[Change of mating type induced by actinomycin D in the ciliate Dileptus anser]

Effect of actinomycin D on the expression and inheritance of mating types (MTs) in mature laboratory clones of the ciliate Dileptus anser was studied. Each mature clone of these ciliates isolated from natural population appears to belong to one of the three complementary MTs - MT I, MT II or MT III. Its MT does not vary in the course of further laboratory cultivation of the clone in a series of vegetative generations. However, after treatment with actinomycin D (15 microg/ml, 3 days), such clones became hereditarily unstable for their MTs. At weekly testing for MT over the course of at least 15 weeks after treatment (which corresponds to more than 100 cellular divisions), many subclones of the treated clone reversibly changed their MT for another, and (or) showed temporary state of immaturity or partial maturity. These data testify in favour of our hypothesis of epigenetic MT determination in D. anser. Considering that actinomycin D induces heritable destabilization of some characters in amebas Amoeba proteus which is obviously of epigenetic nature, this antibiotic can possibly be regarded as "epimutagen".