Egg-killing HR is inherited as a single dominant Mendelian locus
We studied the inheritance of P. brassicae egg-induced HR in a crossing scheme derived from a cross between accessions DG1-S1 and SF48-O1 (Fig. 2a) that were selected from our germplasm screening (Fig. 1). The parental accessions consistently showed contrasting phenotypes (Fig. 2a), particularly DG1-S1 showing no/weak HR (score 0-1) and SF48-O1 showing a strong HR (score 2-4) upon treatment with egg wash. Based on our previous studies (Griese 2017, Griese 2021), no/weak HR (score 0-1) did not affect egg survival resulting in “susceptible” (S) plants, while strong HR (score 2-4) caused egg-killing resulting in “resistant” (R) plants. When egg-killing HR was scored as presence/absence in the F1 population (F1-1, n = 150), it segregated with a clear bimodal distribution with a 1:1 ratio between plants without and with HR (χ2 test, p> 0.05) (Fig. 1b, Table 1). Segregation in the F1 was not surprising considering that wild B. nigra are self-incompatible and thus highly heterozygous. This suggests that HR may be heterozygous at least in the R donor accession. A backcross population (BC1-3, n = 66) between a resistant F1 plant and the susceptible parent (DG1-S1) showed again a 1:1 segregation (χ2 test, p> 0.05) (Fig. 2b, Table 1). Recurring of a 1:1 segregation pattern resembled the outcome of a test-cross for a single heterozygous locus. This was confirmed after selfing a resistant BC1plant with HR which resulted in a BC1S1population (BC1S1-1, n = 695) with a 3:1 ratio between R and S plants (χ2 test, p > 0.05) (Fig. 2b, Table 1). Overall, the segregation of P. brassicaeegg-killing HR in our crossing scheme was consistent with a trait controlled by a single dominant Mendelian locus, which should be heterozygous in the HR donor SF48-O1 plant.