Race specific resistance against Phytophthora infestans in potato is controlled by more genetic factors than only R-genes

For RFLP mapping of R-genes, determining resistance to specific races of Phytophthora infestans in tetraploid potato, it is necessary to develop well segregating populations at the 2x level. During mapping studies, evidence was obtained that more genetic factor(s) are involved in the expression of R-genes than conventionally believed. Two experiments are described in which such an additional genetic factor was suppressing or enhancing the expression of unknown R(n) and R(i) factors. R(n) and R(i) appeared to be present in the investigated plant material, containing R4 and RIG, or in one of the susceptible crossing parents. In a third experiment, the expression and the segregation of the well known RI gene was influenced by an additional genetic factor. In that case there were indications for a dominant suppressor. This was established by the selection of susceptible plants carrying a RFLP allele of probe GP21 closely linked to R1. In three of the four F-1 populations, resulting from crosses between such susceptible plants and susceptible tester plants, resistant progenies were found. The resistance appeared to be R1-specific. This clearly indicates that in three of the four investigated susceptible plants, the R1 gene was still present but not expressed.