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Infrequent Transmission of Double-stranded RNA Virus Particles but Absence of DNA Proviruses in Single Ascospore Cultures of Gaeumannomyces graminis

J. J. P. McFadden

Department of Pure and Applied Biology, Imperial College of Science and Technology, London SW7 2BB
and¹ Plant Pathology Department, Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ, U.K.

A field isolate, 3b1a, of the wheat take-all fungus Gaeumannomyces graminis var. tritici, was previously shown to be infected with three serologically unrelated viruses, A, B, and C. It is now shown that virus B can be separated into two distinct strains, designated B1 and B2. All four viruses, A, B1, B2 and C, were faithfully transmitted into conidia. However, six out of eight single ascospore cultures derived from 3b1a single conidial cultures were shown to be virus-free. The remaining two single ascospore cultures each contained only one virus, which appeared to be the same in each culture. This virus was serologically indistinguishable from virus B1, but had dsRNA components of mol. wt. lower than those of virus B1 and had only the smaller of the two capsid polypeptide species of virus B1. No DNA provirus molecules homologous to viruses B1, B2 or C could be detected in two of the virus-free ascospore cultures or in one of the virus-infected ascospore cultures. Very low concentrations of virus particles were detected in hyphal tip isolates of G. graminis. After prolonged storage and subculturing of these isolates, the concentration of virus particles had increased to the level of the parent culture from which the hyphal tip isolates were derived.

Keywords: dsRNA mycoviruses, transmission, provirus absence

Present address: Department of Biochemistry, St Mary's Hospital Medical School, London W2, U.K.

Received 11 August 1982; accepted 16 November 1982.