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Obligate bacterial mutualists evolving from environmental bacteria in natural insect populations

Nature Microbiology volume 1, Article number: 15011 (2016) Cite this article

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Abstract

Diverse organisms are associated with obligate microbial mutualists. How such essential symbionts have originated from free-living ancestors is of evolutionary interest. Here we report that, in natural populations of the stinkbug Plautia stali, obligate bacterial mutualists are evolving from environmental bacteria. Of six distinct bacterial lineages associated with insect populations, two are uncultivable with reduced genomes, four are cultivable with non-reduced genomes, one uncultivable symbiont is fixed in temperate populations, and the other uncultivable symbiont coexists with four cultivable symbionts in subtropical populations. Symbiont elimination resulted in host mortality for all symbionts, while re-infection with any of the symbionts restored normal host growth, indicating that all the symbionts are indispensable and almost equivalent functionally. Some aseptic newborns incubated with environmental soils acquired the cultivable symbionts and normal growth was restored, identifying them as environmental Pantoea spp. Our finding uncovers an evolutionary transition from a free-living lifestyle to obligate mutualism that is currently ongoing in nature.

Symbiosis enables organismal evolution and diversification in a variety of ways1,2. Obligate insect–bacterium symbiotic associations are among the most sophisticated forms of symbiotic intimacy, where the host and the symbiont are integrated into a coherent biological entity and are unable to survive without the partnership3–5. Harboured in specialized host cells or gut-associated structures over evolutionary time, such symbionts tend to evolve a drastically reduced genome, losing many genes for independent life and streamlining for specific biological roles to support their hosts5–11. Despite their present-day extreme lifestyle specialization, such symbionts must have been derived originally from free-living ancestors residing in the environment. How have highly specialized obligate symbiotic bacteria arisen from less specialized free-living bacteria? Here, we address this evolutionary issue by reporting the discovery of currently ongoing evolution of obligate symbiotic bacteria from environmental free-living bacteria in natural insect populations.

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Figure 1: Plautia stali and its gut symbiotic bacteria.
Figure 2: Symbiont polymorphism in natural populations.
Figure 3: Effects of symbiont elimination and replacement.
Figure 4: Acquisition of symbiotic bacteria from environmental soil.
Figure 5: Effects of interspecific symbiont replacement.

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Acknowledgements

The authors thank M. Baba, N. Baba, Y. G. Baba, D. Haraguchi, H. Hirayama, M. Hironaka, S. Kada, N. Kaiwa, Y. Kikuchi, K. Kizaki, S. Kudo, T. Makino, M. Moriyama, N. Nagata, S. Ohno, T. Ohtani, M. Ono, M. Sakakibara, H. Toju, K. Tsuji, N. Tsurusaki, T. Uesato, R. Ukuda, H. Watanabe and T. Yamaguchi for insect samples; J. Makino, N. Tanifuji, U. Asaga and W. Kikuchi for technical assistance; J. P. McCutcheon for comments on the manuscript; and Y. Nakajima and Y. Kikuchi for logistic and technical support. This study was supported by a JSPS KAKENHI grant (25221107) to T.F. and by the University of the Ryukyus Foundation to T.H., and Y.I. was supported by a JSPS Fellowship for Young Scientists.

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Author notes

  1. Takahiro Hosokawa

    Present address: †Present address: Faculty of Science, Kyushu University, Fukuoka 819-0395, Japan.,

Authors and Affiliations

  1. Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 305-8566, Tsukuba, Japan

    Takahiro Hosokawa, Yoshiko Ishii & Takema Fukatsu

  2. Tropical Biosphere Research Center, University of the Ryukyus, 903-0213, Okinawa, Japan

    Takahiro Hosokawa

  3. Department of Liberal Arts, The Open University of Japan, 261-8586, Chiba, Japan

    Naruo Nikoh

  4. DNA Sequencing Section, Okinawa Institute of Science and Technology Graduate University, Onna, 904-0495, Okinawa, Japan

    Manabu Fujie

  5. Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, 904-0495, Okinawa, Japan

    Nori Satoh

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  1. Takahiro Hosokawa
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Contributions

T.H. and T.F. designed the study. T.H. performed most experiments and data analyses. Y.I. conducted some symbiont-replacing experiments. N.N. analysed the symbiont genome data. M.F. and N.S. determined the draft genome sequences of the symbionts. T.H. and T.F. wrote the paper with input from all authors.

Corresponding author

Correspondence to Takema Fukatsu.

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Supplementary Figures 1–8 and Tables 1–5. (PDF 7326 kb)

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Hosokawa, T., Ishii, Y., Nikoh, N. et al. Obligate bacterial mutualists evolving from environmental bacteria in natural insect populations. Nat Microbiol 1, 15011 (2016). https://doi.org/10.1038/nmicrobiol.2015.11

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  • DOI: https://doi.org/10.1038/nmicrobiol.2015.11

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