Stable changes of mating type after abortive conjugation in Paramecium bursaria

Abstract

A means of bringing about stable changes of mating type without nuclear reorganisation in syngen 4 of Paramecium bursaria is described. “Separating” clones have been collected, individuals of which can begin to conjugate with individuals of other mating types; but the pairs fall apart prematurely. Normal paramecia, after engaging in abortive conjugation with separating paramecia, frequently acquire the “separating” phenotype and undergo a change of mating type. A reversible swelling of the micronucleus, but no nuclear reorganisation, is observed in normal paramecia after abortive conjugation. The new phenotype acquired after abortive conjugation is stable; and newly-transformed paramecia are capable of inducing further changes of phenotype in normal paramecia. The hypothesis that transformation is the result of the transfer of an infective genetic agent from the “separating” to the normal paramecium during abortive conjugation is discussed.