The Transmembrane Protein Semi1 Positions Gamete Nuclei for Reciprocal Fertilization in Tetrahymena

During sexual reproduction in the ciliate, Tetrahymena thermophila, cells of complementary mating type pair (“conjugate”) undergo simultaneous meiosis and fertilize each other. In both mating partners only one of the four meiotic products is “selected” to escape autophagy, and this nucleus divides mitotically to produce two pronuclei. The migrating pronucleus of one cell translocates to the mating partner and fuses with its stationary pronucleus and vice versa. Selection of the designated gametic nucleus was thought to depend on its position within the cell because it always attaches to the junction with the partner cell. Here we show that a transmembrane protein, Semi1, is crucial for attachment. Loss of Semi1 causes failure to attach and consequent infertility. However, a nucleus is selected and gives rise to pronuclei regardless of Semi1 expression, indicating that attachment of a nucleus to the junction is not a precondition for selection but follows the selection process.