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Genetic and Cytological Evidence for a Diploid Life Cycle in Pythium aphanidermatum. C. W. Dennett, Graduate Research Assistant, Department of Plant Pathology, University of Arizona, Tucson, AZ 85721; M. E. Stanghellini, Professor, Department of Plant Pathology, University of Arizona, Tucson, AZ 85721. Phytopathology 67:1134-1141. Accepted for publication 11 March 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1134.

A cytological and genetic examination was undertaken to determine the location of meiosis in the life cycle of Pythium aphanidermatum. The hyphal nuclei were examined in order to compare them with meiotic figures in the gametangia. A new model is suggested to explain the nonclassical fungal mitosis using the assumption of chromatin-nuclear-membrane attachment. Diplotene-diakinesis figures were photographed in both the antheridia and oogonia. A chloramphenicol-tolerant mutant was induced with ethyl methansulfonate and the oospore progeny segregated one drug-sensitive to 5.83 drug-tolerant. Zoospores from the same mutant failed to segregate, indicating that heterokaryosis was not responsible for oospore segregation. These results are most simply the product of heterozygous diploid somatic nuclei and gametangial meiosis. Both the cytological and genetic data suggest a life cycle with diploid nuclei in the mycelium.