marker-assisted selection in wheat - ictsd

126Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishMABC is certainly the form of MASwith the most immediate and obvious benefitsfor maize breeding. MABC is usedfor three main purposes: selection of transgenes(or of native DNA sequences of themaize genome, whether genes or QTL),elimination of unwanted regions of thedonor-parent genome linked to the transgeneand selection of unlinked regions ofthe recurrent-parent genome. With theexception of DNA markers and transgenes,these have been the same goals of backcrossbreeding since the inception of that methoddecades ago (Fehr, 1987). Of course, DNAmarkers enable breeders to identify progenythat contain the desired recombinantchromosomes and donor-parent genome ina more direct manner. Also, MABC facilitatesthe process of combining more thanone transgene in a given inbred line (e.g.“gene or trait stacking or pyramiding”).This reduces the number of generationsneeded to reach certain stages of a breedingprogramme and reduces the time needed toproduce commercial hybrids for the market.Generic MABC schemes suitable for maizebreeding programmes have been describedin detail for single genes (Hospital, Chevaletand Mulsant, 1992; Ragot et al., 1995;Frisch, Bohn and Melchinger, 1999a, 1999b;Frisch and Melchinger, 2001a; Hospital,2001; Ribaut, Jiang and Hoisington, 2002),for QTL (Hospital and Charcosset, 1997;Bouchez et al., 2002) and for gene stacks(Frisch and Melchinger, 2001b). Versions ofsuch schemes have been used in maize breedingprogrammes in the private sector, oftenat their continuous nurseries (Ragot et al.,1995). Most recently, MABC has also beenadopted as a tool to develop sets of nearisogeniclines (NILs) for genomics research(Peleman and van der Voort, 2003).Theoretical and simulation studies havebeen conducted to identify the most efficientMABC protocols. Parameters mostcommonly studied include the number ofindividuals genotyped at each generation,the number of markers used, relative selectionpressure for recombination around thetarget locus or global recovery of recurrentparent genome and the number of individualsselected at any generation. Optimalvalues for each of the above depend onthe objective of the MABC approach interms of quality (required level of recurrentparent genome recovery), speed(fastest possible conversion or set numberof generations) and resources (unlimitedor limited). While the fastest and highestquality MABC approaches have themost expensive protocols, less intensiveapproaches can result in significant timesavings and quality improvements whencompared with conventional backcrossingapproaches and at a fraction of the cost ofthe most expensive MABC protocols.Frisch, Bohn and Melchinger (1999b)showed that to minimize linkage dragaround the target locus (loci), selection ofrecombination events close to the targetlocus (loci) should be conducted in theearly backcross generations. Frisch andMelchinger (2001a) and Ribaut, Jiang andHoisington (2002) further demonstratedthat minimizing linkage drag around thetarget locus requires very large numbersof individuals (possibly hundreds) to begenotyped. Hospital and Charcosset (1997)proposed a selection scheme based onselecting a single individual to be backcrossed.By contrast, Frisch and Melchinger(2001a) proposed selecting several individualsand determining the family sizeof their backcross progeny based on theindividuals’ genotypes. By using varyingrather than constant numbers of individualsor markers at the different backcrossgenerations, it was shown that the number