The ciliate Euplotes octocarinatus and some close relatives of it are triggered by predator-released substances to undergo morphogenetic changes that inhibit their engulfment. The changes occur within a few hours and do not require cell division. They are perpetuated during reproduction so long as the concentration of the morphogen is maintained. The ability of Euplotes to respond to predator-produced signals by a defensive change in cell architecture probably provides an effective mechanism for damping population oscillations ofboth prey and predators andfosters coexistence. The signal-induced cell transformation merits study for its own sake because of its developmental implications.

No abstract

Freshwater species of the genus Euplotes (Protozoa, Ciliophora) change their morphology in the presence of some of their predators. The ciliates develop extended lateral 'wings' as well as dorsal and ventral projections which make engulfment by predators more difficult. In a series of laboratory experiments ingestion rates of four protozoan predators, the ciliates Lembadion bullinum, Dileptus anser, Stylonychia mytilus and Urostyla grandis, and one metazoan predator, the turbellarian Stenostomum sphagnetorum, on three species of Euplotes (E. octocarinatus, E. patella and E. aediculatus) were determined. It was calculated that the probability of rejection by a predator changed from 1:1 for ovoid morphs of Euplotes to about 2:1-20:1 for 'winged' morphs of Euplotes, dependent on the prey and predator species that were combined. The nutritional condition of the prey also had some influence. In mixed-species cultures of prey and predators, transformed cells of E. octocarinatus survived for several months.

The complex pattern of reactions between cells of ten different mating types of Euplotes octocarinatus, a fresh water ciliate closely related to E. patella,is reported. Mixture of cells of two different types leads in the majority of combinations to the formation of both heterotypic and homotypic pairs after a waiting period of about 3 hours. In a few combinations only homotypic pairs of one of the two mixed types are formed. Both homotypic and heterotypic pairs undergo meiosis and complete conjugation. Cross breeding experiments show that the ten mating types are determined by four codominant alleles (mt', mt2, mt3, and mt4) that control the production of four matinginducing signal-substances or gamones (GI, G2, G3, and G4). These substances are secreted into the medium and induce mature cells of certain other mating types to transform into cells that are able to unite and form conjugant pairs. We suggest that the transformation of the cells into cells ready for conjugation is triggered by the binding of gamone molecules to gamone-specific receptors and that mature cells usually expose receptors for all gamones except the ones they produce themselves.