Mitochondrial plasmids do not determine host range in crucifer‐infecting strains of Fusarium oxysporum

The role of mitochondrial plasmids in determining host range in Fusarium oxysporum was tested by using protoplast fusion and fluorescence‐assisted cell sorting to produce hybrid cells between two strains (ATCC 9990 and ATCC 16601) of the fungus which differ in host specialization to cabbage and radish. Three nuclear restriction fragment length polymorphism‐defined loci and mitochondrial chromosomal RFLP markers were identified which distinguish the parental strains. In addition, parents were differentiated by mitochondrial plasmids, one unique to each strain and previously correlated to host specialization. In single‐spored cultures derived from cell fusions (‘fusion cultures’), only parental combinations of the three nuclear genetic markers were detected. However, non‐parental combinations of nuclear and cytoplasmic markers were recovered. One mitochondrial plasmid type was not found in fusion cultures; the other plasmid type was detected in association with both parental mitochondrial chromosome types. Several fusion cultures lacked detectable plasmid sequences of either type. Chromosome‐sized DNAs from parent strains and fusion cultures were separated using contour‐clamped homogeneous electric field (CHEF) electrophoresis. Electrophoretic karyotypes of the parent strains differed greatly, with eight and 11 chromosome‐sized bands detected for ATCC 9990 and ATCC 16601, respectively. Electrophoretic karyotypes of fusion cultures generally were indistinguishable from one or the other parental types. Host specialization was not correlated with mitochondrial plasmid type, but rather with nuclear genotype.