Dominance at the Tox1 locus controlling T-toxin production by Cochliobolus heterostrophus

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Heterokaryons of Cochliobolus heterostrophus were constructed which contained approximately equal numbers of nuclei bearing either the tox1+ allele for presence of T-toxin or the tox1− allele for absence of T-toxin in culture filtrates. The heterokaryons were grown in a liquid medium known to support production of T-toxin; the resulting culture filtrates were assayed for T-toxin activity using 2 different bioassays: seedling root growth and dark CO2 fixation. Both assay procedures detected the presence of T-toxin in culture filtrates of tox1+tox1− heterokaryons; levels of activity were slightly less than those found in filtrates from tox1+tox1+ heterokaryons. Since these heterokaryons are known to function as diploids, the simplest interpretation of the results is that the tox1+ allele, which determines high virulence as well as T-toxin production, is either dominant or semidominant.