Novel aberrant nuclear behavior in abortive conjugation of Tetrahymena thermophila: joint selection of a meiotic product and macronucleus during nuclear selection.

In conjugation of Tetrahymena thermophila, the paroral zone, cortical cytoplasm in the vicinity of the cytostome, is the site where nuclear selection occurs; one of the four meiotic products is selected in this site prior to the production of gametic pronuclei. During inbreeding cross experiments, several sterile strains were obtained which showed aberrant nuclear behavior. Conjugants of these strains normally underwent meiosis, resulting in the generation of four meiotic products. They, however, failed to complete the process of nuclear selection and aborted the subsequent conjugation sequences. During nuclear selection, macronucleus was frequently selected instead of a meiotic product. A novel aberrant nuclear behavior was observed: Macronucleus and a meiotic product were jointly selected and the both nuclei simultaneously attached to the same paroral zone. When this simultaneous attachment occurred in one partner cell of a pair, nuclear selection was never observed in the other partner cell. This result suggests that a conjugating pair has only two attachment sites on the paroral zone during nuclear selection, and that the distribution of the sites is occasionally distorted in abortive conjugation.