marker-assisted selection in wheat - ictsd

218Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishHallerman and Soller, 1990; Mackinnonand Georges, 1998);• selection of bull sires without a PT, basedon half-sib records and genetic markers(Spelman, Garrick and van Arendonk,1999);• selection of sires in a half-sib scheme,based on half-sib records and geneticmarkers (Spelman, Garrick and vanArendonk, 1999);• use of genetic markers to reduce errorsin parentage determination (Israel andWeller, 2000).Meuwissen and van Arendonk (1992)found that inclusion of marker informationto increase the accuracy of sire evaluationsincreased the rate of genetic gain byonly 5 percent when the markers explained25 percent of the genetic variance. Thisresult is not surprising considering that theaccuracy of sire evaluations based on a PTof 50 to 100 daughters is already quite high.In “open” and “closed” nucleus breedingschemes, rates of genetic gain were increasedby 26 and 22 percent, respectively. Theadvantage of MAS in this case is greater,because young sires are not progeny tested,and their reliabilities based only on half-sibinformation are much lower.Mackinnon and Georges (1998) proposed“top-down” and “bottom-up” strategies toapply the third scheme listed above, preselectionof young sires prior to PT. In the“top-down” strategy, QTL genotypes aredetermined for the elite sires used as bullsires by a granddaughter design. If a densemarker map is available, it will then be possibleto determine which QTL allele is passedto each son. Elite bulls from among thesesons are then selected as bull sires for thenext generation. If the original sire was heterozygousfor a QTL, it can be determinedwhich of his sons received the favourableallele. Sons of these sires are then genotypedand selected based on whether they receivedthe favourable grandpaternal QTL alleles.It is assumed that the dams of the candidatesires are also genotyped, and that these cowswill be progeny of the sires evaluated by agranddaughter design. Thus, grandpaternalalleles inherited via the candidates’ dams canalso be traced. A disadvantage of this schemeis that only the grandpaternal alleles are followed.Some of the sons of the original siresthat were evaluated by a granddaughterdesign will also have received the favourableQTL allele from their dams, but not via thegenotyped grandsires. However, young sireswill be selected based only on the grandpaternalhaplotypes.In the “bottom-up” scheme, QTL genotypesof elite sires are determined bya daughter design. These sires are thenused as bull sires. The candidate bullsare then pre-selected for those QTL heterozygousin their sires, based on whichpaternal haplotype they received. As theQTL phase is evaluated on the sires of thebull calves (the candidates for selection),no selection pressure is “wasted” as inthe “top-down” scheme. In addition, thisdesign can be applied to a much smallerpopulation, because only several hundreddaughters are required to evaluate each bullsire. On the negative side, more daughtersthan sons must be genotyped to determineQTL genotype. Mackinnon and Georges(1998) assumed that in either scheme it willnot be necessary to increase mean generationinterval above that of a traditional PTprogramme, although this will probablynot be the case (Weller, 2001).Kashi, Hallerman and Soller (1990),Mackinnon and Georges (1998), and Israeland Weller (2004) all addressed the problemthat QTL determination will be subjectto error. Deciding that a specific sire ishomozygous for the QTL when in fact