The Hologenome Concept

Abstract

The hologenome theory of evolution considers the holobiont (the animal or plant with all of its associated microorganisms) as a unit of selection in evolution. The hologenome is defined as the sum of the genetic information of the host and its microbiota. The theory is based on four generalizations, each of which is supported by a large body of empirical data: (1) All animals and plants establish symbiotic relationships with microorganisms; often the genetic information of the diverse microbiota exceeds that of the host. (2) Cooperation between the host and the microbiota contributes to the fitness of the holobiont. (3) Variation in the hologenome can be brought about by changes in either the host or the microbiota genomes; under environmental stress, the symbiotic microbial community can change rapidly by a variety of mechanisms including microbial amplification, horizontal gene transfer, and acquisition of new microorganisms from the environment. (4) Symbiotic microorganisms are transmitted between generations. These points taken together suggest that the genetic wealth of diverse microbial symbionts can play an important role both in adaptation and in evolution of higher organisms. During periods of rapid change in the environment, the diverse microbial symbiont community can aid the holobiont in surviving, multiplying, and buying the time necessary for the host genome to evolve. The distinguishing feature of the hologenome theory is that it considers all of the diverse microbiota associated with the animal or the plant as part of the evolving holobiont. The hologenome theory contains Lamarckian aspects within a Darwinian framework, accentuating both cooperation and competition within the holobiont and with other holobionts.