Closely related microorganisms often cooperate, but the prevalence and stability of cooperation between different genotypes remain debatable. Here, we track the evolution of pellicle biofilms formed through genetic division of labour and ask whether partially deficient partners can evolve autonomy. Pellicles of Bacillus subtilis rely on an extracellular matrix composed of exopolysaccharide (EPS) and the fibre protein TasA. In monocultures, ∆eps and ∆tasA mutants fail to form pellicles, but, facilitated by cooperation, they succeed in co-culture. Interestingly, cooperation collapses on an evolutionary timescale and ∆tasA gradually outcompetes its partner ∆eps. Pellicle formation can evolve independently from division of labour in ∆eps and ∆tasA monocultures, by selection acting on the residual matrix component, TasA or EPS, respectively. Using a set of interdisciplinary tools, we unravel that the TasA producer (∆eps) evolves via an unconventional but reproducible substitution in TasA that modulates the biochemical properties of the protein. Conversely, the EPS producer (ΔtasA) undergoes genetically variable adaptations, all leading to enhanced EPS secretion and biofilms with different biomechanical properties. Finally, we revisit the collapse of division of labour between Δeps and ΔtasA in light of a strong frequency versus exploitability trade-off that manifested in the solitarily evolving partners. We propose that such trade-off differences may represent an additional barrier to evolution of division of labour between genetically distinct microorganisms.
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All data sets generated and analysed during this study are available from the corresponding author on request.
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We thank S. West for his comments on our manuscript. This work was funded by the Deutsche Forschungsgemeinschaft (DFG) to Á.T.K. (KO4741/2.1) within the Priority Program SPP1617. A.D. and C.F.G. were supported by fellowships from the Alexander von Humboldt Foundation and Consejo Nacional de Ciencia y Tecnología (CONACyT), respectively. The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 713683 (COFUNDfellowsDTU). This work was also supported by the DFG through project B11 in the framework of SFB863 granted to O.L., and a start-up grant from the Technical University of Denmark to Á.T.K.
The authors declare no competing interests.
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Supplementary Tables 1–5, Supplementary References, Supplementary Figures 1–11, Supplementary Results.
List of mutations detected in evolved single isolates and populations.
Effects of cysteine-containing TasA on wetting behaviour of B. subtilis pellicles.
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Dragoš, A., Martin, M., Falcón García , C. et al. Collapse of genetic division of labour and evolution of autonomy in pellicle biofilms. Nat Microbiol 3, 1451–1460 (2018). https://doi.org/10.1038/s41564-018-0263-y
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