marker-assisted selection in wheat - ictsd

88Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishcausal agent of angular leaf spot (ALS),and Colletotrichum lindemutheanum,which induces anthracnose. In both cases,pathogen isolates tend to be more virulenton host genotypes of the same gene pool(Andean or Mesoamerican) and less so onhost genotypes from the contrary genepool. Resistance genes of utility to onehost gene pool thus tend to originate in theother gene pool and require introgressionfrom one gene pool to the other. MAS hasgreat potential for introgression as DNApolymorphisms are maximized in widecrosses across gene pools, and markers areavailable for this purpose for both ALS(Carvalho et al., 1998; Sartorato et al.,1999; Nietsche et al., 2000; Ferreira et al.,2000; Mahuku et al., 2004) and anthracnose(Young et al., 1998; Awale and Kelly 2001;Vallejo and Kelly, 2001).Other cases of wide crosses in whichMAS can be of use include those for theselection of genes for resistance to a storageinsect, the Mexican bean weevil (Zabrotessubfasciatus [Boheman]) derived from wildbean accessions from Mexico. Selection forresistance has also been achieved by analysisfor the active resistance agent, a seedprotein called arcelin, by either antibodyreaction or electrophoresis, but MAS is simplerand more efficient than either of theseanalyses that require protein extraction.Even wider crosses of common bean withPhaseolus acutifolius have recovered resistanceto common bacterial blight (causedby Xanthomonas axonopodis pv. phaseoli)(Muñoz et al., 2004) and markers have alsobeen developed for these resistance genes(Jung et al., 1997; Miklas et al., 2000b; Parket al., 1999; CIAT, unpublished data). Inthese cases also, the fact of deploying genesfrom relatively wide crosses favours maintaininga state of DNA polymorphism inrelation to the target genotypes.Complex multigenic traitsIn addition to the studies previously discussed,several attempts have been carriedout to tag quantitative trait loci (QTL)for abiotic stress tolerance or insect resistancein common bean, although most ofthese traits might better be described asoligogenic, as results usually suggest that alimited number of loci (from three to six)are involved in their genetic control.One example is tolerance to low soilphosphorus that was investigated in thelandrace G21212. Linkage group b08proved to be especially important to yieldunder low phosphorus, with as many asthree important and loosely linked QTL(Beebe, Velasco and Pedraza, 1999; Miklaset al., 2006). Interestingly, these same QTLwere linked to QTL for resistance to Thripspalmi Karny derived from the same source(Frei et al., 2005). This is a promising candidatefor applying MAS in the short term forabiotic stress tolerance, although anothernotable attempt was also made for droughttolerance breeding with MAS through ajoint programme between Michigan StateUniversity and the National Institutefor Forestry, Agriculture and LivestockResearch (INIFAP) in Mexico (Schneider,Brothers and Kelly, 1997).In theory, a breeder would prefermarkers for low heritability quantitativetraits that are difficult to select throughphenotypic selection. However, in general,markers for polygenic or oligogenic traitshave not moved into the application phase.The same problems that make phenotypicselection difficult apply in some degree toMAS. Multiple minor genes that are oftenassociated with poor heritability also implythat it is difficult to identify QTL withhighly significant effects and that merit theinvestment of MAS. Furthermore, goodgenome coverage is usually necessary to