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… - 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.

The genomes of long-term obligate symbionts often undergo irreversible gene loss and deterioration even as hosts evolve dependence on them. In some cases, animal genomes may have acquired genes from symbionts, mirroring the gene uptake from mitochondrial and plastid genomes. Multiple symbionts often coexist in the same host, resulting in coadaptation among several phylogenetically distant genomes.

Life cycles that incorporate discrete, morphologically distnct phases predominate among animals.