ent as more genomes are sequenced for host species with long histories of symbiosis, such as aphids (Brisson and Stern, 2006). The Carsonella genome is also extreme in its base composition (16.5% GC content) and in the rate of sequence evolution of proteins; it is remarkable that the insect hosts are dependent on an organism that appears so degenerate.

Long-term coadaptation of hosts with symbionts can enforce dependence beyond the original basis for the symbiosis. For example, aphids require Buchnera for normal embryonic development and are unable to reproduce in the absence of Buchnera even when diets are supplemented with the nutrients that Buchnera normally provides.

For an animal host, one potential consequence of acquiring a bacterial symbiont might be that it would serve as a portal for ongoing acquisition of novel genes, which is far more common in bacterial than in animal genomes. Although the bacteriome-associated nutritional symbionts provide the most extreme cases of genome stasis known in Bacteria and do not acquire novel genes (Tamas et al., 2002; Degnan et al., 2005), some heritable bacteria continue to undergo recombination, to harbor phage, and to incorporate foreign genes into their chromosomes. In many cases, these symbionts confer benefits such as protection against natural enemies (parasitoids and pathogens) (Oliver et al., 2005; Scarborough et al., 2005) or against variable abiotic conditions such as thermal stress (Russell and Moran, 2006). Although nutritional symbionts usually live in a specialized organ and are strictly required for normal host development, these bacteria are facultative for hosts and more varied in their locations within host bodies. Although they are maternally transmitted with high fidelity, they can also be transferred horizontally, sometimes through paternal transmission (Moran and Dunbar, 2006). As a result, different strains sometimes coinfect the same host individual, resulting in opportunity for recombination and transfer of phage and genes among strains (Moran et al., 2005a). In the case of the symbiont Hamiltonella defensa, which provides aphid hosts with protection against parasitoid wasps, phage-borne genes appear to contribute to defensive strategies that are observed to vary among symbiont strains (Oliver et al., 2005; Moran et al., 2005b). These symbionts use some of the same mechanisms for interacting with hosts as do mammalian pathogens, and many of these mechanisms are linked to capacity for gene uptake (Dale and Moran, 2006).

Mutualism is an obvious route for spread of heritable symbionts in a host population and has been the focus of this work. But heritable symbionts can spread among host lineages without conferring a benefit, by manipulating host reproduction to favor their own increase (Werren,