Development in electrofused conjugants of Tetrahymena thermophila

Electric shock can create parabiotic fusions of living Tetrahymena cells. I n this study, cells were mated and successful pairs were electrofused with either vegetatively growing cells or other mating pairs. In particular, we electrofused pairs from normal [diploid X diploid] matings with vegetativel y dividing cells in G- or M-phase of the cell cycle. We also fused [diploid x diploid] conjugants with mating pairs involving an aneuploid partner [di ploid x "star"], which typically undergo an abortive conjugal pathway terme d genomic exclusion. Using such parabiotic fusions we identified and charac terized two developmentally critical landmarks: I) the "abort" signal, whic h is initiated in pairs with nuclear defects (this first becomes evident so on after the completion of Meiosis I or the beginning of Meiosis II): and 2 ) the "terminal commitment point", a developmental stage in normal [diploid x diploid] pairs after which conjugation no longer responds to a parabioti cally transmitted abort signal (this correlates with the onset of the secon d postzygotic nuclear division). Finally we demonstrate that a conjugal-arr est-activity varies with the vegetative cell cycle, reaching its highest le vel of activity during M-phase and dropping just after cytokinesis.