Abstract
Incompatibility reactions of somatic cells in the myxomycete Didymium iridis are controlled by several loci displaying simple dominance. Phenotypically dissimilar plasmodia generally undergo a temporary fusion which is quickly terminated by a cytotoxic reaction, whereas phenotypically similar ones undergo fusion which is not followed by such reactions. The size of the killed areas varies from microscopic up to a few square centimeters and is directly correlated with the amount of protoplasmic mixing which occurs. The amount of mixing itself is controlled by the incompatibility loci through regulation of the speed of killing. Each locus allows a characteristic amount of mixing and when two or more loci act together the amount is reduced by a subtractive effect. This results in a smaller killed area because of the more rapid cytotoxic reaction. Such a reaction is also unidirectional for each locus, with killing taking place in the recessive phenotype. Reversion of some heterokaryons to a homokaryotic state within 24 hours is considered evidence for the existence of unidentified weak incompatibility loci which act very slowly.