Content area
Full text
About the Authors:
Jonathan G. Lundgren
* E-mail: [email protected]
Affiliation: North Central Agricultural Research Laboratory, United States Department of Agriculture–Agricultural Research Service, Brookings, South Dakota, United States of America
R. Michael Lehman
Affiliation: North Central Agricultural Research Laboratory, United States Department of Agriculture–Agricultural Research Service, Brookings, South Dakota, United States of America
Introduction
Microbes affect the phenotypes of their symbiotic hosts in myriad ways, especially the host's ability to rely nutritionally on certain foods. Nutritional symbioses between microorganisms and animals evolve when a major component of the animal's diet lacks sufficient quantities of specific nutrients, or when nutrients present in the diet are inaccessible because the animal lacks the requisite metabolic tools to fully digest their food [1], [2], [3], [4], [5]. Most research on nutritional symbioses has focused on how obligate relationships between microbes and their animal hosts evolve and are maintained [4], [6], [7]. Less understood are the functions of more transient or facultative bacterial communities that invariably reside within animal guts, which could contribute to the diet diversification of the host [1], [2], [3], [8].
Microbial-based nutritional symbioses are particularly well studied in insects with highly restricted diets of limited nutrition (e.g., blood, plant sap, wood, etc.) [4]. In these systems, bacteria or fungi help in nitrogen processing, sulfate assimilation, fatty acid metabolism, and help to contribute deficient sterols, vitamins (especially B-vitamin groups), digestive enzymes and essential amino acids to their insect hosts [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Insects that feed on high quality foods (i.e., predators) or that display dietary plasticity (i.e., omnivores) were once believed to rely less on microbial symbionts, because these insects are able to self-select nutritionally optimal diets from their environment [20]. But even those insects that ordinarily consume diets of high nutritional quality often must ingest foods of marginal quality, either because high quality foods are temporally or spatially scarce or because “low quality foods” are superior in certain nutrients. The result of this is that most insects are best described as omnivores [5], [21], [22], and they must confront the physiological and structural hurdles associated with occasionally consuming certain suboptimal foods to attain a balanced diet [2], [3], [8]. Microbial symbioses are known to play a role in...