Abstract
AbstractOne class of caryonides in stock 210, syngen 5 of P. aurelia, designated mc S, was found to be a persistent selfer. Cells of complementary mating type (IX and X) regularly appeared in cultures of the selfer caryonides. Type IX and type X cells arose independently of each other as mating reactivity began in the cultures. While the type X cells remained type X during their period of sexual reactivity, the type IX cells in some cases changed to type X. This unidirectional change of mating type in single cells occurred within an hour. Cell division was not required for the change. Under certain conditions, the expression of type IX was suppressed and cultures of mc S caryonides contained solely type X cells.Similar unidirectional changes of mating type (from odd to even) occur in other syngens of P. aurelia. In these cases, as well as in those species of ciliates where persistent selfing occurs, variable gene activity seems the most likely explanation for the unstable mating type expression.
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