Figure 4. Transfer of a pathogenicity chromosome.

a, Tomato plants infected with Fol007, Fo-47 or double drug resistant Fo-47⁺ strains (1A through 3C) derived from this parental combination, two weeks after inoculation as described for b. b, Eight of nine Fo-47⁺ strains derived from pairing Fol007 and Fo-47 show pathogenicity towards tomato. Average disease severity in tomato seedlings was measured 3 weeks after inoculation in arbitrary units (a.u.). The overall phenotype and the extent of browning of vessels was scored on a scale of 0–4: 0, no symptoms; 1, slightly swollen and/or bent hypocotyl; 2, one or two brown vascular bundles in hypocotyl; 3, at least two brown vascular bundles and growth distortion (strong bending of the stem and asymmetric development); 4, all vascular bundles are brown, plant either dead or stunted and wilted. c, The presence of SIX genes and ORX1 in Fom, Fo-47 and Fol isolates and in double drug-resistant strains derived from co-incubation of Fol/Fom and Fol/Fo-47, assessed by PCR on genomic DNA. Co-incubations were performed with the isolates shown in bold. Three independent transformants of Fom and Fo-47 with a randomly inserted hygromycin resistance gene (H1, H2, H3) were investigated. d, Fo- 47⁺ strains derived from a Fol007/Fo-47 co-incubation have the same karyotype as Fo-47, plus one or two chromosomes from Fol007. Protoplasts from Fol4287, Fol007 (with BLE on chromosome 14), three independent HYG transformants of Fo-47 (lane Fo-47 H1, H2 and H3) and nine Fo-47⁺ strains (lane 1A to 3C, the number 1, 2 or 3 referring to theHYG resistant transformant from which they were derived) were loaded on a CHEF (contour-clamped homogeneous electric field) gel. Chromosomes of S. pombe were used as a molecular size marker. Arrows 1 and 2 point to additional chromosomes in the Fo-47⁺ strains relative to Fo-47. e, Southern blot of the CHEF gel shown in d, hybridized with a SIX6 probe, showing that chromosome 14 (arrow 1 in d) is present in all strains except Fo-47 (H1, H2 and H3).