stability among known genomes, lacking chromosomal rearrangements and, most significantly from the point of view of host biology, lacking any genes newly acquired from exogenous sources. The only substantial source of divergence is rapid sequence evolution affecting ancestral genes and elimination or inactivation of genes in individual lineages. The latter is of particular interest because such losses are irreversible given the lack of gene uptake; they represent loss of functions that cannot be reinstated. In several cases, eliminated genes are ones that affect host nutrition, such as those underlying pathways for sulfur fixation, arginine biosynthesis, and tryptophan biosynthesis (Tamas et al., 2002; van Ham et al., 2003; Perez-Brocal et al., 2006). The hosts must obtain these compounds from enriched diets available from certain plant species, from manipulation of plant phloem chemistry, or from additional symbionts that have been acquired subsequent to the acquisition of Buchnera >100 million years ago, as hypothesized for the smallest Buchnera genome sequenced to date (Perez-Brocal et al., 2006).
This genome degradation is not dependent on being intracellular, but rather it reflects long history of obligate host dependence and lack of recombination among strains, enforced by strict maternal transmission. The importance of population genetic structure rather than cellular location is confirmed by the observation that the symbiont, Ishikawaella capsulata, of plataspid stinkbugs (Hemiptera) shows reduced genome size and rapid protein evolution despite its location in the gut lumen rather than within cytoplasm of specialized cells (Hosokawa et al., 2006). Ishikawaella transmission, which occurs when progeny ingest an inoculum deposited on eggs by the mother, is strictly maternal, resulting in single infections and consequent lack of recombination among lineages. As for intracellular symbionts of other insect groups, Ishikawaella shows long-term parallel evolution with hosts, indicating an ancient origin (Hosokawa et al., 2006). These features of transmission enforce asexuality and small population size, as for intracellular symbionts such as Buchnera of aphids, and Ishikawaella shows similar patterns of gene and genome evolution.
The most extreme known case of degradation of a symbiont genome (other than those of organelles) occurs in Carsonella ruddii, the obligate symbiont of psyllids (a sap-feeding insect group related to aphids and whiteflies) (Baumann, 2005). This 160-kb genome contains only 182 protein-coding genes, a number considerably smaller than the proposed minimum gene number for cellular life, based on those required for essential metabolic and informational processes (Nakabachi et al., 2006). One of the most plausible explanations of how this symbiont functions with so few genes is that some genes have been stably transferred to the host genome, with their products reimported to the symbiont cellular compartment. The extent of gene transfer from symbionts to animal hosts will become appar-