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Nat Commun. 2017 Jul 4;8:15973. doi: 10.1038/ncomms15973.

The evolution of host-symbiont dependence.

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Department of Ecological Science, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085-1087, 1081 HV Amsterdam, The Netherlands.
Department of Zoology, University of Oxford, Oxford OX1 3PS, UK.
School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
Department of Biology, Lund University, Sölvegaten 37, 223 62 Lund, Sweden.


Organisms across the tree of life form symbiotic partnerships with microbes for metabolism, protection and resources. While some hosts evolve extreme dependence on their symbionts, others maintain facultative associations. Explaining this variation is fundamental to understanding when symbiosis can lead to new higher-level individuals, such as during the evolution of the eukaryotic cell. Here we perform phylogenetic comparative analyses on 106 unique host-bacterial symbioses to test for correlations between symbiont function, transmission mode, genome size and host dependence. We find that both transmission mode and symbiont function are correlated with host dependence, with reductions in host fitness being greatest when nutrient-provisioning, vertically transmitted symbionts are removed. We also find a negative correlation between host dependence and symbiont genome size in vertically, but not horizontally, transmitted symbionts. These results suggest that both function and population structure are important in driving irreversible dependence between hosts and symbionts.

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