marker-assisted selection in wheat - ictsd

120Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishin the nuclear genome. As described below,the methods used to detect RFLPs wereincompatible with the magnitude, speedand efficiency of all but a few aspects ofselection in maize breeding programmes.Gradually, however, the methods used todetect DNA polymorphisms and to createmeaningful information from DNA markerand phenotypic data sets have evolved to thepoint where they are routine componentsof some maize breeding programmes in theprivate sector.Selection occurs at various stages inmaize breeding programmes. The firstopportunity arises when choosing inbredlines to mate as parents of new populations.In some programmes, all such inbreds aregenotyped systematically at DNA markerloci (Smith and Smith, 1992). If the markerloci are sufficiently close on genetic orphysical maps then reasonably good inferencesmay be made about the inbred’shaplotype. Such information is used toestablish identity, resolve disagreementsrelated to germplasm ownership and acquisition,enforce laws intended to encouragegenetic diversity of the hybrids and avoidusing inbreds that contain transgenes whichmay violate regulatory considerations andrestrictions. These selection practices, whileadmittedly not conventional MAS, haveled to improvements in the maize crop byenabling more informed stewardship anddeployment of genetic resources and byproviding a degree of protection of intellectualproperty and related investments inmaize breeding.Unquestionably, the most pervasive anddirect use of MAS in maize by the privatesector has been with backcrossing oftransgenes into elite inbred lines, the directparents of the commercial hybrids (Ragotet al., 1995; Crosbie et al., 2006). Currently,the most widely deployed transgenes andcombinations thereof (i.e. gene stacks) arefor resistance to herbicides or insects (e.g.Ostrinia and Diabrotica). As the commercialmaize crop of any region, maturityzone, market or country is not yet uniformor homogeneous for any transgene,maize breeders have elected to developnear-isogenic versions (transgenic and nontransgenic)of elite inbreds and commercialhybrids in order to satisfy combinations oflicensing agreements, agronomic practices,regulatory requirements, market demandsand product development schemes. Thishas required companies to have twoparallel maize breeding programmes, transgenicand non-transgenic. In this manner,marker-assisted backcrossing (MABC) oftransgenes, and to a lesser degree, of nativegenes and quantitative trait loci (QTL) forother traits, has expedited the developmentof commercial hybrids.More recently, marker-assisted recurrentselection (MARS) schemes and infrastructurehave been developed for “forwardbreeding” of native genes and QTL forrelatively complex traits such as diseaseresistance, abiotic stress tolerance and grainyield (Ribaut and Betrán, 1999; Ragot et al.,2000; Ribaut, Jiang and Hoisington, 2000;Eathington, 2005; Crosbie et al., 2006).Simulation studies suggested that MAScould be effective for such traits under certainconditions (Edwards and Page, 1994;Gimelfarb and Lande, 1994), but the initialempirical attempts at such selection werenot successful (Stromberg, Dudley andRufener, 1994; Openshaw and Frascaroli,1997; Holland, 2004; Moreau, Charcossetand Gallais, 2004) except in the specialcase of sweetcorn (Edwards and Johnson,1994; Yousef and Juvik, 2001). The successreported for sweetcorn is due to the fact thatthe genetic base of sweetcorn is extremelynarrow relative to dent or flint maize; thus