HETEROKARYON INCOMPATIBILITY BLOCKS VIRUS TRANSFER AMONG NATURAL ISOLATES OF BLACK ASPERGILLI

The extent of heterokaryon (also termed somatic or vegetative) incompa tibility among black Aspergillus strains was examined using nitrate no n-utilising mutants selected on chlorate medium. Pairings of complemen tary mutants showed that somatic compatibility between different strai ns is exceptional in natural populations of the asexual black Aspergil li. Mycoviruses are present in a considerable fraction of the sampled natural population, but surprisingly, horizontal transfer of mycovirus es only occurs - at least under laboratory conditions - between the (v ery rare) compatible combinations of strains. Thus, unlike other funga l species, somatic incompatibility in black Aspergilli efficiently blo cks virus transfer. Viruses present in black Aspergillus isolates are highly efficiently transmitted to asexual progeny.