marker-assisted selection in wheat - ictsd

Chapter 9 – Molecular marker-assisted selection for resistance to pathogens in tomato 157Figure 1Number of generations reached in backcross schemes carried out between four susceptiblerecurrent genotypes and five resistant donor genotypes5432MomorOkitzuOntarioPyrellaStevens10Sel 8 137 PI15 AD17simplified, enabling a faster and cheapermarker system, i.e. a sequence characterizedamplified region (SCAR; Kawchuk,Hachey and Lynch, 1998) marker, whichonly requires one PCR reaction to detectpolymorphism between the resistant andthe susceptible genotypes, to be set up.(Barone et al., 2004).The second strategy was followed todesign primers and enzymes suitable fortargeting three resistance genes (I2, Pto andVe2). This strategy allowed gene-assistedselection to be achieved through the simplePCR procedure. Finally, the third strategywas applied in the case of one CAPSmarker targeting the resistance gene Frl;it was derived from one RFLP tomatomarker (TG101) linked to the gene (Fazio,Stevens and Scott, 1999).The markers found were used to selectresistant genotypes in backcross breedingschemes, while the process itself allowedthree generations to be screened annually.At present, for some cross combinations,the BC 5 generation has been reached, forothers the BC 2 -BC 3 (Figure 1). Where aBC 5 generation was already available, thebreeding programme continued by selfingBC 5 resistant genotypes. In all other casesthe backcross programme will continue upto the fifth backcross generation. At theend of each backcross scheme, the resistantBC 5 F 3 genotypes, selected through molecularmarker analysis, will also be testeddirectly for resistance by inoculating thepathogen and monitoring signs of disease.This will allow verification that no linkagebreakage and loss of resistance geneoccurred.This procedure was already adopted inthe case of one backcross scheme aimedat transferring a resistance gene to tomatospotted wilt virus (TSWV) to the susceptiblegenotype AD17 (Langella et al., 2004).The in vivo test performed on F 1 BC 5 ,F 2 BC 5 and, F 3 BC 5 generations confirmedthe introgression of the resistance trait andrevealed that the resistance gene Sw5 was