Bacteriocyte-Associated Endosymbionts of Insects
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Bacteriocyte-Associated Endosymbionts of Insects PAUL BAUMANN, NANCY A. MORAN AND LINDA BAUMANN
Introduction Intracellular associations between bacteria and insects are widespread in nature (Baumann and Moran, 1997; Buchner, 1965; Dasch et al., 1984; Douglas, 1989; Houk and Griffiths, 1980). Extensive studies of the natural history of such associations have led to the conclusion that they are commonly found in insects that utilize diets containing an excess of one class of compounds but a deficiency of some essential nutrients (Buchner, 1965; Dadd, 1985). It was thought that the function of the endosymbionts was to rectify this imbalance by the synthesis of these essential nutrients for the host. Extensive compilations of the occurrence of endosymbionts in different groups of insects are found in Buchner (1965) and Dasch et al. (1984). Because most of the prokaryotes involved in such associations are not cultivable on common laboratory media, their characterization had to await the development of recombinant DNA methodology. The past 10 years have witnessed the initiation of studies on the intracellular association of prokaryotes with a variety of insect hosts. In this chapter we will provide an overview of the evolution, and where possible, genetics and physiology of such recently studied associations. A summary of some of their features is presented in Table 1, and the phylogeny of the endosymbionts based on 16S rDNA is presented in Fig. 1. The diversity of symbiotic associations and problems of definitions have been previously discussed and will not be considered here (Smith and Douglas, 1987; Werren and O’Neill, 1997). Some of the phylogenetic studies have included few host taxa and are thus not entirely conclusive; nevertheless, current results suggest that most of the associations considered in this chapter have common features and represent a relatively well-defined type. To aid presentation we will describe these common features, which are established from recent, largely molecular, studies as well as from older investigations based on morphological analyses. References to the earlier studies are found in Buchner (1965), who arrived at similar conclusions. References to
recent studies are given as each association is considered. The associations listed in Table 1 and Fig. 1 are the results of infections of various insect lineages with different prokaryotes. These associations became stable, resulting in the emergence of a new composite (of host and endosymbiont) organism. The endosymbiont became heritable through the acquisition of mechanisms ensuring vertical, maternal transmission to progeny. The association also became obligate, or beneficial, for host growth. Because the host depended on the association, and because horizontal or infectious transmission between hosts did not occur, the phylogeny of the endosymbionts is congruent with the phylogeny of the hosts. With some exceptions, heritable associations tend to become mu
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