VEGETATIVE COMPATIBILITY BETWEEN AND WITHIN EPICHLOE SPECIES

Epichloe species (Clavicipitaceae, Ascomy- cota) are biotrophic symbio nts of cool-season grasses and grow slowly on defined media. Nine Epic hloe biological species (mating populations) have been identified. Veg etative compatibility within or between Epichloe species has not yet b een investigated. We selected chlorate-resistant strains that were nit rate non-utilizing (nit) mutants from ten isolates representing four b iological species. Each mutant grew appressed on a defined medium with nitrate as the sole nitrogen source, in contrast to the abundant aeri al mycelia produced by the prototrophic wild type isolates on the same medium. Each mutant was putatively identified as nit1 (nitrate reduct ase structural gene mutant), nit3 (mutant in the pathway-specific regu latory locus or nitrite reductase gene), or NitM (mutant affecting mol ybdenum cofactor production) by growth phenotypes on medium with nitri te, hypoxanthine, uric acid or ammonium as the sole nitrogen source. W hen appropriate nit mutants were paired, heterokaryon formation and co mplementation were indicated by a zone of prototrophic growth. Complem entation occurred between complementary mutants of most isolates, even from different species; however, complementation did not occur in all replications. Interspecific heterokaryon formation was confirmed by r ecovery of parental phenotypes and cosegregation of beta-tubulin gene polymorphisms among single-conidiospore isolates from the complementat ion zone. There was no indication that mating type interactions affect ed heterokaryon formation, and no discrete vegetative compatibility gr oups were identified.