Abstract
Epulopiscium sp. type B (Lachnospiraceae) is an exceptionally large, highly polyploid, intestinal symbiont of the coral reef dwelling surgeonfish Naso tonganus. These obligate anaerobes do not form mature endospores and reproduce solely through the production of multiple intracellular offspring. This likely makes them dependent on immediate transfer to a receptive host for dispersal. During reproduction, only a small proportion of Epulopiscium mother-cell DNA is inherited. To explore the impact of this unusual viviparous lifestyle on symbiont population dynamics, we investigated Epulopiscium sp. type B and their fish hosts collected over the course of two decades, at island and reef habitats near Lizard Island, Australia. Using multi-locus sequence analysis, we found that recombination plays an important role in maintaining diversity of these symbionts and yet populations exhibit linkage disequilibrium (LD). Symbiont populations showed spatial but not temporal partitioning. Surgeonfish are long-lived and capable of traveling long distances, yet the population structures of Epulopiscium suggest that adult fish tend to not roam beyond a limited locale. Codiversification analyses and traits of this partnership suggest that while symbionts are obligately dependent on their host, the host has a facultative association with Epulopiscium. We suggest that congression of unlinked markers contributes to LD estimates in this and other recombinant populations of bacteria. The findings here inform our understanding of evolutionary processes within intestinal Lachnospiraceae populations.
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Acknowledgements
We thank Rick Harrison and Lynn Johnson for their advice about study design and statistical analysis. We also thank Amy Dapper, Xavier Didelot, and Melanie Smee for advice on our population analyses. We thank Zanah Francis for help screening samples and David Sannino for insightful discussions. We are grateful to Daniel Buckley for the use of his server to run analyses. Fish were collected under the James Cook University Ethics approvals A503 and A1641 and with approval of the Great Barrier Reef Marine Park Authority permits G01-356 and G10-33239.1. This study was supported by National Science Foundation grants MCB1244378 and IOS1354911.
Data availability
The sequences obtained in this study have been deposited in GenBank under Accession No. MH259595-MH259695 (16S rRNA genes), MH268406-MH269196 (Epulopiscium sp. type B MLSA genes), and MH282951-MH283005 (N. tonganus MLSA genes).
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Arroyo, F.A., Pawlowska, T.E., Choat, J.H. et al. Recombination contributes to population diversification in the polyploid intestinal symbiont Epulopiscium sp. type B. ISME J 13, 1084–1097 (2019). https://doi.org/10.1038/s41396-018-0339-y
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DOI: https://doi.org/10.1038/s41396-018-0339-y
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