Inheritance of killer phenotypes and double-stranded RNA in Ustilago maydis.

Three different killer specificities in U. maydis are inherited cytoplasmically and transmitted by cell fusion. Each killer generates low frequencies of specifically immune forms in crosses with sensitive strains. The properties of immunity to each killer are also inherited cytoplasmically and transmitted by cell fusion. Killer strains carry virus-like particles about 41 nm in diameter. Each killer possesses distinct double-stranded RNA components that range in molecular weight from 0.46 X 10(6) to 2.9 X 10(6). Two components are shared by all three killers. Immune strains possess new forms. Crosses and heterokaryons between different killers revealed unilateral or mutual restrictions that prevent inclusion of two killer specificities in the same cell.