marker-assisted selection in wheat - ictsd

100Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 5MAS scheme to improve local varieties of cassava in the United Republic of Tanzaniausing improved disease and pest resistant introductions from Latin AmericaYear1Improved introductions(≈90)Local varieties(≈60)Year2Crossing block(Polycross design)Year3Seedling trial(60 000 seedlings)MASYear4 Combining ability studiesSingle row trial(≈10 000 genotypes)Year5The scheme is now in its second year.Farmer participatory trials(≈ 600 genotypes)the population size and a small set of genotypeswith the “minimum criteria” forsuccessful cassava production are evaluatedin a single season in the corresponding agroecologyand then evaluated over two cyclesin collaboration with end-users (rural communitiesand cassava processors). Figure 5describes the scheme of the United Republicof Tanzanian MAS and PPB project. CMDresistant F 1 generated by MAS at CIATwere crossed to BC 1 derivatives of M. esculentasub spp. flabellifolia, showing goodresistance to CGM, to produce progeniesthat combine some CMD and CGMresistance (Kullaya et al., 2004). The progenieswere established from embryo axesas in vitro plants to aid shipment to Africa.Molecular markers associated with resistanceto CMD and phenotypic evaluationfor CGM resistance were used to screenand select progenies that combine resistanceto CMD and CGM. Resistant plants(300 genotypes and ten plants per genotype),were shipped to the United Republicof Tanzania as in vitro plantlets for useas improved parents. A selection basedon harvest index, a highly heritable trait,and total biomass was made and 80 genotypesselected. These were planted in thesecond year in a controlled crossing blocktogether with 54 local germplasm from theeastern and southern zones of the country.Emphasis was placed on local varieties with,or tolerance to, CBSD, which is a majordisease of cassava in coastal east Africa andMozambique. Over 40 000 crosses weremade between the improved genotypesand the local varieties producing more than60 000 seeds.Sexual seeds obtained from crossingimproved and local genotypes were plantedin the screen house and transferred tothe field 40 days after planting. Parentallines were also planted in the screen housefrom woody stakes. DNA was isolatedfrom parental lines using the rapid minipreparationmethod and evaluated withthe five markers associated with the