Prokaryotic Symbionts of Amoebae and Flagellates
Amoebae and flagellates have long been known to be associated with both extracellular and intracellular symbionts (Hall, 1969 ; Kirby, 1941a ; Lee et al., 1985 ). The presence of prokaryotic symbionts on and in flagellates and in some amoebae, as observed
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Prokaryotic Symbionts of Amoebae and Flagellates KWANG W. JEON*
Amoebae and flagellates have long been known to be associated with both extracellular and intracellular symbionts (Hall, 1969; Kirby, 1941a; Lee et al., 1985). The presence of prokaryotic symbionts on and in flagellates and in some amoebae, as observed by light microscopy, was reported by several authors during the late 1800s and the early part of this century, as was comprehensively reviewed by Kirby (1941a). Symbiontbearing flagellates were chiefly found in termite guts, and only a few free-living flagellates were found to have adhering symbionts. Hall (1969) extensively reviewed the literature on symbionts of protozoa published since 1941. Both in flagellates and amoebae, the suspected presence of some of the small bacterial symbionts had to be confirmed later by more sophisticated methods such as electron microscopy and specific staining. Amoebae and flagellates represent two very diverse groups of protozoa and it is not possible to cover all known prokaryotic symbionts in depth. In this chapter, we shall simply list known symbionts described in the above two reviews in a tabular form (Tables 1 and 2) and then consider newly found symbionts or results of recent studies on earlier symbionts in some detail. The significance of symbiotic relationships remains obscure in most cases and in only a few instances has the host-symbiont relationship been studied in detail. Some disagreements remain about whether the term “symbiosis” should be limited to associations where definite benefits have been proven to exist or not. In this chapter, we shall use the broader definition of symbiosis to include parasitism, commensalism, and mutualism, as did Kirby (1941a) and Hall (1969). Thus, the list of prokaryotic symbionts will include those whose relationships to their hosts are not known or may not be mutually beneficial. The heightened interest in cellular symbiosis in recent years has been stimulated, in part, by the notion that eukaryotic cell organelles such as
*This chapter was originally planned to appear in volume 1, but due to technical reasons it has been placed here to ensure inclusion.
mitochondria, chloroplasts, and microtubules may have originated from endosymbionts, i.e., the Serial Endosymbiosis Theory (Sagan, 1967; Margulis, 1970, 1981; Taylor, 1974; papers in Lee and Fredrick, 1987). Some authors have felt that the role of endosymbiosis in the origin of eukaryotic cell organelles has not yet been clearly established (Gray and Doolittle, 1982), but the theory is gaining wider support in view of recent results on the close relationship between the ribosomal RNAs of prokaryotes and those of chloroplasts and mitochondria (e.g., Watson et al., 1987). It should be noted that an opposing view has existed, according to which such organelles evolved as a result of autogenous intracellular differentiation without involving symbionts (Cavalier-Smith, 1975; Raff and Mahler, 1972; Uzzell and S
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