The histophagous ciliate Tetrahymena rostrata was found as a parasite in the renal organs of the land snails Zonitoides nitidus and Cochlicopa lubrica. A starvation medium induced encystment, meiosis, autogamy, and development of new macronuclei. The cell division rate declined linearly with number of divisions from the last autogamy until senescence. The senescing strains were rejuvenated by further encystment-induced autogamy. It is expected, that these processes contribute to genetic variability among the local, small, and isolated T. rostrata populations. Consistent with this expectation, small divergences in the cox-1 sequences appeared even among these strains, which had been isolated from different specimens of the same host species at the same site. The divergences in this gene between our T. rostrata strains from Z. nitidus and other strains from C. lubrica, Helix aspersa, and Deroceras reticulatum in Spain (Segade et al. 2009, Parasitology 136:771-782), were beyond the limits of intra-species variability within the genus Tetrahymena. However there is the lack of inter-strain differences in the life history and cytology among our, the Spanish, and those T.rostrata strains, that are not available for "barcoding" anymore. Therefore, variability in the life history and morphology within T .rostrata is constrained by natural selection.
Keywords: Tetrahymena rostrata; encystment; rejuvenation by autogamy.; senescence.
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