A 971-bp insertion in the rns gene is associated with mitochondrial hypovirulence in a strain of Cryphonectria parasitica isolated from nature

In the chestnut-blight fungus Cryphonectria parasitica, cytoplasmically transmissible hypovirulence phenotypes frequently are elicited by double-stranded RNA (dsRNA) virus infections. However, some strains manifest cytoplasmically transmissible hypovirulence traits without containing any mycovirus. In this study, we describe an altered form of mtDNA that is associated with hypovirulence and senescence in a virus-free strain of C. parasitica, KFC9, which was obtained from nature and has an elevated level of cyanide-resistant respiration. In this strain, a 971-bp DNA element, named InC9, has been inserted into the first exon of the mitochondrial small-subunit ribosomal RNA (rns) gene. Sequence analysis indicates that InC9 is a type A1 group II intron that lacks a maturase-encoding ORF. RT-PCR analyses showed that the InC9 sequence is spliced inefficiently from the rRNA precursor. The KFC9 strain had very low amounts of mitochondrial ribosomes relative to virulent strains, thus most likely is deficient in mitochondrial protein synthesis and lacks at least some of the components of the cyanide-sensitive, cytochrome-mediated respiratory pathway. The attenuated-virulence trait and the splicing-defective intron are transferred asexually and concordantly by hyphal contact from hypovirulent donor strains to virulent recipients, confirming that InC9 causes hypovirulence.