marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 95and plants representing them through thedifferent stages. Table 1 also provides an ideaof the selection pressures generally applied.Strong emphasis on highly heritabletraits (plant type, branching habits andreaction to diseases, harvest index and drymatter content) is applied during the earlyphases of selection (F 1 and CET), (Hahn,Howland and Terry, 1980; Hahn, Terry andLeuschner, 1980; Hershey, 1984; Kawano,2003; Ceballos et al., 2004). As the numberof plants representing each genotypeincreases, the weight of selection criteriashifts towards low heritability traits suchas root yield. The clones that show outstandingperformance in the regional trialsare released as new varieties and, eventually,incorporated as parents in the crossingnurseries. With that the selection cycle isfinished and a new one begins. The wholeprocess has the following characteristics(Ceballos et al., 2004):• the process is indeed phenotypic selectionbecause no family data are involved;• no data are collected in the early stagesof selection. Therefore, data regardinggeneral combining ability effects(∼ breeding value) are not available for abetter selection of parental materials;• there is no proper separation betweengeneral (GCA ∼ additive) and specific(SCA ∼ heterotic) combining abilityeffects. The outstanding performanceof selected materials is likely to dependon positive heterotic effects that cannotbe transferred to the progenies that aresexually derived from them;• no inbreeding is incorporated purposelyin the selection process. Therefore,large genetic loads are likely to remainhidden in cassava populations and usefulrecessive traits are difficult to detect;• several stages of selection are based onunreplicated trials. A large proportion ofgenotypes is eliminated without properevaluation.For the above-mentioned reasons, cassavabreeding is difficult, expensive andto a certain degree inefficient (Perez et al.,2005a; Cach et al., 2005a, b). Kawano et al.(1998) mention that, during a 14-year periodabout 372 000 genotypes derived from4 130 crosses were evaluated at the CIAT-Rayong Field Crop Research Center. Onlythree genotypes emerged from the selectionprocess to be released as official varieties.Similar experiences have been observedat the International Institute of TropicalAgriculture (IITA), CIAT-Colombia andBrazil. Therefore, the development andadaptation of molecular tools for cassavagenetic improvement offer importantadvantages to make the process more efficientand effective.MAS in cassava breedingCassava genetic improvement can be mademore efficient through the use of easilyassayable molecular genetic or DNAmarkers (MAS) that enable the preciseidentification of genotype without theconfounding effect of the environment,thereby increasing heritability. MAS canalso contribute to the efficient reduction oflarge breeding populations at the seedlingstage based upon “minimum selection criteria”.This is particularly important giventhe length of the growing cycle of cassavaand the expense involved in the evaluationprocess. Therefore, a pre-selection at the F 1phase (see Table 1) could greatly enhancethe efficiency of the CET experiments.The selection of progenies based on geneticvalues derived from molecular marker datasubstantially increases the rate of geneticgain, especially if the number of cycles ofevaluation or generations can be reduced(Meuwissen, Hayes and Goddard, 2001).