marker-assisted selection in wheat - ictsd

108Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishuntil some benefit is extracted from it.On the other hand, far too much efforthas been expended in the identification ofmarkers for traits without carrying thesethrough to application. Often gene taggingis a component of a short-term project, anddoes not receive the necessary follow up inimplementation. In each case, the essentialquestion is: what are the key genes foreach crop? And once defined, which genesmerit the investment to develop molecularmarkers? For investments in molecularmarker development to yield results, itis important that traits are chosen forwhich molecular breeding has both a clearadvantage over field-based selection and isfeasible in the short to medium term. It isalso important that emphasis be given toselecting the relevant crosses, pedigrees andpopulations in which to practise MAS, andto have in place appropriate phenotypingstrategies for the confirmation of MASresults. In this regard, the use of parentalsurveys of many of the genotypes involvedin a given breeding programme is animportant first step in implementing MAS.Short- and long-term research relatedto MASThe present research structure that is normallybased on short-term projects, usuallyof three years’ duration, can seldom beexpected to deliver results of usable markersfor complex traits. Such short-term projectsthat seek to establish the basis for MASor to implement selection should limittheir objectives to simply inherited traits.On the other hand, longer-term fundingeither of a programmatic or successiveproject funding nature, must be obtainedto address more complex traits governedby QTL as these would normally requireat least two phases of three-year projects.The first phase might be expected to revealthe inheritance of a given trait, establishingthe location and numbers of QTL, while asecond phase would be required to validatethese over more environments and to findmarkers that are polymorphic over a widenumber of genotypes and therefore widelyuseful for breeding, as well as adapted torapid laboratory techniques. A mediumtolong-term investment likewise impliescareful prioritization of such traits, withregard to potential impact and the eventualneed for MAS. These reflections are basedupon presently available laboratory techniques,but as techniques for more detailedand widespread evaluation of loci and genotypesare developed (e.g. gene chips foranalysis of multiple loci), conclusions couldchange significantly.Scaling-up technologiesAfter the development of molecular markersfor a trait and their initial implementation,a period of scaling-up in use of the specificmarkers is necessary. Sometimes thisinvolves changes to MAS protocols, inthe marker detection technique or in themarkers themselves. Marker re-design hasbeen a common element of scaling-up exercisesand can involve something as simpleas changing a PCR fragment size to implementinga SNP assay for the actual sequencedifferences between alleles. Technologiesthat speed up the implementation processand lower the costs associated with scalingupare crucial to the success of MAS and areoften neglected.Development of markers that areuseful in a large number of crossesOften a marker developed for a particulartrait in one or a few related genotypes willnot work for other genotypes with highvalue of the trait due to differences in geneor allelic effects. Unravelling the genetics of