A cytoplasmically transmissible hypovirulence phenotype associated with mitochondrial DNA mutations in the chestnut blight fungus Cryphonectria parasitica.

Abstract

Mutations causing mitochondrial defects were induced in a virulent strain of the chestnut blight fungus Cryphonectria parasitica (Murr.) Barr The ascomycetous fungus Cryphonectriaparasitica is the causal agent of the blight responsible for the virtual disappearance of chestnut trees (Castanea dentata) from North American forests. Spontaneous natural control of this disease occurs in areas where the fungus is infected by hypovirulence-causing double-stranded RNA (dsRNA) viruses (1-3). However, some attenuated isolates from recovering chestnut trees exhibit cytoplasmically transmissible hypovirulence but do not contain dsRNAs (4, 5). Unlike virulent and dsRNA-containing hypovirulent strains, a high proportion of the virus-free hypovirulent isolates constitutively express high levels of cyanide (CN)-resistant, salicylhydroxamate-sensitive respiration (alternative oxidase activity) (5). Such strains resemble dsRNA-containing hypovirulent strains in that, relative to virulent strains, they grow more slowly, produce fewer conidia, and form morphologically abnormal colonies. These traits suggest that this type of hypovirulence may be affiliated with mitochondrial dysfunctions similar to those found in cytoplasmic mutants of well-characterized filamentous fungi (6, 7), particularly Neurospora, Podospora, and Aspergillus (8,9). Mitochondria also might be involved in the attenuation of Cryphonectria and other fungi by prolonged asexual propagation in the laboratory (10). However, direct evidence for the association of mtDNA mutations with the spontaneous appearance of hypovirulence in dsRNA-free strains of C. parasitica is still lacking.To explore the possible involvement of mitochondrial genes in the attenuation of virulence in C. parasitica, mutations that cause respiratory defects were induced and selected in a highlyThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Linkage between Mitochondrial Hypovirulence and Viral Hypovirulence in the Chestnut Blight Fungus Revealed by cDNA Microarray Analysis

“…Subsequent studies (14,23) revealed that this and other dsRNA-free hypovirulent C. parasitica strains collected from the same general geographic area exhibited high levels of alternative oxidase activity, a hallmark of mitochondrial dysfunction that is often associated with fungal senescence (15). Monteiro-Vitorello and colleagues (23) were able to provide direct evidence that mutation of mitochondrial DNA (mtDNA) in the laboratory could induce transmissible hypovirulence. Conidia of the highly stable C. parasitica strain EP155 were exposed to UV mutagenesis, and mutants were selected for a cyanide (CN)-resistant respiratory phenotype.…”

“…Fulbright et al (14) reported the first isolation of a dsRNA virus-free hypovirulent strain of C. parasitica recovered from a healing canker in the Kellogg Forest, Mich. Subsequent studies (14,23) revealed that this and other dsRNA-free hypovirulent C. parasitica strains collected from the same general geographic area exhibited high levels of alternative oxidase activity, a hallmark of mitochondrial dysfunction that is often associated with fungal senescence (15). Monteiro-Vitorello and colleagues (23) were able to provide direct evidence that mutation of mitochondrial DNA (mtDNA) in the laboratory could induce transmissible hypovirulence.…”

“…Monteiro-Vitorello et al (23) noted several phenotypic similarities between mitochondrial hypovirulent and viral hypovirulent C. parasitica strains. Both hypovirulent strains generally grew slower, produced fewer conidia, and formed abnormal colony morphologies relative to their corresponding virulent strains.…”

“…The EP155/mit2 mutant produced colonies with a considerable level of orange pigmentation and very irregular margins, while the colonies produced by EP155/CHV1-EP713 were white, with only slightly irregular margins. Nevertheless, both EP155/ mit2 and EP155/CHV1-EP713 have significantly reduced virulence (23). The availability of a list of genes that are differentially expressed as a result of the mit2 mutation provided an opportunity to test for possible molecular linkages between mitochondrial hypovirulence and viral hypovirulence.…”

“…The term "mitochondrial hypovirulence" refers to a cytoplasmically transmissible form of virulence attenuation (hypovirulence) in the chestnut blight fungus, Cryphonectria parasitica, that is associated with mitochondrial defects (23). The term was introduced to distinguish this form of hypovirulence from the more commonly observed form caused by doublestranded RNA (dsRNA) viruses, most notably viruses in the family Hypoviridae (hypoviruses).…”