Prokaryotic Symbionts of Ciliates

Prokaryotes living in ciliates were first noticed over a century ago by J. Muller (1856). Rod-shaped structures were observed in the macro-nuclei and micronuclei of a number of ciliates, and less commonly, in their cytoplasm. In the beginning, it was not clear whether they were parasites or spermatozoa because the micronucleus was considered to be a testis and the macronucleus an ovary, while chromosome filaments and endonuclear symbionts were mistaken for spermatozoa. This view was corrected by Butschli (1876), who also wrote the first review on parasites in ciliates (Butschli, 1889). Accounts of early observations of bacteria in protozoa that followed this initial period were reviewed by Kirby (1941), Wichterman (1953), and Ball (1969).