"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."

“Heteroecy appears to be a consequence of the seasonal polymorphism of aphids, which causes some morphs to be more evolutionarily constrained than others in their abilities to acquire hosts. Some aphid lineages have escaped this constraint by replacing the ancestral fundatrix morph and remaining all year on former secondary hosts, thus becoming secondarily autoecious. Most of the large and species-rich groups of Aphidinae on herbaceous angiosperms probably are derived from ancestors showing such life-cycle reduction.”

“Biogeographical and paleobotanical evidence suggests that the aphid subtribe Melaphidina has been associated with its sumac host plant since the early Eocene when these plants were continuously distributed across the Bering land bridge. Transfer experiments indicate that the American species, Melaphis rhois, shows an unusual complex life cycle, similar to that known in Chinese melaphidines, with some generations feeding on mosses as alternate host plants. As with the association with sumac, this complex life cycle may have been established in the melaphidine lineage before the southward retreat of sumac from Alaska 48 million years ago. This example suggests that the interactions and life histories shown by modern populations may be determined, in large part, by evolutionary commitments made in the distant past.”

“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.”

“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 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 .”

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