marker-assisted selection in wheat - ictsd

274Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishMAS for that particular trait. Estimates ofheritability for a trait might be useful togive an initial clue. Intuitively, the probabilityof major genes existing for traitsof low heritability is lower than for traitsof high heritability. However, this shouldnot be taken as a measure to discard possibleQTL mapping experiments as, evenwith low heritabilities, traits might still displaymajor QTL, and MAS would have thegreatest impact particularly in such cases.Conclusions and perspectivesThe successful application of molecularbreeding in Eucalyptus will dependheavily on first demonstrating and validatingthe clear-cut association between aDNA polymorphism and a quantitativelyinherited phenotypic trait. In highly heterogeneouseucalypts, while conventionalQTL mapping can reveal useful markersto be exploited in within-family selectionpractices, only a more direct LD mappingapproach can uncover populationwide applicable marker-trait associations.Such studies based on candidate genes havebegun and the first candidate gene associationfor MFA was detected. However, thisassociation explains only a small proportion(3.4 percent) of the variation to bereally exciting news to breeders (Thummaet al., 2005). One of the key issues whenembarking on an association mapping experimentis the selection of candidate genes.Maximizing the probability of choosing theproper genes requires levels of knowledgeof biochemistry, physiology and developmentthat are generally not yet availableeven for well defined phenotypes and/orknown metabolic pathways.Following the path taken in humangenetics, co-localization of candidate genesand QTL for relevant traits on linkagemaps together with integrative expression-QTL mapping (Kirst et al., 2004) could be apowerful way forward, although choosingthe correct candidate depends heavily onthe precision of the QTL localization. Atthe moment, there are two possibilities forcircumventing the dilemma of choosingcandidate genes correctly. The first is microarray-basedgenotyping with ultra-densearrays of short (25 nt) oligonucleotides(Borevitz et al., 2003; Hazen and Kay, 2003;West et al., 2006) that would allow sufficientthroughput for association geneticanalysis of thousands of genes at a time.Such an array format could later turn out tobe a useful instrument for MAS once validatedmarker-trait associations have beenestablished. The second would be to haveaccess to a whole genome sequence so thatcandidate genes in a fine mapping intervaldelimited by markers flanking a QTL withcentimorgan resolution could be mined,reannotated and then analysed in associationmapping experiments.A draft genome of E. camaldulensis is currentlybeing sequenced at the Kazusa DNAResearch Institute in Japan (T. Hibino, personalcommunication), and the possibilityexists that a fully public 4X draft of theE. grandis genome will be sequenced bythe Joint Genome Institute of the UnitedStates Department of Energy within thenext years (J. Tuskan, personal communication)following a proposal recentlysubmitted by an international group ofEucalyptus geneticists (www.ieugc.up.ac.za/DOE%20proposal%20-%20final%20-%2026%20July%202006.pdf) who recentlyformed the International EucalyptusGenome Network (EUCAGEN) (www.ieugc.up.ac.za; Myburg, 2004). Such publiccollaborative efforts should contributegreatly to the advancement of Eucalyptusgenetics, genomics and molecular breedingby bringing together existing private data-