The aphid Pemphigus betae typically shows a complex life cycle, with annual alternation between cottonwood trees, where it forms leaf galls, and herb… - Nancy A. Moran

Symbioses are central in the evolution of complexity; have evolved many times and are critical to the lifestyles of many animals and plants and also to whole ecosystems, in which symbiotic organisms are key players. The primary reason that symbiosis research is suddenly active, after decades at the margins of mainstream biology, is that DNA technology and genomics give us enormous new ability to discover symbiont diversity, and more significantly, to reveal how microbial metabolic capabilities contribute to the functioning of hosts and biological communities.

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).

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.