marker-assisted selection in wheat - ictsd

134Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishto a disease or insect then it should bepossible initially to select resistant progenyin the absence of the disease or insectby using the DNA data at continuousnurseries. The selected progeny couldthen be evaluated using relatively moreexpensive bioassays with the pest(s) in thetarget environment. This shift in resourcesis inherent to MARS schemes for complextraits (Edwards and Johnson, 1994; Johnson,2004; Crosbie et al., 2006). By enrichingpopulations through rapid cycles of MARSat continuous nurseries, breeders shouldderive a higher frequency of progeny withfavourable alleles and haplotypes that arethen evaluated in the target environment.Without MARS, resources for evaluation inthe target environment would be diluted bythe inclusion of too many progeny with anundesirable genetic constitution.Concerns about reduced genetic diversityamong commercial maize hybrids anddepletion of genetic diversity in gene poolsused in breeding may be partially alleviatedby successful implementations ofMAS. MABC may revive interest in usingessentially untapped maize exotic germplasmas a source of favourable alleles forimprovement of elite varieties. Very smalland targeted chromosomal segments ofexotic origin can be introgressed into eliteinbred lines with limited risk of carryingalong undesirable characteristics. Suchan approach could be beneficial in maizealthough no accounts of its implementationhave been reported despite the manyyears as reports of its successful use intomato (Tanksley et al., 1996; Bernacchi etal., 1998a, b; Robert et al., 2001), rice (Xiaoet al., 1998), and soybean (Concibio et al.,2003). MARS, in turn, may also contributeto increasing genetic diversity among commercialmaize hybrids because, by focusingon selecting specific recombination events,it will result in the development of genuinelynew genomic rearrangements. AsQTL identified in any experiment representonly a fraction of the loci responsiblefor the phenotypes of complex traits, onecan assume that breeding programmes indifferent private companies will conductMARS based on their different geneticmodels and select for different genomicrearrangements. As a result, hybrids ofsimilar and high performance might bedeveloped that are based on different setsof favourable alleles at different loci, representingdistinct “genetic solutions” andcontributing to increased genetic diversityin farmers’ fields.An indirect but important advantage ofMAS and its underlying information andtechnology relates to intellectual property.Some maize breeding programmes havecreated a form of wealth through their collectionand knowledge of maize germplasm.Significant investments have been made inmaize breeding as exemplified by the billionsof United States dollars that wereused to purchase a few private programmesbetween 1995 and 2005. Protecting andmaximizing returns on such investmentshave always been important but are nowof greater concern. Information from MASshould be advantageous for addressingissues concerning ownership and derivationof germplasm, relatedness among germplasmand for the formation of some claimsin patents and similar documents.Perhaps one of the greatest advantagesof MAS is that, for the first time, maizebreeders have the means of learning someof the genetic details about germplasm andthe response to selection. Some maize programmesin the private sector have startedthis process (Niebur et al., 2004). As realfunctions become associated with the manycandidate genes and other DNA sequences,