Dominance and recessiveness at loci for virulence against potato and tomato in Phytophthora infestans

In this study we investigated the genetic control of virulence in the diploid fungal pathogen, Phytophthora infestans, against host resistance genes R1, R2, R3, and R4 (potato) and Ph1 (tomato). For four of these virulence traits, the presence or absence of segregation indicated conclusively which phenotype was dominant. We observed a 3∶1 (virulent∶avirulent) segregation on R2 in the progeny of parents which were both virulent, suggesting that virulence is dominant and both parents are heterozygous. In a cross in which one parent was virulent and the other avirulent on potato gene R3, all progeny tested were avirulent, so avirulence against R3 is dominant. The same virulent parent crossed with a different avirulent parent produced virulent and avirulent progeny in a 1∶3 ratio, indicating that a second locus may be involved. The progeny of two parents virulent on R4 segregated for virulence and avirulence, so virulence against R4 is dominant. For Ph1, a 1∶3 segregation in the progeny of two avirulent parents showed that the avirulent phenotype is dominant, and a 3∶1 ration in a second cross suggested the involvement of a second locus. The segregations for virulence against R1 did not indicate which phenotype was dominant, but did suggest singlelocus control.