marker-assisted selection in wheat

Chapter 3 – Molecular markers for use in plant molecular breeding and germplasm evaluation 41Yoshimura et al., 1998; Wang et al., 1999;Bryan et al., 2000; Sun et al., 2004) andmany other genes for disease resistancehave been tagged with linked markers. Thisopens the door for targeted approaches toMAS (Valent et al., 2001). While the diseaseresistance literature is too vast to summarizehere, it is important to note that advances inthis area are having an impact on varietalimprovement programmes (www.syix.com/rrb/98rpt/MarkerAssist.htm). Pyramidingof resistance genes into a single varietyand the construction of multiline varieties,each with one or more R genes (resistancegenes) that can be used in various combinations,are under way to develop moredurable forms of disease and insect resistance(Yoshimura et al., 1992; Yoshimura etal., 1995; Hittalmani et al., 1995; Blair andMcCouch, 1997; Ndjiondjop et al., 1999;Davierwala et al., 2001; Su et al., 2002;Conaway-Bormans et al., 2003; Lorieux etal., 2003; Hayashi et al., 2004).Marker-based selection is also helpfulin attempts to transfer genes from exoticgermplasm into cultivated lines. In rice,several workers have used RFLP andSSR markers to monitor introgression ofbrown planthopper resistance from O.officinalis (Kochert, Jena and Zhao, 1990),bacterial blight resistance from O. longistaminata(Ronald et al., 1992), aluminumtolerance or yield and quality-related traitsfrom O. rufipogon (Nguyen et al., 2002;Thomson et al., 2003; Septiningsih et al.,2003a, b) or from other wild species such asO. glumaepatula (Brondani et al., 2002) orO. glaberrima (Jones et al., 1997; Lorieuxet al., 2003) into cultivated O. sativa backgrounds.Marker-assisted introgressionstrategies have also been used in a numberof livestock breeding programmes but,because of longer generation intervals andlower reproductive rates, this is generallyfeasible for genes of large effect (Dekkers,2004; Chapter 10). Identifying the recombinantswith the least amount of donorDNA flanking the genes of interest isenhanced by the use of molecular markers(Monna et al., 2002; Takeuchi et al., 2003;Blair, Panaud and McCouch, 2003). Inthese examples, MAS offers a powerfulstrategy for making efficient use of thewealth of useful genetic variation that existsin the early landraces and wild speciesof cultivated food crops (Tanksley andMcCouch, 1997).As this kind of information accumulates,MAS permits rapid identification of individualsthat may contain only one geneticcomponent of a complex trait. Once identified,such an individual can be crossedwith another individual in a breeding programmeso that multiple, complementarygenes are combined to optimize a quantitativelyinherited trait. Individuals containingonly one gene of interest often defy accuratephenotypic identification wherepolygenic traits are concerned because varioustypes of epistasis, or gene interaction,may be required to generate the phenotypeof interest (Yamamoto et al., 2000; Zhenget al., 2000).Linkage disequilibrium (LD) mappingis another marker-assisted approachthat provides important informationthat is immediately relevant to breedingprogrammes (Remington, Ungererand Purugganan, 2001; Flint-Garcia,Thornsberry and Buckler, 2003). Usingcollections of distantly related germplasmaccessions rather than populations derivedfrom bi-parental crosses allows researchersto explore the relationship betweenphenotype and genotype in materials thathave been amply tested over years andenvironments, often as part of an appliedbreeding programme. This provides