Gnotobiotic experimentation helps define symbiogenesis in vertebrate evolution

The common presence of microbial communities (microbiotas) that form life-long associations with particular body sites of vertebrates are considered to represent symbiotic partnerships. These multi-organismal associations are different than 'holobiont' partnerships defined by Margulis which have restricted diversity and feature high fidelity, intergenerational transmission of symbionts. Comparison of the nutritional and physiological properties of germfree, defined microbiota, and conventional mice help to reveal the extent of the symbiosis between the vertebrate host and microbial communities of high diversity. The mouse-microbiota association is not obligate, but gut microbiota function enables additional energy harvest from the diet. This represents a major evolutionary innovation and therefore could be considered to be an example of symbiogenesis. However, this advantage is balanced by the expenditure of energy required to maintain structures and physiology that prevent invasion of murine tissues by the microbiota. Extrapolation of observations from gnotobiotes to humans helps to develop an appreciation of the human-gut microbiota relationship with emphasis on factors that may have assisted human survival.