VIEW ARTICLE

Genetics

Evidence for Heterokaryosis in Phytophthora megasperma var. sojae. Margaret Long, Staff Research Associate, Department of Plant Pathology, University of California, Riverside CA 92502; N. T. Keen, Associate Professor, Department of Plant Pathology, University of California, Riverside CA 92502. Phytopathology 67:670-674. Accepted for publication 17 November 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-670.

Heterokaryosis in Phytophthora megasperma var. sojae has been proven by the synthesis and degradation of heterokaryons from various auxotrophic and drug-resistant mutants. Three auxotrophic mutants (his, thr, and lys = histidine-, threonine-, and lysine-requiring mutants, respectively) were obtained from wild-type strains of race 1, and two (met and pur = methionine- and purine-requiring mutants, respectively) were obtained from race 3. Fluorophenylalanine-resistant mutants were recovered from races 1, 2, and 3 and cycloheximide-resistant mutants from races 1 and 3. All drug-resistant mutants had pathogenicity, growth rates, and sporulation that were indistinguishable from the corresponding wild types. However, none of the auxotrophic mutants produced normal oospores; only two (his and met) were pathogenic on soybeans, and only met produced normal zoospores. A heterokaryon synthesized from a drug-resistant mutant and its corresponding wild type behaved normally. However, although several heterokaryons were synthesized from auxotrophs, only met + pur heterokaryons were pathogenic and had growth rates and sporulation properties similar to the wild types.