marker-assisted selection in wheat - ictsd

190Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishtious viral disease caused by a memberof the herpes virus family and costs thepoultry industry about US$1 000 millionper annum. An F 2 cross between resistantand susceptible lines was challenged experimentallyand genotyped, providing the datafor a QTL analysis that resulted in a totalof seven QTL for susceptibility to MD(Vallejo et al., 1998; Yonash et al., 1999).Subsequently, the founder lines of the F 2cross were used for a micro-array studyto identify genes that were differentiallyexpressed between the two lines followingartificial infection. Fifteen of these geneswere mapped onto the chicken genomeand two of them mapped to a QTL regionfor resistance to MD (Liu et al., 2001a). Atthe same time, protein interaction studiesbetween a viral protein (SORF2) and achicken splenic cDNA library revealed aninteraction with the chicken growth hormone(GH) (Liu et al., 2001b). This ledto the detection of a polymorphism in theGH gene that was associated with differencesin the number of tumours betweenthe susceptible and the resistant line (Liu etal., 2001b). GH coincided with a QTL forresistance and was differentially expressedbetween founder lines (Liu et al., 2001a).Alongside the various genome scans forQTL, a large number of candidate genestudies have been carried out. The majorityof studies summarized in Table 2 havebeen conducted on White Leghorn strainsand have utilized restriction fragmentlength polymorphisms (RFLPs), SNPs orsingle strand conformation polymorphisms(SSCPs). These techniques require boththat the gene is known and that the experimenteris able to sequence part of the geneto detect polymorphisms that distinguishthe experimental lines.Candidate gene studies have been usedin two ways. First, candidate genes maybe used merely as a marker for a trait(typically disease) based on prior knowledgeand, second, and much less often,to search for the mutation within a genethat is associated with phenotypic variationin a trait. Currently, potential (candidate)genes for a QTL may be obtained froma knowledge of physiology (Dunn et al.,2004) or comparative linkage maps (i.e.locating genes that are in the location of theQTL based on common areas of the generichgenomes of different species, usuallyhuman and mouse). There are likely to bemany more of the second type of candidategene studies as information from large-scalegene expression and proteomic experimentsbegin to suggest novel gene candidates fortraits of commercial and biological importance.It should also be noted that thereis good evidence that genetic variation isnot limited to genomic DNA: associationsbetween polymorphisms in mitochondrialgenes and MD resistance, body weight andegg shell quality were reported by Li et al.(1998a, b).Despite great enthusiasm for breedingcompanies to be involved in functionalgenomics research in poultry, there arevery few applications of MAS in commercialpoultry breeding. One existingexample is the use of blood group markersto improve resistance to MD where selectionof haplotypes B 21 and B 12 based onconventional serological tests has beenwidely used (McKay, 1998). In discussionswith the industry it is clear that mostinterest is in QTL or candidate genes forresistance to diseases like MD or ascites, agenetic condition associated with pulmonaryhypertension, leading to mortalityin fast growing birds. There is also considerableinterest among breeders of layerlines for egg quality, especially egg shellquality because of its importance for food