marker-assisted selection in wheat - ictsd

Chapter 9 – Molecular marker-assisted selection for resistance to pathogens in tomato 159Table 4Resistant heterozygous plants obtained in some cross combinations realized for pyramiding ofresistance genesCrossPyramidedgenesGenerationAnalysed plant(number)Resistant plant(number)(137 x Momor) F 1 BC 3 xTm2a – Frl(137 x Ontario) F 1 BC 3 Ve – PtoF 1 50 7(AD17 x Okitzu) F 1 BC 4 xTm2a – Frl(AD17 x Stevens) F 1 BC 4Sw5F 1 24 5(AD17 x Ontario) F 1 BC 4 x(AD17 x Stevens) F 1 BC 4Pto – Sw5 F 1 24 6(PI15 x Stevens) F 1 BC 4 x(PI15 x Ontario) F 1 BC 4Sw5 - Pto F 2 52 29Indeed, once a marker has been set up,its use on large populations for resistancescreening is then routine. Technical facilitiesare today available for screening manysamples simultaneously and also costs forequipment are decreasing. In addition, therapid development of new molecular techniques,combined with the ever-increasingknowledge about the structure and functionof resistance genes (Hulbert et al.,2001), will help to identify new molecularmarkers for MAS, such as single nucleotidepolymorphisms (SNPs). Moreover, thanksto the International Solanaceae GenomeProject (SOL), sequencing of the tomatogenome is in progress, and in a few yearsthis will enhance information on resistancegenes. This in turn will facilitate thedevelopment of molecular markers fromtranscribed regions of the genome, therebyallowing large-scale gene-assisted selection(GAS) to be achieved.Over the coming years, selection forpathogen resistance in tomato will beunderpinned by research aimed at: mappingother resistance genes for new pathogens;developing PCR-based functional markers(Andersen and Ludderstedt, 2003); designingthe most suitable breeding schemes(Peleman and van der Voort, 2003), especiallyfor transferring QTL resistances;large-scale screening through automation;allele-specific diagnostics (Yang et al.,2004); and DNA microarrays (Borevitzet al., 2003). In effect, the combination ofnew knowledge and new tools will lead tochanges in the strategies used for breedingby exploiting the potential of integrating“omics” disciplines with plant physiologyand conventional plant breeding, a processthat will drive the evolution of MASinto genomics-assisted breeding (Morganteand Salamini, 2003; Varshney, Graner andSorrells, 2005).ACKNOWLEDGEMENTSThe authors wish to thank Mr MarkWalters for editing the manuscript andAngela Cozzolino for her technical assistance.This research was partially financedby the Ministry of Agriculture andForestry of Italy in the framework ofproject PROMAR (Plant Protection by theUse of Molecular Markers). Contributionno. 119 from the Department of Soil, Plantand Environmental Sciences.REFERENCESAmmiraju, J.S.S., Veremis, J.C., Huang, X., Roberts, P.A. & Kaloshian, I. 2003. The heat-stableroot-nematode resistance gene Mi-9 from Lycopersicon peruvianum is localized on the short armof chromosome 6. Theor. Appl. Genet. 106: 478–484.