The genus Sodalis as a resource for understanding the multifaceted evolution of bacterial symbiosis in insects

Insects can establish a variety of symbiotic associations with bacteria that can have a significant impact on their evolutionary ecology. Some bacterial lineages are particularly pervasive as symbiotic associates. This is the case of the Sodalis genus, whose members have established independent, maternally transmitted symbioses in diverse insect taxa. The first members of the genus were isolated and studied some thirty years ago in tsetse flies, where they evolved as heritable facultative symbionts. Since then, numerous symbiotic associations involving members of the genus have been documented, some of which have evolved into strictly host-dependent mutualistic associations. The genus also includes members circulating freely in the environment, which can be pathogenic, have extensive metabolic capabilities and constitute a potential reservoir of new insect symbionts. In this review, we cover more than thirty years of literature to highlight how the diversity of the Sodalis genus described so far embodies the different degrees of host dependence and anatomical integration that bacteria can experience over the course of their evolution with insects. We discuss the propensity of Sodalis bacteria to embrace an endosymbiotic lifestyle, how this feature can be used to understand the nascent stages of bacterial endosymbiosis, and how Sodalis bacteria can be used to address fundamental and applied research issues. Throughout the review, emphasis is placed on research gaps that need to be filled to better address these aspects. We also draw attention to previously overlook facets of the genus that deserve further investigation, such as the potential role of Sodalis bacteria in wood digestion in certain insects, or the nature of their interaction with plants.