Mapping of the S. demissum late blight resistance gene R8 to a new locus on chromosome IX

Abstract

The use of resistant varieties is an important tool in the management of late blight, which threatens potato production worldwide. Clone MaR8 from the Mastenbroek differential set has strong resistance to Phytophthora infestans, the causal agent of late blight. The F1 progeny of a cross between the susceptible cultivar Concurrent and MaR8 were assessed for late blight resistance in field trials inoculated with an incompatible P. infestans isolate. A 1:1 segregation of resistance and susceptibility was observed, indicating that the resistance gene referred to as R8, is present in simplex in the tetraploid MaR8 clone. NBS profiling and successive marker sequence comparison to the potato and tomato genome draft sequences, suggested that the R8 gene is located on the long arm of chromosome IX and not on the short arm of chromosome XI as was suggested previously. Analysis of SSR, CAPS and SCAR markers confirmed that R8 was on the distal end of the long arm of chromosome IX. R gene cluster directed profiling markers CDP(Sw5)4 and CDP(Sw5)5 flanked the R8 gene at the distal end (1 cM). CDP(Tm2)1-1, CDP(Tm2)1-2 and CDP(Tm2)2 flanked the R8 gene on the proximal side (2 cM). An additional co-segregating marker (CDP(Hero)3) was found, which will be useful for marker assisted breeding and map based cloning of R8.

Fig. 1

Positions of NBS profiling markers and R gene homologs on chromosome 9. Markers in large font indicate the NBS profiling markers that were linked to R8. Potato genome sequences, the tomato genome sequences and the marker sequence database from the SGN were searched using the NBS5a6H and NBS1M by BLAST analysis. The bars on the left indicate S. phureja scaffolds (PGSC v3). In the middle are the tomato EXPEN 2000 genetic map and the tomato SL2.40 Ch9 physical map (SGN). On the right positions of tomato genome sequences with homology to R genes are shown. Horizontal and diagonal lines indicate corresponding marker positions in the different maps

Fig. 2

Comparison of different genetic maps of chromosome IX. From left to right the potato SHxRH map (Van Os et al. 2006), the R8 map produced in this study and the combined Solanaceae pathogen resistance map as extracted from the GABI website (May 20th, 2011). Only the long arms of chromosome IX are shown. The dotted arrows indicate relative positions of studied markers shared between the different maps

Fig. 3

Morphology of marker CDP^Hero3. Part of a LI-COR gel containing co-segregating marker CDP^Hero3 which was obtained with Hero4064F1/HaeIII primer/enzyme combination. Pr, Ps, R, S, and M indicate the resistant parent, susceptible parent, resistant and susceptible F1 genotypes, and molecular weight marker, respectively. The arrow on the left points at the CDP^Hero3 band, the right arrow points at molecular weight marker