Dynamics of asexual transmission of a mitochondrial plasmid in Cryphonectria parasitica

In the chestnut blight fungus Cryphonectria parasitica, as in most fungi, l ittle is known about the efficiency of the asexual transmission of optional mitochondrial plasmids, vertically through conidia, and horizontally throu gh hyphal anastomoses. In this paper, we show that pCRY1, a circular mitoch ondrial plasmid, is transmitted vertically with 100%-efficiency through con idia. Moreover, the plasmid is transmitted horizontally through hyphal cont act from donor strains to vegetatively compatible and most incompatible str ains. An allelic difference between the donor and recipient strain, at only one of the five nuclear incompatibility genes that were tested strongly in hibited, but did not absolutely prevent, the transfer of pCRY1 through hyph al fusions. In contrast, allelic differences in any one or several of the o ther four heterokaryon-compatibility loci suppressed the transmission of th e plasmid only partially or not at all. The plasmid was also transmitted am ong incompatible strains by protoplast fusion without the concomitant trans fer of mitochondrial DNA (mtDNA). A comparison of plasmid-bearing with plas mid-free isogenic strains revealed that pCRY1 significantly diminishes the pathogenic potency of some strains of the fungus, but does not affect the v irulence of others. Collectively, the observations indicate that the introd uction of deleterious mitochondrial genetic elements into natural populatio ns may be a means for managing fungal pathogens.