marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 91Figure 3Application of co-dominant and dominant markers during the breeding cyclein common beansMethodGeneration of crossing or inbreedingMass selection F 1 F 1:2 F 1:3 F 1:4 F 1:5 F 5:6Pedigree selection F 1 F 1:2 F 2:3 F 3:4 F 4:5 F 5:6 F 5:7Backcrossing BC 1 BC 2 BC 3 BC 4 BC 4 F 1:2Gamete selection BC x F 1:2 BC x F 1:3 BC x F 3:4MC 1 F 1:2 MC 1 F 1:2 MC 1 F 2:3 Adv. lineNote: Asterisks indicate co-dominant marker, while open circles indicate dominant marker in repulsion andclosed circles indicate dominant marker in coupling.In other cases where phenotypic selectionmethods are available, the advantageof MAS resides in its simplicity. This isthe case in the selection of arcelin, whichcan be achieved through protein extractionfollowed by antibody detection or electrophoresis,but both of these are laboriouswhile MAS can be applied more rapidly andwith much greater throughput. Similarly,markers for common blight resistance andanthracnose have the advantage of obviatingthe need for field inoculations that aresometimes ineffective if environmental conditionsare not favourable. The advantageof MAS is much greater if a single DNAextraction can serve for the evaluation ofseveral markers, as in the multiplexing ofbgm-1 and SW12.700 markers.In spite of attempts to apply MAS tocomplex traits, examples of successfulapplication are still limited to relativelysimple traits. This is contrary to someprevious expectations that markers wouldbenefit mostly traits of low heritability.However, experience has shown that theability to manipulate even one importantgene with confidence can make a breedingprogramme more efficient, if that gene ishighly desirable and valuable for advancedmaterials.Meanwhile the disadvantages of MAScompared with phenotypic selection arebased on effectiveness and cost considerations.The effectiveness of MAS is relativeto the ease of applying a given marker,its reliability and its level of linkage withthe gene of interest. Although molecularmarkers theoretically have a heritabilityof 1.0, variability among laboratories oramong runs within a laboratory makemarkers less than 100 percent reliable. Thisis especially true for RAPD markers forwhich band amplification is dependent onDNA concentration and quality, annealingtemperature and thermocycling conditions,Taq polymerase concentration andthe relative proportion of various otheringredients to the PCR cocktail. In comparison,SCAR markers are much morereliable and repeatable and therefore havehigher heritability than RAPD markers.Linkage distance between a marker for a