Inherited symbionts have been traditionally classified into those required for host function (primary symbionts), and those where the host is competent to survive and reproduce without them (secondary symbionts). Primary symbionts are generally defined as being present in all individuals, combined with a strongly deleterious impact following treatment of the host with antibiotics.
Primary symbionts exist in a wide variety of taxa, but are particularly important in host species that live on nutritionally depauperate diets: phloem or xylem feeders; those feeding on wood; and those feeding throughout their life on vertebrate blood (Moran and Baumann, 2000). Within these taxa, the symbionts variously synthesize essential amino acids where the host diet lacks particular elements, and they are thought to be involved with the supply of B vitamins, and also in nitrogen metabolism. They are often co-adapted into host physiology, being present in large numbers in a special organ within the host (bacteriome for bacteria; mycetome for yeasts). They also commonly have very long associations with particular groups. Concordance of the phylogeny of aphid host and Buchnera sym-biont (co-cladogenesis) indicates that Buchnera has passed vertically through aphid lineages for 200 million years (Moran et al., 1993).
Secondary symbionts, defined by their non-essential nature, are of more recent ancestry: they are rarely shared by pairs of related species. In contrast to primary symbionts, they all show at least occasional movement between host species. They vary in the quantity of intraspecific horizontal transmission. An appreciable number show significant levels of intraspecific horizontal transmission in addition to their maternal inheritance. Horizontal transmission may occur, either directly—for example, following sexual contact (Moran and Dunbar, 2006) or through honeydew (Darby and Douglas, 2003)—or indirectly through a secondary host such as a plant or vertebrate. Secondary symbionts may be found in a bacteriome if one is present for housing primary symbionts, but more commonly are found diffusely among tissues (Cheng and Aksoy, 1999; Moran et al, 2005b).
These symbionts may be maintained by one or more of four factors. First, they may have sufficient horizontal (infectious) transmission to balance any inefficiency in vertical transmission. This transmission may be either direct (such as sexual transmission), or through an intermediary host such as a mammal (through blood feeding) or plant (through phloem feeding). Second, they may provide a direct fitness benefit to the host. This is covered extensively below. Third, they may manipulate host sex ratio towards the production of infected daughters: sex-ratio distorters. Finally, they reduce the fitness of uninfected females through making them incompatible with infected males (cytoplas-mic incompatibility). These latter two classes are termed reproductive parasitisms, as they involve the manipulation of host reproduction by a maternally inherited symbiont that has no interest in the production or fitness of male hosts. A brief summary of the distribution of reproductive parasitic phenotypes for various inherited bacteria is given in Table 8.1.
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