Introduction: The Holobiont Imperative
This book is being written at a time when fundamental shifts in thinking are occurring in the life sciences, but when the metaphorical ground has not yet settled under our feet. There are no germ-free animals in nature. Epithelia in contact with the envir
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Introduction: The Holobiont Imperative
This book is being written at a time when fundamental shifts in thinking are occurring in the life sciences, but when the metaphorical ground has not yet settled under our feet. There are no germ-free animals in nature. Epithelia in contact with the environment are colonized by microbial communities, and all multicellular organisms must be considered an association of the macroscopic host in synergistic interdependence with bacteria, archaea, fungi, and numerous other microbial and eukaryotic species. We refer to these associations that can be analyzed, measured, and sequenced, as “holobionts” or “metaorganisms” (Fig. 1.1). Half a century ago, Lynn Margulis (1993) popularized the idea that symbiosis has been an important factor in evolution, but much of the immediate interest was on the most obvious and significant eukaryote–eukaryote symbioses such as corals and giant clams, the only symbioses involving prokaryotes to receive significant attention being lichens and rhizobia. By contrast, there is now a growing realization of the importance and ubiquity of associations involving prokaryotes and archaea to every aspect of animal life—bacteria not only enable animals to metabolize otherwise indigestible polysaccharides such as lignin and cellulose, but also shape animal development and behavior. This scenario is also playing out within the field of “traditional” symbioses, so that whereas 20 years ago, the coral symbiosis was viewed simply as a cnidarian– dinoflagellate association, current thinking has the coral “holobiont” beside the photosynthetic algae Symbiodinium also including bacteria and also viruses. Interactions between the members of the holobiont, i.e., bacteria, eukaryotic symbionts, and host cells, have probably been critical to enabling the key transitions in animal evolution. However, the reciprocal is also true—animals have dramatically transformed the physical environment that is available for bacterial colonization. Animals also provide niches that simply do not exist elsewhere—for example, the rumen and the vertebrate gut, the light organ of the bobtail squid, or the intracellular environment of an ascidian. Animals also exercise enormous selective forces on bacterial populations—think only of the spread of multidrug-resistant (MDR)
© Springer-Verlag Wien 2016 T.C.G. Bosch, D.J. Miller, The Holobiont Imperative: Perspectives from Early Emerging Animals, DOI 10.1007/978-3-7091-1896-2_1
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Introduction: The Holobiont Imperative
Fig. 1.1 Any multicellular organism must be considered a holobiont or metaorganism, a complex community of many species which have been, and are being, evolved
strains of Staphylococcus aureus, or the evolution of bacteria capable of degrading completely novel chlorinated hydrocarbons. Since their Precambrian origins, the Metazoa have transformed the physical environment, but always in collaboration with bacteria. Along the way, some animals have also formed close relationships with other eukaryotes, but these macrosymbiotic
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