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Effects of Genetic Transformation on Fitness of Cochliobolus heterostrophus. N. P. Keller, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Current address: U.S. Department of Agriculture-Agricultural Research Service, Southern Regional Research Center, P.O. Box 19687, New Orleans, LA 70179; G. C. Bergstrom, and O. C. Yoder. Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 80:1166-1173. Accepted for publication 23 April 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1166.

The fitness of genetically transformed strains of the southern corn leaf blight fungus Cochliobolus heterostrophus was compared to that of the isogenic wild-type progenitor in competitive pathogenicity tests on maize plants. Fitness (an individual?s contribution to the gene pool of the next generation) was assessed by monitoring the frequency of the transformed phenotype in mixed populations with the wild type after successive disease cycles. Transformants were less fit than the wild type in 92% of the competition tests (P = 0.05). However, when the wild type was mock transformed (the transformation procedure without addition of plasmid DNA), transformants were less fit than the wild type in only 40% of the tests (P = 0.05). Thus, the transformation protocol alone accounted for some of the reduced fitness although there was evidence that this condition may be transient; an effect of the presence of plasmid DNA on fitness was not ruled out. In contrast to the effect of protocol, several factors had no apparent effect on fitness: size and copy number of the transforming plasmid, site of plasmid integration into chromosomal DNA, temperature at which plant assays were performed, proportion of initial inoculum composed of transformant conidia, and recipient fungal strain.