marker-assisted selection in wheat - ictsd

130Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishIn contrast to the abundance of QTLmapping reports, very few accounts ofMARS experiments are found in theliterature. Moreau, Charcosset and Gallais(2004) compared phenotypic, markeronly,and combined recurrent selection forgrain yield and grain moisture at harvestover several cycles and years in maize.Combined selection was based both onphenotypic and marker information whilemarker-only selection was based on markerinformation only. Both the marker-onlyand the combined selection methodsconstitute MARS approaches. Severalcombinations of these three methods ofselection were applied to the segregatingpopulation that served to map the QTLused in marker-based selection indices.Over the six years of the experiment,two cycles of phenotypic selection, twocycles of combined selection, one cycle ofcombined selection followed by two cyclesof marker-only selection, and one cycle ofmarker-only selection were conducted inparallel. A reassessment of the positionsand effects of QTL was conducted after thefirst cycle for the three schemes containingmultiple cycles. All MARS methods weremore efficient than phenotypic selectionto increase the frequency of favourablealleles at QTL. Nevertheless, Moreau,Charcosset and Gallais (2004) reportedno significant difference between markerassistedand phenotypic selection on themultitrait performance index, although allMARS methods resulted in genetic gain forboth grain yield and grain moisture whilephenotypic selection resulted in genetic gainfor grain yield but an unfavourable evolutionof grain moisture. This disappointingresult was tentatively explained by thehigh heritability of the traits, favourable tophenotypic selection, while the percentageof total phenotypic variance explained bythe QTL detected for both traits was onlyabout 50 percent. One very encouragingresult of this experiment, although Moreau,Charcosset and Gallais (2004) failed topresent it as such, was that the first cycleof marker-only selection was as efficientas phenotypic or combined selection indelivering genetic gain. Two conclusionscan be drawn from this observation.First, the QTL identified in the initialexperimental population were in generalnot artefacts. Second, selection pressureapplied at these QTL, and aimed at fixingalleles identified as favourable, resulted ina change in performance of the selectedpopulation in the desired direction whencompared with the initial population.A similar experiment, although basedsolely on marker-only recurrent selection,was reported by Openshaw and Frascaroli(1997). They conducted MARS in maizesimultaneously for four traits, for each ofwhich about ten QTL had been identified.They showed that genetic gain had beenachieved in the first cycle of MARS, butthat later cycles did not result in any gain.Possible explanations given for these resultsincluded uncertainties about QTL parameters(location and effect), interaction effects(epistasis, genetic x environment interaction),and the fact that selection was basedon single markers rather than chromosomalsegments (Openshaw and Frascaroli, 1997).Recent communications from severalprivate MARS research programmes (Ragotet al., 2000; Eathington, 2005; Crosbieet al., 2006) revealed large-scale successfulapplications in maize. Accounts weregiven of commercial maize hybrids forwhich at least one of the parental lineswas derived through MARS. Eathington(2005) and Crosbie et al. (2006) reportedthat the rates of genetic gain achievedthrough MARS were about twice those