marker-assisted selection in wheat - ictsd

392Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishfor the purposes of this chapter it is importantto point out that although establishingcapacity to develop new molecular markersrequires substantial investment, establishingthe capacity to use freely available existingmolecular markers requires only a modestinvestment.Variable cost of MASAt CIMMYT the capacity to carry outMAS on a reasonable scale has been developed,but the need now is to make thetechnology work on a high-throughputscale to reduce the cost per data point,while being able to handle large quantitiesof assays per growing season. In this regard,there are several challenges to consideras markers are not always cost-effectiveeven when they improve the precision ofselection. Depending on the nature of thetarget trait (quantitative or qualitative), thetype of gene (major or minor), the form ofgene action that controls expression of thetrait (dominant or recessive effect), and theease with which the trait can be measured(visually detected or more expensive fieldor laboratory analysis required), conventionalselection may be cheaper than MAS.The desirability of using genetic markerstherefore depends in part on the costsof genotypic versus phenotypic screening,which vary among applications.Information about the cost of usingMAS at CIMMYT for specific breedingprojects is available from case studies.For example, Dreher et al. (2002, 2003)examined the costs and benefits of usingMAS for a common application in maizebreeding. This study generated three noteworthyconclusions.First, for any given breeding project,detailed budget analysis is needed todetermine the cost-effectiveness of MASrelative to conventional selection methods.Although the costs of field operations andlaboratory procedures required for molecularmarker analysis may remain relativelyconstant across applications, every breedingproject is likely to involve unique phenotypicevaluation procedures whose costswill frequently differ.Second, direct comparisons of unitcosts for phenotypic and genotypic analysisprovide useful information to researchmanagers, but in many cases technologydecisions are not made solely on the basisof cost. Factors other than cost often influencethe choice of screening methods. Timeconsiderations are often critical, as genotypicand phenotypic screening methodsmay differ in their time requirements. Evenwhen labour requirements are similar, forapplications in which phenotypic screeningrequires samples of mature grain, genotypicscreening can often be completed muchearlier in the plant growth cycle.Third, conventional and MAS methodsare not always direct substitutes. Usingmolecular markers, breeders may be ableto obtain more information about what isgoing on at the genotypic level than they canobtain using phenotypic screening methods.For example, in conventional backcrossbreeding or line conversion projects (seesection Manipulation of qualitative traits),background molecular markers can be usedto identify those plants among a set ofprogeny that not only possess a desirableallele but also closely resemble the recurrentparent at the genetic level. Based onthis additional information, breeders areoften able to modify their entire breedingstrategy, with potentially significantimplications in terms of cost and/or timerequirements (this issue is discussed in thenext section).The CIMMYT case study thus confirmedwhat many practising plant breeders