marker-assisted selection in wheat - ictsd

434Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishthere are many problems that detract fromits usefulness. Principal among these isthe poor state of current tree breeding ingeneral, and in developing countries inparticular. Simons (28) listed a numberof problems concerning genetic improvementof tropical trees, including dioecy,undocumented origins and uncertainty ofgenetic control of traits. However, Galvez(10) mentioned that MAS had been usedto assist in selection of coconut parentsfor breeding. Priyadarshan (11) consideredMAS to be helpful for rubber improvement,at least theoretically, and Badr (47)seemed to be looking forward to MASreducing the time needed for evaluationof fruit trees in Egypt, obviating the needfor grafting to see the products of breedingefforts. Forest trees, perhaps more thanother genetic resources used by humans,are at, or still very near, their wild state(Muralidharan, 7), which indicates thattremendous improvement can probablybe made quite rapidly based on selectionamong existing genotypes. Muchugi (49)recognized the potential of MAS for treespecies improvement, seeing it as a techniquebest placed to help select and upgradetropical tree species where the first fruitingmay take as long as twenty years.Technical details of MAS useThere were several contributions to theconference regarding technical aspects ofMAS, and how to use MAS effectively ingenetic improvement programmes. Mota(14) raised the issues of molecular markerslocated far from the target gene, increasingthe probability of recombination takingplace between them, resulting in reducedefficiency of MAS and, secondly, of falsepositive marker-gene associations. Dulieu(23) also emphasized the importance oftight marker-gene linkage to minimizelosses through recombination. Weller (15)acknowledged the importance of both issuesraised by Mota (14) and proposed that thebest solution to the problem of false positivesis to employ the false discovery rate,to get an idea about the expected numberof false positives. De Koning (16) supportedthe use of the false discovery rateand also referred to recent research resultssuggesting there were benefits in MASfrom using a relaxed threshold for QTL(quantitative trait loci) detection. Mota(36) concluded that developing countriesshould only use MAS in their breeding programmeswhen there is complete linkagebetween the marker and the gene of interest,to avoid wasting precious resources. Dulieu(42) commented on this, pointing out theadvantages of using flanking markers (i.e.where markers are located on either side ofthe gene of interest) in MAS.Singh (44) described the usefulness ofMAS in backcrossing programmes, bygrowing large BC1 populations (BC1 isthe first backcross generation), rejecting50–60 percent based on phenotype (conventionalscreening) and analysing the remainderwith MAS. This could be repeated in thesecond backcross population, saving considerabletime and resources. The usefulnessof this approach was confirmed by Dulieu(53), and Sridhar (54) explained how threegenes for rice bacterial blight resistancewere pyramided into adapted germplasmusing MAS in a backcrossing programme.Which traits for MAS?Referring to crop improvement, Murphy(1) noted that not all crops and traits wereamenable to MAS. A Dutch perspectiveon the type of traits amenable to MASto date was provided by De Lange (57),who indicated that single gene controlledtraits had received most attention, but little