marker-assisted selection in wheat - ictsd

Chapter 8 – Marker-assisted selection in maize 127of marker data points required could bereduced and thus the efficiency of MABCimproved (Hospital, Chevalet and Mulsant,1992; Frisch, Bohn and Melchinger, 1999b).Several studies also showed that using alimited number of markers on non-carrierchromosomes was sufficient to recovermore that 95 percent of the recurrent parentgenome in three or fewer backcross generations(Hospital, Chevalet and Mulsant,1992; Visscher, Haley and Thompson, 1996;Servin and Hospital, 2002).One of the most important lessons fromthe various theoretical and simulation studiesof MABC is that the effects of thedifferent MABC parameters are not independentof each other. With maize, largebackcross populations can be generatedfrom a single plant when that plant is usedas the male and recurrent parent plants areused as females. Marker systems in maizeare also such that very large amounts ofmarker data can be generated on plantsbefore flowering. Potential MABC protocolsare almost endless in maize andidentifying the most efficient is only possibleon a case-by-case basis. For example,while achieving almost complete recovery ofthe recurrent parent’s genome is necessaryfor registering backcross-derived lines andhybrids in many European countries, partialrecovery might be sufficient to improvethe agronomic performance of varieties indeveloping countries. The optimal MABCprotocols for these two strikingly differentobjectives will be very different. Protocolsfor the first objective will involve backgroundselection and the use of backgroundmarkers very close to the target locus (loci).Protocols for the second objective mightinvolve markers for the target locus (loci)only, while relying on successive backcrossgenerations to recover an adequate amountof recurrent parent genome.Successful examples of MABC in maizeinclude backcrossing of transgenes (Ragotet al., 1995), and QTL for insect resistance(Willcox et al., 2002), flowering maturity(Ragot et al., 2000; Bouchez et al.,2002) and grain yield (Ho, McCouch andSmith, 2002).Methods of “forward breeding” withDNA markers have also been proposed andimplemented by maize breeding programmes.As with the pedigree-based methods ofmaize breeding favoured by the private sector,many of the “new” methods that utilizegenetic data from DNA markers integratedwith phenotypic data are essentially a formof recurrent selection, a method that hasbeen in use for several decades (Hallauerand Miranda, 1981). The key advantages ofthe new versions of recurrent selection are,of course, the availability of genetic data forall progeny at each generation of selection,the integration of genotypic and phenotypicdata, and the rapid cycling of generations ofselection and information-directed matingsat continuous nurseries.At least two distinct forms of forwardbreeding with MAS have been proposed:single large-scale MAS (SLS-MAS) (Ribautand Betrán, 1999) and MARS (Edwardsand Johnson, 1994; Lee, 1995; Stam, 1995).A key difference between the methods isthat SLS-MAS employs DNA markers atonly one generation and attempts to retaingenetic variation in regions of the genomeunlinked to the DNA markers, whileMARS uses markers at each generation,exhausting genetic variation in mostregions of the genome. Versions of bothSLS-MAS and MARS have been used bybreeding programmes in the private sector(Johnson, 2004; Eathington, 2005; Crosbieet al., 2006).SLS-MAS is of particular interest inpedigree breeding as it consists of screening