marker-assisted selection in wheat

86Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishlines. MAS for this trait has also been practisedat the University of Puerto Rico andat the Biotechnology Institute of Cuba.Bean common mosaic virus and beancommon mosaic necrotic virusBean common mosaic virus (BCMV) andthe related necrotic strains (bean commonmosaic necrotic virus [BCMNV]) areaphid-transmitted potyviruses that arefound worldwide and are seed-borne fromseason to season. BCMNV resistance isvery important in Africa where necroticstrains are prevalent and has become arenewed priority for parts of the Caribbeanwhere necrotic strains have been discovered.BCMV is also endemic in the Andeanregion where it persists in farmer-saved seedand long-season climbing beans. Climbingbeans are grown in both intensive (trellised/staked monoculture) and extensive (intercroppingwith maize) farming systems. Inboth systems the need to protect the cropfrom easily transmitted viral diseases suchas BCMV or BCMNV is great; however,very few climbing beans have been bred forresistance to BCMV. A number of BCMV/BCMNV resistance genes have been taggedincluding the dominant I gene (with whichthe necrotic strains interact to producenecrosis) and the recessive bc-3, bc-2 andbc-1 2 genes (Haley, Afanador and Kelly,1994; Melotto, Afanador and Kelly, 1996;Johnson et al., 1997; Miklas et al., 2000a).The genes can be distinguished by inoculationwith different viral isolates, and a rangeof molecular marker tags are available foreach gene (reviewed in Kelly et al., 2003;Miklas et al., 2006). The dominant I genewas incorporated into a wide range of smallseeded bush beans at CIAT, while resistantbush beans of the bush bean resistant toblack root (BRB) series carrying recessivegenes were developed in the 1990s andhave been widely distributed as breedingparents. The need to reselect the recessivegenes with confidence from segregatingpopulations makes MAS a priority.CIAT started a collaborative project withthe Colombian national bean programmebased at the Colombian AgriculturalResearch Corporation (CORPOICA) in2002 to introgress BCMV resistance genesfrom BRB lines into local landraces andimproved genotypes of Andean climbingbeans (CIAT, 2002, 2003, 2004; Santana etal., 2004). During the breeding programmefor BCMV and over the course of fouryears, MAS was used extensively basedprimarily on the SCAR marker ROC11developed for the bc-3 gene (Johnson etal., 1997) and the SCAR marker SW13 forthe I gene (Melotto, Afanador and Kelly,1996) along with virus screening to confirmthe selection of resistant progeny.The programme was successful in movingbc-3 resistance into a background of creammottled and red mottled seed types forboth highland areas (known as Cargamantocommercial class) as well as mid-altitudeareas through triple-, double- and backcrosses.Although virus resistance was alsoscreened phenotypically, the frequency ofescape, the complex interaction of multiplegenes and the recessive nature of mostof these made MAS the best option forbreeding resistant varieties rapidly. In addition,as climbing bean breeding is a moretime-consuming and expensive endeavourthan bush bean breeding due to the longerseason, wider plant spacing and need forstaking material, MAS was also found to bea very effective measure to reduce breedingcosts and save on breeding nursery space.The implementation of MAS for BCMVwas based on a combination of the previouslydeveloped SCAR markers previouslymentioned and techniques developed at