In the ciliate Euplotes crassus, mixed cells of different compatible mating types pass through an induction period before agglutinating with each other by means of cilia in the mating reaction. We examined the ciliary membranes of cells involved in the mating reaction by the freeze-fracture technique and detected at least 5 distinct types of specialization, each indicated by a special congregation of intramembrane particles. Near to the necklace, at the ciliary base, we observed a set of several parallel transverse rows of 10–15 nm particles; a longitudinal row of 12 nm particles appeared more distally from the necklace, preferentially in replicas of intermediate regions of the ciliary shaft. These 3 specializations were common to both mating and vegetative cells. The other 2 appeared as more dynamic specializations found exclusively at least in their patterned organization, in the ciliary membranes of mating cells. Taking on the aspects of rosette-like arrays and patches, respectively, the former were positioned regularly on ciliary membrane bulges (containing an electron-opaque granule) and consisted of 8–9 nm particles; the latter had an elliptic shape and contained up to 50 closely packed 9–10 nm particles.

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