marker-assisted selection in wheat - ictsd

10Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishof the major commercially used pine andEucalyptus species. Molecular markers aretherefore primarily an information tool andare used to locate DNA/genes that can be ofinterest for genetic transformation, or informationon population structure, matingsystems and pedigree confirmation.”FishMolecular marker maps have been constructedfor a number of aquaculture species,e.g. tilapia, catfish, giant tiger prawn,kuruma prawn, Japanese flounder andAtlantic salmon, although their density isgenerally low. Density is high for the rainbowtrout, where the map published in 2003has over 1 300 markers spread throughoutthe genome – the vast majority are AFLPsbut it also includes over 200 microsatellitemarkers (Nichols et al., 2003). Some QTLsof interest have been detected (e.g. for coldand salinity tolerance in tilapia and for specificdiseases in rainbow trout and salmon).In a recent review of MAS in fish breedingschemes, Sonesson (2003) suggested thatMAS would be especially valuable for traitsthat are impossible to record on the candidatesfor selection such as disease resistance,fillet quality, feed efficiency and sexualmaturation, and concluded that MAS is notused in fish breeding schemes today andthat the lack of dense molecular maps is thelimiting factor.ConclusionsMolecular marker maps, the necessaryframework for any MAS programme,have been constructed for the majorityof agriculturally important species but thedensity of the maps varies considerablyamong species. Currently, MAS does notplay a major role in genetic improvementprogrammes in any of the agriculturalsectors. Enthusiasm and optimism remainconcerning the potential contributionsthat MAS offers for genetic improvement.However, this seems to be tempered bythe realization that it may be more difficultand therefore take longer than originallythought before genetic improvement ofquantitative traits using MAS is realized. Theconclusions from the review by Dekkers andHospital (2002) are a good reflection of this:“Further advances in molecular technologyand genome programmes will soon create awealth of information that can be exploitedfor the genetic improvement of plants andanimals. High-throughput genotyping,for example, will allow direct selection onmarker information based on populationwideLD. Methods to effectively analyseand use this information in selection are stillto be developed. The eventual applicationof these technologies in practical breedingprogrammes will be on the basis of economicgrounds, which, along with cost-effectivetechnology, will require further evidence ofpredictable and sustainable genetic advancesusing MAS. Until complex traits can befully dissected, the application of MASwill be limited to genes of moderate-tolargeeffect and to applications that donot endanger the response to conventionalselection. Until then, observable phenotypewill remain an important component ofgenetic improvement programmes, becauseit takes account of the collective effect of allgenes.”Some Factors Relevant toApplying MAS in DevelopingCountriesIn the debate on the role or value of MASas a potential tool for genetic improvementin developing countries, some of thepotential factors that should be consideredare described briefly below, as they mayinfluence applications of the technology.