Symbiotic conversations are revealed under genetic interrogation

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Abstract
Microbial symbiosis is a central factor in the evolution of all animal species and in the survival of all individuals. Advances in molecular genetics are allowing the development of invertebrate–bacteria associations as model systems for understanding how the host and microorganism communicate. Five natural systems have been particularly well described, including three that are binary (the host and one bacterial species) and two that consist of simple bacterial consortia of fewer than a dozen species. The development of these experimental symbiosis models has allowed us to ask distinct sets of questions about host–microorganism communication and has provided a breadth of opportunities that would not be available from any one system. In the past 20 years, advances in our understanding of host–microorganism communication have centred on several common themes: surface structures and specificity; bacterial behaviour and gene regulation; adaptation to host defences; induction of host development; and nutritional and metabolic accommodation. Although parallels can be drawn between the systems, there are also intriguing differences that reflect the special biology of each host–microorganism association. A number of newly recognized symbioses between bacteria and invertebrate hosts are emerging as model systems in which molecular genetic approaches are only now being applied.