Cytoplasm Rescues an Arrhythmic Mutant on the Circadian Rhythm of Mating Reactivity in Paramecium bursaria

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Abstract
Cells of an unusual Paramecium bursaria stock (Sj2) expressed rhythmic mating reactivity in a light/dark cycle (LD) and under continuous illumination (LL). When placed in continuous darkness (DD), did not show rhythmicity but rather demonstrated a continuous high mating reactivity. However, mating reactivity was reduced following exposure to a 6‐h light pulse interrupting the DD, and then recovered to its former condition. Genetic analysis showed the arrhythmicity in DD to be a dominant character inherited in a Mendelian ratio. On the other hand, a clone (MCIw) that did not show the rhythmicity in either DD or LL was isolated from the parent stock Sj2w following a 5‐h treatment with 2 μg/ml nitrosoguanidine (MNNG). The MCIw cells expressed weak rhythmicity in LD, but were insensitive to a 6‐h light pulse in DD. The arrhythmicity in LL was inherited cytoplasmically. In addition to this, rhythmicity in LL could be recovered by injection of cytoplasm from the wild‐type cell when the recipient cell was homozygous for the wild‐type nuclear gene (+/+). The cytoplasmic components or factors are assumed to control the functional circadian system and genetically determine the rhythmicity of mating reactivity.