sax, 1923), the discovery reportedby Stuber and Moll (1972) was very significant.Stuber and Moll (1972) described forthe first time associations between molecularmarkers and quantitative traits whileprevious associations had been based onmorphological markers (Sax, 1923). Theadvantages of molecular over morphologicalmarkers soon became obvious anddetailed descriptions of these advantageswere published by Tanksley et al. (1989)and Stuber (1992).Two of these advantages are of particularimportance. First, molecular marker genotypescan usually be obtained from anyplant tissue, even from young seedlings orkernels, while morphological markers frequentlyrequire the observation of whole,mature plants. Selection can therefore occurearlier in the plant’s cycle when usingmolecular markers than when using morphologicalmarkers. The ability to conductearly selection, possibly before flowering,can have a tremendous impact on the rateof genetic gain of a breeding programmeand therefore constitutes a very significantadvantage of molecular over morphologicalmarkers.Second, molecular markers are neutralmarkers. They are not affected by environmentalor growing conditions. Theyare not affected by the genetic backgroundeither, nor do they affect phenotypes. Theexpression of morphological traits, by contrast,can be dependent on environmentalor growing conditions. In addition, epistasicinteractions are often observed amongmorphological marker loci or betweenmorphological marker loci and the geneticbackground. These epistatic interactionsprevent distinguishing all genotypes associatedwith morphological markers and furtherlimit the number of morphological markersthat can be studied simultaneously.Although isozyme markers had manyadvantages over morphological markers,the lack of a sufficient number of polymorphicloci limited their use for MAS(Goodman et al., 1980). Nevertheless, isozymemarkers are still used for qualitycontrol during seed production.RFLPs (Botstein et al., 1980) are basedon DNA polymorphisms detected throughrestriction nuclease digestions followedby DNA blot hybridizations. The abundanceand high level of polymorphism ofRFLPs, especially in maize, allowed theconstruction of extensive maize geneticmaps (Helentjaris et al., 1986; Burr et al.,1988; Hoisington, 1989; Coe et al., 1995;Davis et al., 1999) as well as the identificationand mapping of many QTL.Being robust, reproducible and codominant,RFLPs are perfectly suited forgenetic studies as well as for MAS applications.Their two main disadvantagesare the large quantities of DNA required,and the difficulty to miniaturize and automate.Nevertheless, RFLPs were quicklyadopted and represented the marker systemof choice for many plant species includingmaize throughout the 1980s and duringmuch of the 1990s.The development of the polymerasechain reaction (PCR) (Saiki et al., 1988)turned out to be a major breakthrough in