marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 97• for definition of average heterozygosityin the selection of partially inbred linesfor tolerance to inbreeding;• for identification of the male parentin elite germplasm derived from polycrossesby fingerprinting. This tool isalso useful for checking the identityof different genotypes to eliminateduplication in germplasm collections.Best results are achieved when MAS iscombined with phenotypic data as comparedwith either approach independently(Hospital, Chevalet and Mulsant, 1992).Phenotypic data would reduce the cost ofgenotyping especially if phenotypic evaluationis conducted on early generations(Gimelfarb and Lande, 1994). This not onlyreduces the cost of MAS but also increasesits efficiency. Some examples of MAS incassava breeding conducted at an internationalcentre and national programmes aredescribed below.Molecular MAS for CMD resistance at anIARCAn ideal target for MAS is breeding fordisease resistance in the absence of thepathogen. This is the case of CMD in theAmericas, where the disease does not occur.CMD is a viral disease first reported byWarburg in 1894 in eastern Africa (quotedby Storey and Nichols, 1938). Several variantsof the disease (East Africa cassavamosaic virus [EACMV], South Africa cassavamosaic virus [SACMV], Indian cassavamosaic virus [ICMV]) have been reported(Swanson and Harrison, 1994) and areendemic in all cassava growing regions ofAfrica and southern India, where it is themost severe production constraint. Thewhite fly vector of CMD, Bemisia tabacibiotype A, does not colonize cassava inthe New World but recently a new biotypeof B. tabaci, biotype B (also referred to asB. argentifolia), has become widespreadin the Americas and has a wide host rangeincluding cassava (Polston and Anderson,1997), increasing the possibility that CMD,EACMV, SACMV, ICMV or a nativeAmerican gemini virus will become establishedon cassava in the neo-tropics. Thisis a frightening prospect for cassava productionin Latin America, considering thatmost Latin American cassava germplasm isvery susceptible to CMD (Okogbenin etal., 1998). The susceptibility of neo-tropicalgermplasm to CMD also limits the utilizationof germplasm from the crop’s centreof diversity in the neo-tropics for these keycassava production regions. Breeding forresistance to CMD in Latin America, wherethe disease does not exist and is unlikely tobe introduced due to very strict quarantinecontrols, requires the tools of MAS.Evaluations at IITA identified an excellentsource of resistance to CMD in someNigerian landraces (A.G.O. Dixon 1989,unpublished data), namely TME3, TME7,TME5, TME8, TME14 and TME28. Thisresistance is effective against all knownstrains of the virus, including the virulentUgandan variant (UgV) (Akano et al.,2002; CIAT, 2001). CIAT, in collaborationwith IITA in Ibadan, Nigeria, and withsupport from the Rockefeller Foundation,devel-oped several molecular markers forthis source of CMD resistance, revealedto be controlled by a single dominantgene designated as CMD2 (Akano et al.,2002). At least five markers tightly associatedto CMD2 have been developed, theclosest being RME1 and NS158 at distancesof four and seven cM respectively.The dominant nature of CMD2 and itseffectiveness against a wide spectrum ofviral strains makes its deployment veryappealing for protecting cassava againstthe actual or potential ravages of CMD