Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 237

Monogenic resistance is most exploited in tomato breeding programs. Modern tomato cultivars may harbor resistances to more than 10 pathogens. Thus, the second step is to study the inheritance of the resistance identified in the wild tomato species. For this purpose, resistant plants were selected and crossed to a susceptible cultivar, S. lycopersicum cv. Moneymaker to produce populations (usually F2 populations) for inheritance study. Crosses between S. lycopersicum and wild tomato species can be easy but sometimes require strategies such as embryo rescue, especially for the self‐incompatible species like S. peruvianum.

By using F2 populations, inheritance of resistance identified in several wild species was characterized. Monogenic resistance to O. neolycopersici was found in S. peruvianum LA2172, S. habrochaites G1.1560 and G1.1290, and polygenic resistance in S. neorikii G.16101. Rick, C.M. (1988). Further, by screening these F2 plants with molecular markers, such as RAPD, AFLP, and CAPS, the resistance in these species was mapped onto specific chromosomes. The resistance loci in S. peruvianum LA2172 and S. habrochaites G1.1560 and G1.1290, named Ol‐4, Ol‐1, and Ol‐3, respectively, are all located on tomato chromosome 6. The Ol‐4 locus is on the short arm, while Ol‐1 and Ol‐3 are on the long arm and closely linked if not allelic (Figure B5.2). In addition to these monogenic Ol‐genes, three quantitative trait loci (QTLs) were identified governing the resistance in S. neorickii G1.1601. The Ol‐qtl1 interval overlaps with Ol‐1 and Ol‐3, while the other two linked Ol‐qtls are located on chromosome 12 in the vicinity of the Lv locus that confers resistance to another powdery mildew species, Leveillula taurica. Markers with close linkage to these loci were generated and can be applied in marker‐assisted selection (MAS) in breeding programs.

Figure B5.2 The chromosome locations of tomato loci for resistance to tomato powdery mildew caused by Oidium neolycopersici. On the left, genetic distance in cM is shown. On the right, map positions of markers and resistance loci are shown on tomato chromosome 6 and 12, respectively. The donors for Ol‐1, Ol‐3, Ol‐5 are Solanum habrochaites G1. 1560, G1. 1290, and PI247087, respectively; for Ol‐4 is S. peruvianum LA2172 and for Ol‐qtls is S. neorickii G1.1601. Ol‐6 is identified from an advanced breeding line with unknown source. As to Ol‐qtls, bars indicate the QTL interval for which the inner bar shows a one‐LOD support and the outer one shows a two‐LOD support interval.