Hormonal Heterothallism in Phytophthora parasitica: A Novel Mode of Sexual Reproduction?

Summary: All the single-spore cultures originating from zoospores and oospores of wild-types of Phytophthora parasitica were sensitive to chloramphenicol and streptomycin. Cultures from zoospores and oospores of chloramphenicol-resistant mutants were resistant to chloramphenicol but not streptomycin, while those from zoospores and oospores of streptomycin-resistant mutants were resistant to streptomycin but not chloramphenicol. When chloramphenicol-resistant A1 (6133Cr) of P. parasitica was paired directly with streptomycin-resistant A2 (6134Sr) of the same species, all single-oospore cultures from three crosses were resistant to either chloramphenicol or streptomycin but not both, suggesting absence of genetic exchange in the progeny. Appearance of A2 resistant to chloramphenicol and A1 resistant to streptomycin was due to a mating-type change during oospore formation because oospores produced by A1 through hormone stimulation also gave rise to A2 cultures, and vice versa. Similar results were obtained when reciprocal crosses involving the same direct and membrane-separated pairings between streptomycin-resistant A1 and chloramphenicol-resistant A2 were studied. When A1 wild-type (6133), sensitive to both chloramphenicol and streptomycin, was paired with A2 double mutant 6134CrSr, resistant to chloramphenicol first and streptomycin later, all single-oospore cultures from three crosses were either sensitive to both chloramphenicol and streptomycin or resistant to both antibiotics, indicating that all single-oospore cultures from direct pairing were of uniparental origin. When A2 isolate 6134Sr, resistant to streptomycin, was paired directly with its A1 variant 6134VCr, resistant to chloramphenicol, all single-oospore cultures from three crosses were resistant to either chloramphenicol or streptomycin but not both, indicating an apparent absence of genetic exchange in the progeny. This ruled out the possibility that absence of genetic exchange in the crosses between two different isolates of P. parasitica might be due to genetic divergence between the strains crossed. We interpret the results of this study to show that all progenies from direct pairings between different mating types of P. parasitica were of uniparental origin, although close linkage or mitochondrial location of the markers cannot as yet be eliminated as possible explanations of the apparent absence of genetic exchange. We propose the term “hormonal heterothallism” to denote this suggested novel phenomenon.