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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 7785

John O. Corliss; Tetrahymena and some thoughts on the evolutionary origin of endoparasitism. Trans.Amer.Micros.Soc. 91(4):566-573, 1972

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Traditionally, only two possible modes of origin of obligate endoparasitism seem to have been supported. One is that endoparasites arose from ectoparasites by invasion of the latter into the tissues or digestive tract of the host. The other is that free-living forms, accidentally ingested, have quickly and successfully adapted to their new, internal environment. Proposed here is a hypothesis endorsing a third possible mode of evolutionary origin of the obligately parasitic habit, one suggesting that endoparasitism exhibited by a number of animal groups today may be the end result of a process involving facultative parasitism as a transition between a stictly free-living existence and the endoparasitic life-style. "Living" examples are cited from the conditions found among the 10 species comprising the holotrich ciliate genus Tetrahymena. Grouped by degrees of parasitism (from none to obligate endoparasitism), four species (T. setifera, T. patula, T. vorax, T. paravorax) are obligately free-living; two (T. pyriformis, T. rostrata) are facultatively parasitic; two (T. chironomi, T. corlissi) are facultatively free-living; and two (T. stegomyiae, T. limacis) are obligately parasitic. Also discussed are two additional, interrelated topics: evolution of Tetrahymena species within and among their three taxonomic complexes; and, independently, evolution and development, to some degree, of the parasitic habit in six of these species.