Genetics of Phytophthora : evidence for hybridization

Abstract

Single-zoospore cultures from two isolates (A1 and A2) of Phytophthora infestans after metalaxyl treatment consisted of A1 and A2. Metalaxyl also caused mating type change in 3 A1 of g. parasitica. The conversion of A1 to A2 by metalaxyl is postulated as a possible origin of A2 of g. infestans in Europe. When g. parasitica was treated with metalaxyl for 6 weeks, many of its zoospore progeny became resistant to metalaxyl. Exposure to metalaxyl also caused zoospore progeny to change colony morphology, growth rate, ability to produce sporangia and ability of zoospores to form colonies, indicating that metalaxyl is mutagenic. The optimum conditions for activation of oospores of g. infestans were to treat oospores with 0.25% KMn04 for 15 min. Light was required for oospore germination during germination but not during maturation. Isozyme patterns of E. infestans show that selfed progeny from isolates heterozygous at PEP and GPI-1 loci segregated 1:2:1, and that all selfed progeny from homozygous isolates were identical with their respective parents indicating that £. infestans is diploid in vegetative state. All sexual progeny from the cross 903SrX947Cpr were resistant to either chloramphenicol or streptomycin and were hybrid at PEP locus suggesting that streptomycin-resistance and chloramphenicol-resistance genes were located in cytoplasm. Selfed progeny from metalaxyl-resistant (Mr) mutants of g. parasitica segregated 3 resistant : 1 sensitive. Selfed progeny from chloroneb-resistant (Cnr) mutants also segregated 3 resistant : 1 sensitive, indicating that metalaxyl and chloroneb resistance in these mutants are each conferred by a single dominant gene in heterozygous condition. Progeny from the pairing between homozygous Mr and wild type consisted of selfs and hybrids. Progeny from the pairing between homozygous Cnr resistant to chloramphenicol and resistant to streptomycin consisted of hybrids resistant to either chloramphenicol or streptomycin, suggesting that chloramphenicol-resistance and streptomycin-resistance genes are present in cytoplasm. Progeny from the pairing between homozygous Mr, Cpr A1 and homozygous Cnr, Sr A2 consisted of 4 selfs from Al, 6 selfs from A2, 46 hybrids from the union of Al oogonium with A2 antheridium, and 92 hybrids resulting from the union of A2 oogonium with Al antheridium.