marker-assisted selection in wheat

178Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishIn general, for the purpose of increasedgenetic change of economically importantquantitative traits, and in the contextof well recorded and efficient breedingprogrammes, there is no need to haveknowledge of functional mutations sincenearby markers will have a high predictivevalue about genetic merit. Moreover,the benefit from the extra investment andtime spent on finding functional mutationsmight be superseded by the geneticchange that can be made in the breedingprogramme in the meantime.Implementation of marker-assisted geneticevaluationIt is important to note that, for most ofthe gene marker tests currently on themarket, integration with existing systemsfor genetic evaluation is not obvious. Thisis because the gene testing is either fora Mendelian characteristic, or it predictsphenotypic differences for traits that arenot the same as those in current geneticevaluation. Moreover, breeders would notonly be interested in more accurate EBVsbased on gene markers, but they wouldalso want to know the actual QTL genotypesfor their breeding animals. Thisinformation on individual genotype willbecome less relevant if more gene testsbecome available and if testing becomescheaper and more widespread. This mightstill take some years. Thus, as gene markertesting is gradually introduced, it is morelikely to create additional selection criteriato consider and it will take sometime before QTL information is seamlesslyand optimally integrated in existing geneticevaluation programmes. In particular, ifgenetic evaluation is based on informationfrom many different breeding units, suchas in cattle or sheep, genotyping informationwill initially be available for only asmall proportion of the breeding animals,possibly not justifying a total overhaul ofthe system for genetic evaluation. Simplead hoc procedures where QTL effects areestimated and presented separately as additionaleffects are initially a more likelyroute to implementation.Solutions for fixed QTL genotypeeffects, along with genotype probabilitiesas outputs of genetic evaluation, mightbe interesting to breeders and, comparedwith random QTL effects, may be morelikely to be presented and used separatelyfrom polygenic EBVs. This would alsobe the case for genotypic information onMendelian characters, where there is nopolygenic component.Incorporating MAS in selectionprogrammesMolecular information can be used toenhance both the processes of integratingsuperior qualities of different breeds andwithin-breed selection. These strategies arefurther described below.Between-breed selectionCrossing breeds results in extensive LD,which can be capitalized upon using MASin a number of ways. If a large proportionof breed differences in the trait(s) ofinterest are due to a small number of genes,gene introgression strategies can be used.If a larger number of genes is involved,MAS within a synthetic line is the preferredmethod of improvement.Marker-assisted introgressionIntrogression of the desirable allele at atarget gene from a donor to a recipient breedis accomplished by multiple backcrossesto the recipient, followed by one or moregenerations of intercrossing. The aim ofthe backcross generations is to produce