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Obligate bacterial mutualists evolving from environmental bacteria in natural insect populations


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.

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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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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.

Author information




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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The authors declare no competing financial interests.

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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).

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