Demonstration of a cell-free factor involved in cell interactions during mating in Tetrahymena

Abstract

CELL interactions play a major part in the development of a metazoan organism. An ideal system for studying cell interactions is one in which two and only two homogenous cell populations are involved. This condition is met in unicellular systems in which cells of two differing mating types may be induced to enter the sexual phase of their life cycle and conjugate. Conjugation in Tetrahymena is characterised by an association of cells of complementary mating types into pairs joined at their anterior tips. Pairing does not begin until ∼1 h after mixing starved cells of complementary mating types (Fig. 1). It has recently been shown that a cell interaction called costimulation occupies this 1-h period; costimulation is required for and distinct from cell pairing^1,2. A block at the level of costimulation necessarily blocks pairing. Therefore, the absence of pair formation may arise from conditions which affect costimulation, or pairing, or both. We demonstrate here, for the first time, a requirement for an extracellular factor during the costimulation interaction and show that the factor behaves neither like a gamone nor like an agglutinin. Its properties suggest a role in mediating a direct interaction between cells, and involving physical proximity or contact between participating cells.