marker-assisted selection in wheat - ictsd

90Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 2Potential of MAS in crosses of varying genetic diversityMAS potentialMethodPurposeLow – due to poorpolymorphismNarrowelite x eliteMass/PedigreeElite crossesHigh – due to goodpolymorphismIntermediateInter-racialGamete/RecurrentGenepyramidingWidewild/secondary/tertiarygene poolsAdv. / Rec. / Cong.BackcrossIntrogressionPotential of MASWidth of crossesModified from Kelly, Schneider & Kolkman, 1999 and Singh, 1999by MAS will give the greatest cost/benefitratio. This would probably be early inthe breeding programme for the pedigreemethod or for gamete selection while itwould be later in the programme for bulkmethod or mass selection (Figure 3). In thecase of early generation selection, eliminationof plants without the gene(s) will avoidunproductive investment.Advantages and disadvantages ofMASMAS provides real advantages where theconditions are not favourable for phenotypicselection, for example, in the case ofBGYMV, which does not exist at epiphytoticlevels in CIAT. Indeed, bgm-1 behavesas a recessive gene, so phenotypic selectionin early generations would be inefficientin recovering the gene in the heterozygousstate.The same principle would apply to therecessive bc-3 gene, although the lack of amarker linked in coupling to this gene hasbeen a serious drawback and has limited theeffectiveness of MAS to advanced generationswhen the gene is fixed by inbreeding.In this case, early generation selection withMAS would be limited to negative selectionagainst homozygous dominant andheterozygous plants, and this eliminatespotentially useful allele-carrying genotypes.Indeed, MAS is impossible in generationssuch as the F 1 or BC 1 F 1 to the susceptibleparent when no homozygous recessiveplants exist at all.