The evolutionary history of symbiotic associations among bacteria and their animal hosts: a model
A. Moya1,2, R. Gil1,2 and A. Latorre1,2 ` ` ` ´ 1) Institut Cavanilles de Biodiversitat i Biologia Evolutiva and Departament de Genetica, Universitat de Valencia, Valencia and 2) CIBER en Epidemiologıa y ´ Salud Publica (CIBERESP), Spain
Amodel to explain the evolutionary history of animal-bacteria obligatory mutualistic symbiosis is presented. Dispensability of genes and genetic isolation are key factors in the reduction process of these bacterial genomes. Major steps in such genome reductive evolution, leading towards primary endosimbiosis, and the possibility of complementation or replacement by a secondary symbiont are alsoindicated. Yet, we need to understand what happens at the beginning of the adaptative process towards an obligate mutualistic relationship. For this purpose, we propose to sequence the complete genome of SOPE, the primary endosymbiont of the rice weevil. Keywords: Complementation, endosymbiosis, genome reduction, replacement, SOPE (Sitophilus oryzae primary endorsement)
Clin Microbiol Infect 2009;15 (Suppl.1): 11–13 Corresponding author and reprint requests: A. Moya, Institut Cavanilles de Biodiversitat i Biologia Evolutiva and Departament de ` ` Genetica, Universitat de Valencia, Apartado Postal 22085, 46071 Val` encia, Spain E-mail: email@example.com
The establishment of symbiosis among bacteria and their animal hosts
Many bacteria maintain symbiotic associations with eukaryoticcells; this association can be mutualistic, commensal, or parasitic. In some cases, the relationship is so close that the bacteria live inside the host cell, an association called endosymbiosis. Bacterial endosymbioses are widespread among higher life forms. Insects have been particularly well studied, where the establishment of symbiotic associations with bacteria has allowed them to grow onimbalanced food resources (e.g. plant sap, cereals, or blood), poor in essential nutrients but with the nutrients being provided by the bacteria [1,2]. The symbiotic bacteria reside in specialized host cells called bacteriocytes, and are vertically transmitted from the mother to the offspring. Transition to an obligate intracellular lifestyle in bacteria triggers a cascade of changes that shape thegenome structure and content, leading to a reduction in genome size and an increase in the A + T content, among other features. Fig. 1 shows a model for the establishment and evolution of symbiotic associations. The ﬁrst step
towards the establishment of an obligate endosymbiosis occurs when a free-living bacterium infects a host. From this point, both organisms co-evolve to adapt to the newsituation. The host develops specialized cells to harbour the bacterium, which, in turn, provides essential nutrients. In this new stable situation, the bacterium suffers an evolutionary genome reductive process (see next section). Eventually, a second bacterial species might join the consortium. Although, initially, this new association might be facultative, if the second bacterium provides beneﬁtsto the association, a new stable association can appear and both bacteria will subsequently co-evolve. As the reductive process continues, and new genes are rendered unnecessary owing to redundancy, two possible outcomes can occur. Either both bacteria will be needed to keep a healthy consortium (complementation) or, depending upon which genome is affected by the loss of genes needed for thesynthesis of molecules essential for the association, one bacterium can begin an extreme degenerative process, which may end with its extinction; in this case, the retained bacterium will continue the reductive process alone (replacement).
Factors affecting the process of genome reduction
As a consequence of their access to an intracellular environment, free-living bacteria alter their genome...