marker-assisted selection in wheat - ictsd

Chapter 8 – Marker-assisted selection in maize 133of which are related to time and resourceallocations.MABC clearly provides the informationneeded to reduce the number of generationsof backcrossing, to combine (i.e.“stack”) transgenes, “native” genes or QTLinto one inbred or hybrid quickly, and tomaximize the recovery of the recurrentparent’s genome in the backcross-derivedprogeny. In several private breeding programmes,MABC has enabled the numberof backcrossing generations needed torecover 99 percent of the recurrent parentgenome to be reduced from six to three,reducing the time needed to develop a convertedvariety by one year (Crosbie et al.,2006; Ragot et al., 1995). As a line derivedby MABC can be made to be very similarto the original non-converted line, mostof its attributes, including agronomic performance,can be assumed to be equal orsimilar to those of the original line. Onlylimited phenotyping is therefore necessaryto verify these assumptions, comparedwith the extensive multiyear phenotypingrequired when backcrossing is conductedwithout markers. One or two years can besaved with MABC during post-conversionphenotyping when compared with conventionalbackcrossing, resulting in an overalltime advantage of MABC over conventionalbackcrossing of up to three years.In many situations, the greatest advantagesand profits are realized by those whoare first to the market with their products.Also, for reasons related to the practicesof seed production or legal aspects of cropregistration procedures, it may be quiteimportant to be able to produce nearisogenicversions of inbreds and hybrids;MABC provides such ability at a higherprobability.By contrast with MABC, SLS-MAS andMARS do not necessarily decrease the timeneeded to develop inbred lines. The useof MARS might actually increase it. Theadvantage of SLS-MAS and MARS residesin their ability to increase the rate of geneticgain (Eathington, 2005), which potentiallyresults in higher performing lines andhybrids than can be developed through phenotypicselection only. Both SLS-MAS andMARS increase the frequency of favourablealleles in the population of selectedindividuals. The difference between thetwo approaches is that SLS-MAS operateson few loci while MARS operateson many. When SLS-MAS or MARS areused, the effective size of the populationon which selection operates is increased,either directly for SLS-MAS or indirectlythrough several consecutive generations forMARS when compared with phenotypicpedigree selection. This increase in effectivepopulation size permits the applicationof a greater selection intensity and henceproduces a higher genetic gain. SLS-MASand MARS can also be seen as pre-selectionsteps if conducted prior to phenotypicselection and therefore improve the chancesof evaluating genotypes with a higher frequencyof favourable alleles phenotypicallybecause the truly undesirable portion of thepopulation may have been eliminated priorto phenotyping. Phenotypic selection cantherefore be conducted with higher selectionintensity than would be possible ifno pre-selection had taken place, resultingpotentially in additional genetic gain.Alternatively, the resources used forphenotyping can be allocated differentlybased on whether individuals have beenpre-selected or not with MAS. MASschemes for forward breeding should enablebreeding programmes to reallocate or focusresources for phenotypic evaluation in thetarget environment. For example, if DNAmarkers are linked to genes for resistance