marker-assisted selection in wheat - ictsd

132Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishwith up to four cycles per year, whereasphenotypic recurrent selection is limitedto one cycle per year at most. The impacton the rate of genetic gain of such animplementation of MARS might be verypositive even if MARS did not presentany advantage over phenotypic selectionon a per-cycle basis.• Cost of marker data points. Large privatecompanies have made considerable effortsto reduce both the cost of marker datapoints and the cost of experimental fieldplots. The ratio of cost of marker datapoint to cost of experimental field plot ismost likely lower in large private breedingprogrammes than in most public researchlaboratories or small private programmes,potentially leading to different views onthe economic efficiency of MARS.Marker-based and phenotypic selectioncan be mobilized in many different ways,with respect to each other, in markerassistedbreeding schemes. Marker andphenotypic information can be used eithersimultaneously or sequentially. Selectionof parents for breeding populations canbe made using marker information alone,phenotypic information alone, or a combinationof each. Selection of individuals in abackcross programme can be made on thesole basis of either marker or phenotypicinformation, or using both. Advancementof individuals in a line development programmecan also be made at each generationon the basis of either marker informationonly, phenotypic information only, or acombination of each. In order to maximizethe rate of genetic gain it is likely thatMAS breeding schemes such as MABCand MARS will involve generations ofmarker-only selection conducted at continuousnurseries. The advent of improvedmethods of producing doubled haploidswill certainly further influence the waymarker-based and phenotypic selection aremobilized with respect to each other.In spite of the development ofmarker-only selection and regardless ofthe underlying technology and breedingscheme, high-quality phenotyping remainsvital and without substitute at severalstages; but it may become more focused.Phenotypic evaluation remains the ultimatescreen before any cultivar is released.MAS-derived lines and hybrids that meetphenotypic requirements are selected forfurther evaluation and selection on the basisof their phenotypic value, while those thatdo not are discarded. Phenotypic evaluationis also critical to establish marker-traitassociations or perform the candidate genevalidations required to conduct MAS.Here, high quality phenotyping is necessary.Phenotyping protocols will thereforelikely be different from those commonlyused for phenotypic selection. Experimentsmay be conducted that involve side-by-sidecomparisons of different treatments such aswater stress or nitrogen fertilization levelsto dissect complex traits into their componentsand facilitate the elucidation of theirgenetic basis.Enhancements of such approachesto maize breeding will be based on theincorporation of improved methods ofproducing doubled haploid inbred lines,information from functional genomics andby learning how to incorporate favourablenative genetic variation systematically afterMAS has reduced the genetic variationin the original reference populations tounacceptable levels.Advantages and limitations ofMAS in maize breeding programmesAdvantages of MASFor private breeding programmes, MAShas offered several attractive features, most