Genetics or epigenetics? A peculiar case from the ciliate life

Inheritance of three mating types (MTs) in the ciliate Dileptus anser is described. When reproduced vegetatively, clones of dilepti retain their mating type invariably. In sexual generations (at crosses) the character behaves as controlled by one locus with three alleles in it, manifesting serial dominance. In other words, the character seems to be under the direct genic control. However, after treatment with Actinomycin D, ciliates from clones which stably express this or that MT, become destabilized and start to express in turn all three MTs. It suggests that actually MT of such a clone results from stable epigenetic differentiation of a complex, multipotential locus.