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Genetics

Resistance to Fungicides and Antibiotics in Phytophthora parasitica: Genetic Nature and Use in Hybrid Determination. T. T. Chang, Graduate research assistant, Department of Plant Pathology, Beaumont Agricultural Research Center, University of Hawaii, Hilo 96720; W. H. Ko, Professor, Department of Plant Pathology, Beaumont Agricultural Research Center, University of Hawaii, Hilo 96720. Phytopathology 80:1414-1421. Accepted for publication 13 July 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1414.

Progeny derived from oospores produced through hormonal stimulation by each of four metalaxyl-resistant (M^r) mutants of Phytophthora parasitica segregated in a ratio of 3 resistant: 1 sensitive. Each of two chloroneb-resistant (Cn^r) mutants also gave rise to selfed progeny with a segregation ratio of 3 resistant: 1 sensitive. These results indicate that metalaxyl resistance and chloroneb resistance in these mutants are each conferred by a single dominant gene in the heterozygous condition. Progeny from the pairing between M^r A¹ carrying homozygous dominant resistance alleles and A² wild type consisted of selfs from each parent and hybrids carrying heterozygous dominant resistance alleles. Similar results were obtained when A¹ wild type was paired with M^r A² carrying homozygous dominant resistance alleles. Progeny from the pairing between homozygous Cn^r A¹ resistant to chloramphenicol and A² resistant to streptomycin consisted of hybrids resistant to either chloramphenicol or streptomycin, but not both. Similar results were obtained when A¹ resistant to chloramphenicol was paired with homozygous Cn^r A² resistant to streptomycin. These results suggest that chloramphenicol-resistance and streptomycin-resistance genes are present in the cytoplasm. Progeny from the pairing between homozygous M^r A¹ resistant to chloramphenicol and homozygous Cn^r A² resistant to streptomycin consisted of 4 selfs from A¹, 6 selfs from A², 46 hybrids resulting from the union of an A¹ oogonium with an A² antheridium, and 92 hybrids resulting from the union of an A² oogonium with an A¹ antheridium.