marker-assisted selection in wheat - ictsd

Chapter 8 – Marker-assisted selection in maize 129it was generally superior to phenotypicselection in accumulating favourable allelesin one individual (van Berloo and Stam,1998, 2001; Charmet et al., 1999). MARSappeared to take better advantage of thegenetic diversity present in the populationsto which it was applied than phenotypicselection. Simulation research conductedby van Berloo and Stam (2001) showedthat MARS was between 3 and almost20 percent more efficient than phenotypicselection. The advantage of MARS overphenotypic selection was greater when thepopulation under selection was larger ormore heterozygous (BC 1 s or F 2 s vs. RILs,recombinant inbred lines, or DHs, doubledhaploids). Although van Berloo andStam (2001) limited their simulations topopulations of up to 200 individuals, theirresults seem to indicate that the relativeadvantage of marker-assisted over phenotypicselection would keep increasing aspopulation size increased. The same simulationstudies showed that the advantageof marker-assisted over phenotypic selectionwas larger when dominant QTL wereinvolved in the selection index, or whentrait heritability was low in the case ofselection for a single trait (van Berloo andStam, 1998, 2001). These latter observationsare of little relevance to most commercialmaize breeding programmes, the goalof which is generally the development ofinbred lines improved for several traitsthat will be later combined into superiorhybrid varieties. They should, however,increase the appeal of MARS approachesfor breeding programmes aimed at developingopen-pollinated varieties.Simulations have also addressed theimpact of the amount and quality ofQTL information on selection efficiency.Simulation and empirical studies (Beavis,1994, 1999) showed that QTL mappingexperiments based on segregatingpopulations of less than 500 individualsgenerally revealed only a subset of all QTLaffecting the complex traits segregating inthese populations. Quantitative trait lociinformation used in subsequent MARSwas therefore necessarily incomplete.Van Berloo and Stam (2001) showed thatthe relative advantage of MARS overphenotypic selection decreased rapidlywhen the fraction of the total genotypicvariance explained by the QTL included inthe selection index decreased. By contrast(van Berloo and Stam, 1998; Charmet et al.,1999), the efficiency of MARS seems to berather robust to the well-documented (Lee,1995) uncertainty of QTL genetic locations.The use of genotypic information at markersflanking the QTL possibly explains thisobservation.The cost efficiency of MARS was alsoinvestigated through simulation (Moreauet al., 2000; Xie and Xu, 1998). Whensimulating selection for a single trait,Moreau et al. (2000) found that, irrespectiveof the heritability of the trait, MARS wasalways more cost efficient than phenotypicselection if the cost of genotyping was lessthan that of evaluating one individual inone plot. When simulating simultaneousselection for multiple traits, Xie and Xu(1998) found that MARS was more costefficient than phenotypic selection if thecost of genotyping was less than that ofphenotyping one individual for all traits.These studies were based on a singlegeneration of MARS. Also, they did nottake into consideration any factors besidesgenotyping and phenotyping costs, althoughfactors influencing the length of a selectioncycle or the number of cycles that can becompleted in a year can obviously affect therelative economic merits of marker-assistedand phenotypic selection.