marker-assisted selection in wheat - ictsd

54Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishment length polymorphism (RFLP)-basedmaize genetic map. In the less geneticallyvariable cereals, prominently wheat,the level of polymorphism is not now inpractice likely to represent the major constraintto MAS uptake, although in the pastit was argued that this was the case. Whathas changed in recent times is that currentmarker technology, and systems ofDNA acquisition, laboratory managementand integration into the breeding cycle,have all developed to the extent where thebenefits of MAS can be increasingly realizedin actual practice. As many of theseimprovements are incremental rather thansudden, we argue that the trends in MASapplication in wheat are characteristicallyevolutionary rather than revolutionary.Target traits for MAS innorthwest European winterwheat breedingThe use of MAS to date has a history ofabout 20 years, and until recently involvedthe exploitation of just two non-DNAbasedassays. The first, which has beenretained with only slight modificationssince its inception, exploits a correlationbetween bread-making quality and allelicstatus at the Glu-1 (endosperm storageprotein subunit) loci. It uses electrophoreticprofiles obtained by the straightforward,robust and cheap procedure sodium dodecylsulphate polyacrylamide gel electrophoresis(SDS-PAGE) from crude seed proteinextracts, which have been shown to bepartially predictive of end-use quality. Thesecond is predictive for the presence of thegene Pch1, which confers a high level ofresistance to eyespot, a stem base diseasethat is difficult to screen using conventionalpathology methods. Both these targets havein the meanwhile become assayable bypolymerase chain reaction (PCR)-basedassays, although SDS-PAGE remains inroutine use thanks to its flexibility and costeffectiveness. In recent years, the numberof loci for which DNA-based assays havebeen generated has increased dramatically,the majority using PCR as a technologyplatform. Over 50 of these are described(specifically in a United States of Americacontext) at http://maswheat.ucdavis.edu/,which reports the output of an ongoingUnited States Department of Agriculture(USDA)-funded programme. The focusis heavily on disease and pest resistance,reflecting the generally simple inheritanceof genes conferring these traits.Some of the above traits are of sufficientrelevance to the United Kingdom contextthat identical or equivalent assays havebeen incorporated in a number of breedingprogrammes, where they are used as guidesto parental selection and/or in early generationselection. Prominent among these aremarkers for the genes Rht-1 (responsible forthe “Green Revolution” semi-dwarfism),Pinb (grain texture), Pch1, Lr37/Yr17 (agene complex conferring resistance to twoof the most important leaf fungal pathogens)and the wheat/rye translocation1B/1R (which is associated with high levelsof yield). Emerging MAS targets are necessarilyprogramme-dependent, but the broadfocus is on quantitative trait locus (QTL)targets that could have a major impact onbreeding efficiency. In the United Kingdom,as elsewhere worldwide, current focus is onresistance to the diseases Fusarium headblight (FHB), Septoria tritici blotch (STB)and barley yellow dwarf virus (BYDV), andon durable resistance to yellow rust. Othercurrent targets, more specific to the UnitedKingdom and northwest European context,but in routine use, are resistance to theinsect pest orange blossom midge (OBM),and soil-borne mosaic virus (SBMV).