Inheritance of streptomycin and chloramphenicol resistance in Phytophthora infestans: evidence for co-segregation of mitochondrial DNA and streptomycin resistance

Spontaneous mutant strains of Phytophthora infestans resistant to either streptomycin or chloramphenicol were selected by prolonged exposure of colonies to drug-amended media. Each strain was then crossed with a sensitive strain having different RFLP markers in its mitochondrial (mt) DNA and nuclear (n) DNA. A majority (56 out of 65) of the sexual progeny from the streptomycin mating inherited at least one nRFLP band from each parent. Fifty-two of these hybrids were streptomycin resistant and inherited their mtDNA from the resistant parent, while the four streptomycin sensitive isolates inherited their mtDNA from the sensitive parent. These data are consistent with streptomycin resistance being cytoplasmically inherited, possibly on the mitochondrial genome. The inheritance of chloramphenicol resistance in hybrids did not correlate well with that of mtDNA type but neither was it obviously Mendelian.