marker-assisted selection in wheat - ictsd

Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishusing within-breed or within-line selection,important in livestock); development ofcrossbreds or hybrids (by crossing severalimproved lines or breeds) and introgression(where a target gene is introduced from,for example, a low-productive lineor breed (donor) into a productive line(recipient) that lacks the target gene (astrategy especially important in plants).See Dekkers and Hospital (2002) for moredetails. MAS can be incorporated into anyone of these strategies (e.g. for markerassistedintrogression by using markers toaccelerate introduction of the target gene).Alternatively, novel breeding strategies canbe developed to harness the new possibilitiesthat MAS raises.Current Status of Applications ofMAS in AgricultureBelow is a brief summary of the currentstatus regarding application of MAS inthe different agricultural sectors. For moredetails, a number of case studies for cropsare presented in Section II of the book andfor livestock, forestry and fish in SectionsIII, IV and V, respectively.CropsThe promise of MAS has possibly beengreeted with the most enthusiasm and expectationin this particular agricultural sector,stimulating tremendous investments in thedevelopment of molecular marker mapsand research to detect associations betweenphenotypes and markers. Molecular markermaps have been constructed for a widerange of crop species. Information on majorplant projects (such as the sequencing of theentire rice genome) can be found at www.ncbi.nlm.nih.gov/genomes/PLANTS/PlantList.html.In a recent review, however, Dekkers andHospital (2002) noted that “as theoreticaland experimental results of QTL detectionhave accumulated, the initial enthusiasmfor the potential genetic gains allowed bymolecular genetics has been tempered byevidence for limits to the precision of theestimates of QTL effects”, and that “overall,there are still few reports of successfulMAS experiments or applications.” Theyreported that marker-assisted introgressionof known genes was widely used in plants,particularly by private breeding companies,whereas marker-assisted introgressionof unknown genes had often proved to beless useful in practice than expected. AsYoung (1999) wrote: “even though markerassistedselection now plays a prominentrole in the field of plant breeding, examplesof successful, practical outcomes arerare. It is clear that DNA markers holdgreat promise, but realizing that promiseremains elusive.”There is also considerable divergencewith respect to the applications of MASamong different crop species. For example,Koebner (2003) highlighted the relativelyfast uptake of MAS in maize compared withwheat and barley, arguing that this largelyreflected the breeding structure. Thus,whereas maize breeding is dominated inindustrialized countries by a small numberof large private companies that produceF 1 hybrids, a system allowing protectionfrom farm-saved seed and competitor use,breeding for the other major cereal speciesis primarily by public sector organizationsand most varieties are inbred pure breedinglines, a system allowing less protection overthe released varieties. Progress in arablecrops is nevertheless quite advanced comparedwith horticultural crop species suchas apples and pears, where development ofmolecular marker maps has been slow andonly few QTL have been detected (Tartarini,2003), even if MAS can potentially be very