Defensive function of trichocysts in Paramecium against the predatory ciliate Monodinium balbiani

Defensive function of trichocysts in Paramecium was recently verified against the predatory ciliate Dileptus margaritifer. Until then, however, their defensive function had been repeatedly questioned based on the observation that trichocysts are of little use against Didinium nasutum. We tested whether trichocysts can defend Paramecium against Monodinium, a predatory ciliate closely related to Didinium. Cells of trichocyst-non-discharge (tnd) mutants, artificially induced trichocyst-deficient cells, and intact wild-type cells of Paramecium were compared as prey for M. balbiani. In both P. caudatum and P. tetraurelia, mutant cells were much more vulnerable than wild-type cells (killed 12-40 times more often by the predator under a certain condition). Cells of P. tetraurelia with reduced numbers of trichocysts, obtained by treating wild-type cells with lysozyme, were almost as vulnerable as mutant cells. On the contrary, when tested against D. nasutum, wild-type and mutant cells of P. tetraurelia were equally vulnerable. We conclude that discharge of trichocysts defends Paramecium against M. balbiani but not against D. nasutum. These results strongly suggest that trichocysts in Paramecium function as defensive organelles against many toxicyst-bearing predatory ciliates with a few exceptions such as D. nasutum.