marker-assisted selection in wheat - ictsd

182Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishthe variation in profitability is dominatedby traits that are hard to measure.However, new technologies often lead toother breeding programme designs beingcloser to optimal. Genotypic informationhas extra value in the case of early selectionand where within-family variance can beexploited, which is particularly the case inprogrammes where reproductive technologiesare used. Reproductive technologiesusually lead to early selection and moreemphasis on between-family selection.DNA marker technology and reproductivetechnologies are therefore highly synergisticand complementary (van der Werfand Marshall, 2005) and gene markers havemuch more value in such programmes.Gene marker information is also clearlyvaluable in introgression programmes, asdemonstrated by simulation (Chaiwong etal., 2002; Dominik et al., 2006) as well as inpractice (Nimbkar, Pardeshi and Ghalsasi,2005). Yet, although these examples arefavourable to the value of gene markerinformation, the added value of MAS stillrelies heavily on a high degree of trait andpedigree recording.Opportunities for MAS indeveloping countriesComplete phenotypic and pedigree informationis often only available in intensivebreeding units. Therefore, in the context oflow input production systems, some questionscan be raised concerning the validityand practicality of the simulation studiesdescribed above, and it would be moredifficult to realize the value of markerinformation. It would be harder and moreexpensive to determine the linkage phase inthe case of using linked markers. Moreover,even if the genetic marker were a direct orLD marker, its effect on phenotype wouldhave to be estimated for the populationand the environment in which it is used.This would require phenotypes and genotypeson a sample of a rather homogeneouspopulation to avoid spurious associationsthat could result from unknown populationstratification. Therefore, a gene markerfor a QTL is likely to be most successfulin an environment with intensive pedigreeand performance recording. Nevertheless,in low input environments, direct andLD markers will be more useful than LEmarkers because the latter require routinerecording of phenotypes and genotypes toestimate QTL effects within families.In addition to MAS within local breeds,several other strategies for breed improvementcould be pursued in developingcountries, including gene introgression andMAS within synthetic breeds. This wouldbe most advantageous for introducing specificdisease resistance alleles into breedswith improved production characteristicsto make them more tolerant to the environmentsencountered in developing countries.Gene introgression is, however, a long andexpensive process and only worthwhilefor genes with large effects. MAS withinsynthetic breeds, e.g. a cross between localand improved temperate climate breeds,can allow development of a breed that isbased on the best of both breeds (e.g. Zhangand Smith, 1992). Because of the extensiveLD within the cross, a limited number ofmarkers would be needed. Care should,however, be taken to avoid the impactof genotype x environment interactions ifMAS is implemented in a more controlledenvironment.