Figure 2. Butterfly egg-killing HR segregates as a Mendelian trait and it is mapped to a single locus on chromosome B3 . A) B. nigra parental accessions, five-weeks old plants and relative HR phenotype (above). Crossing scheme used to study inheritance of HR-like induced by P. brassicae e gg wash (below). Magnification bar = 10 cm. B) HR phenotypic distribution within the populations obtained from the crossing scheme. C) Distribution of k -mers unique to resistant (R) samples mapped on chromosome B3 (genome C2). k -mers are plotted on each 1 Mb bin as percentage of the total k -mer set. A single peak consisting of ~73% R-specifick -mers located the PEK locus at interval 3-13 Mb (top panel). Validation of the locus was carried out with KASP markers on the BC1 population (n = 66) and four informative recombinants restricted the PEK locus between 6.11 and 8.45 Mb (bottom panel).
The single locus model was further supported by the phenotypic segregation in other crosses involving selfings of parental plants, F2 and BC1 populations (Supporting information: Table S1). Selfing of the susceptible parent DG1-S1 resulted in progeny with no HR (DG1-S2, n = 15). Similarly, a backcross between F1 plants with no HR and DG1-S1 (BC1-5, n = 70) resulted also in progenies with no HR. These results further suggested that absence of HR resulted from fixing a homozygous recessive allele at a single locus. Conversely, backcrosses between F1 with HR-like and DG1-S1 showed again a 1:1 segregation (BC1-4, n = 69; BC1-6, n = 70), as previously observed. Finally, F2s derived from resistant F1s showed a 3:1 segregation as expected (F2-2, n= 8; F2-3, n = 12).