marker-assisted selection in wheat - ictsd

158Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 2Breeding scheme used for thepyramiding of two resistanthomozygous genes (Sw5 and Pto) in thesusceptible genotype PI15PI15 x Stevens Sw5/Sw5F1BC 1 Sw5/-F1BC 2 Sw5/-F1BC 3 Sw5/-F1BC 4 Sw5/-XPI15 x Ontario Pto/PtoF1BC 1 Pto/-F1BC 2 Pto/-F1BC 3 Pto/-F1BC 4 Pto/-F 1 (F 1BC 4 x F 1BC 4 ) Sw5/-, Pto/-XF 2 (F 1BC 4 x F 1BC 4 ) Sw5/Sw5, Pto/Ptofixed at the homozygous stage at the F 3 BC 5generation.Finally, besides the transfer of one resistancegene to each susceptible genotype, acrossing scheme was undertaken to accumulatetwo or three resistance genes in thesame genotype. In this case, the decisionwas made to stop the backcross scheme atthe BC 3 or BC 4 generation as both parentallines were cultivated varieties and hencegenetically very similar, and therefore therecovery of the recurrent genome couldbe satisfactory. F 1 BC 4 hybrids carrying thesame genetic background in the recurrentparent have been intercrossed, followingthe breeding scheme shown in Figure 2.At the end of each F 1 BC 4 x F 1 BC 4 crossand after selecting the genotypes carryingall the resistant alleles at the heterozygouslevel, one or two selfing generations will becarried out to fix all the resistant genes atthe homozygous level.This strategy has already started in somecases and the first homozygous multiresistantgenotypes have been obtained. Alsoavailable are two F 2 genotypes out of 52 analysedplants, obtained by intercrossing theF 1 BC 4 progeny from PI15 x Stevens withthe F 1 BC 4 progeny from PI15 x Ontario(Table 4). This F 2 generation exhibited twogenotypes carrying both resistant genesSw5 and Pto at the homozygous level aswell as 29 genotypes carrying both genes atthe heterozygous level.The work reported here on transferringresistance genes among tomato genotypesdemonstrates the usefulness of MAS forimproving traditional breeding strategies.The contribution of molecular markerslinked to resistance genes was very efficientin reducing the time and space necessaryfor selection, enabling both early screeningfor resistance and reduced numbers ofgenotypes to be transplanted. The mostchallenging work was the search for suitablemarkers, which often required bothconsiderable time and financial resources.Different strategies were used successfullyto find the most suitable markers to performMAS for transferring eight resistance genesinto superior tomato genotypes; such strategiescould be repeated in tomato for manyother genes due to advanced molecularknowledge of the genome of this species.PERSPECTIVESThe availability of PCR-based markersfor many resistance genes allows MAS forbiotic resistance in tomato to be appliedsuccessfully in any laboratory without theneed for highly sophisticated techniques.