A CYTOPLASMICALLY TRANSMISSIBLE HYPOVIRULENCE PHENOTYPE ASSOCIATED WITH MITOCHONDRIAL-DNA MUTATIONS IN THE CHESTNUT BLIGHT FUNGUS CRYPHONECTRIA-PARASITICA

Mutations causing mitochondrial defects were induced in a virulent str ain of the chestnut blight fungus Cryphonectria parasitica (Murr.) Bar r. Virulence on apples and chestnut trees was reduced in four of six e xtensively characterized mutants. Relative to the virulent progenitor, the attenuated mutants had reduced growth rates, abnormal colony morp hologies, and few asexual spores, and they resembled virus-infected st rains. The respiratory defects and attenuated virulence phenotypes (hy povirulence) were transmitted from two mutants to a virulent strain by hyphal;contact. The infectious transmission of hypovirulence occurred independently of the transfer of nuclei, did not involve a virus, and dynamically reflects fungal diseases caused by mitochondrial mutation s. In these mutants, mitochondrial mutations are further implicated in generation of the attenuated state by (i) uniparental (maternal) inhe ritance of the trait, (ii) presence of high levels of cyanide-insensit ive mitochondrial alternative oxidase activity, (iii) cytochrome defic iencies, and (iv) structural abnormalities in the mtDNA. Hence, cytopl asmically transmissible hypovirulence phenotypes found in virus-free s trains of C. parasitica from mutant forms of mtDNA.