marker-assisted selection in wheat - ictsd

98Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishin both Africa and Latin America. CIATand IITA undertook a project to verifythe utility of these markers for MAS inbreeding CMD resistance by developingcrosses between the sources of TME3 andsusceptible varieties. A total of six families,ranging in size from 36–840 genotypes, anda total of 2 490 genotypes were used. Thecrosses were genotyped with two markersand also evaluated for CMD resistancein a high CMD pressure area in Nigeria.Results of the marker analysis and phenotypicevaluation of CMD resistance inthe field revealed that the markers RME1and NS158 SSR were excellent predictiontools for CMD resistance in some crosses(a prediction accuracy of 70–80 percent). Ina few families, however, the markers werenot polymorphic between the resistant andsusceptible parent and, therefore, were notuseful. This highlights the need to developmany markers around a gene of interest ina MAS programme and then to use thosemarkers to evaluate the parents and identifythe best markers for the different crosscombinations.Eighteen progenies from TME3 carryingthe CMD2 marker were established fromembryo axes and imported to CIAT fromIITA. They were crossed extensively to eliteparents. Seeds harvested from the crosseswere germinated in vitro from embryo axesaccording to standard protocols for cassava(Fregene et al., 1997, CIAT, 2002) to allowsharing the CMD resistant genotypes withcollaborators in Africa and India. Eachplantlet was multiplied after three to fourweeks of growth to obtain three to fiveplants. After another four weeks, leaves of allPhytosanitary conditions for the exchangeof cassava germplasm between Africa and Asiaare very stringent, but appropriately indexed invitro cultures of embryo axes are permitted forexperimental purposes.plants were removed for molecular analysisand the plants multiplied again to obtain10–20 plantlets. DNA isolation was by arapid mini preparation method developedfor rice (Nobuyuki et al., 2000). The DNAobtained is sufficient for 100 reactions andcan be held in the Costar plates for twomonths at –20 o C without any degradation.PCR amplification, polyacrylamide gelelectrophoresis (PAGE) or agarose gelanalysis of SSR markers NS158 and RME1were as described by Mba et al. (2001). Theversatility of spreadsheets makes them theappropriate software to handle the diverseinformation generated by MAS. Gel imagesfrom the marker analysis were entereddirectly into a spreadsheet that containsinformation on the parents, tissue cultureand greenhouse records, and subsequentphenotypic evaluation of the progenies.After molecular analysis, genotypes thatcarry the marker allele associated withCMD2 were further multiplied to obtainat least 30 plants. Ten plants were sent tothe greenhouse for hardening and latertransferred to the breeding programme forevaluation. Five plants were kept in vitro,while 15 plants were shipped to partners inIndia and Africa as shown in the flow chartfor MAS (Figure 4).To date, more than 50 000 progeny havebeen evaluated with CMD linked markersand resistant lines shared with nationalprogrammes in India or Africa, and alsoincorporated into the breeding scheme atCIAT. The cost of a single marker datapoint is US$0.30 and 32 000 samples can beprocessed in a year.MAS for CMD resistance at a NARSAlthough evaluation for CMD resistancein sub-Saharan Africa is relatively easy andmost areas have sufficient disease pressureto permit moderate to high heritability