The genomes of long-term obligate symbionts often undergo irreversible gene loss and deterioration even as hosts evolve dependence on them. In some c… - Nancy A. Moran

Buchnera only have 600 genes, compared to about 4,000 or 5,000 for E. coli ... This is a recurring pattern in the genomes of both bacterial symbionts and pathogens, but why do they get so small? ... while part of the reduction is due to adaptation, a lot of it just reflects genetic drift ... It's just a consequence of long-term evolution in a restricted environment with small population sizes.

Among the many early revelations from molecular phylogenetic studies of bacteria (Woese, 1987) was the recognition that the mycoplasmas represented an evolutionarily derived condition rather than a primitive one, as once believed. Now that phylogenetic relationships and genome sizes are determined for a broader array of organisms, it is clear that the mycoplasmas are just one example of genome shrinkage that has occurred in a variety of obligately host-associated bacteria. Other prominent examples are Rickettsia and related pathogens within the α-proteobacteria; insect symbionts within the γ-proteobacteria, as exemplified by Buchnera aphidicola in aphids; the chlamydiae; and the parasitic spirochetes, such as Borrelia burgdorferi (the agent of Lyme disease).

The guts of honey bee workers contain a distinctive community of bacterial species. They are microaerophilic or anaerobic, and were not clearly deliniated by earlier studies relying on laboratory culture of isolates under atmospheric oxygen levels. Recently, a more complete picture of the potential metabolism and functions of these bacteria have been possible, using genomic approaches based on metagenomic samples, as well as cultured isolates. Of these, most are host-restricted and are generally absent outside adult guts.