Mutants in paramecium tetraurelia defective in their axonemal response to calcium

Six mutants of Paramecium tetraurelia, which display altered axonemal responses to Ca⁺⁺, are described. The mutants, designated atalantas, are impaired in their ability to swim backward when stimulated by ions or heat; instead they spin very rapidly in one place. Three mutants, ataA^1–3, are completely unable to swim backward. The three lines, however, can be distinguished from one another by their forward swimming velocities. The remaining three mutants are leaky. ataB swims backward briefly when stimulated, then stops and spins in place. ataC and ataD are extremely leaky and only display the spinning phenotype at elevated temperatures. An electrophysiological analysis reveals that all six mutants have normal membrane properties, including the Ca⁺⁺ inward current under voltage clamp. When the membrane is disrupted so as to allow the axoneme free access to Ca⁺⁺, wild‐type cells swim backward, but the mutants do not. These data indicate the site(s) of lesion in the mutants is in the axoneme or in some step linking Ca⁺⁺ influx and the axoneme, not within the ciliary membrane. These mutants may be useful in investigating the role of Ca⁺⁺ in the regulation of axonemal motion.