Narnaviruses: novel players in fungal–bacterial symbioses

Abstract

Rhizopus microsporus is an early-diverging fungal species with importance in ecology, agriculture, food production, and public health. Pathogenic strains of R. microsporus harbor an intracellular bacterial symbiont, Mycetohabitans (formerly named Burkholderia). This vertically transmitted bacterial symbiont is responsible for the production of toxins crucial to the pathogenicity of Rhizopus and remarkably also for fungal reproduction. Here we show that R. microsporus can live not only in symbiosis with bacteria but also with two viral members of the genus Narnavirus. Our experiments revealed that both viruses replicated similarly in the growth conditions we tested. Viral copies were affected by the developmental stage of the fungus, the substrate, and the presence or absence of Mycetohabitans. Absolute quantification of narnaviruses in isolated asexual sporangiospores and sexual zygospores indicates their vertical transmission. By curing R. microsporus of its viral and bacterial symbionts and reinfecting bacteria to reestablish symbiosis, we demonstrate that these viruses affect fungal biology. Narnaviruses decrease asexual reproduction, but together with Mycetohabitans, are required for sexual reproductive success. This fungal–bacterial-viral system represents an outstanding model to investigate three-way microbial symbioses and their evolution.

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Fig. 1: Discovery of narnaviruses in Rhizopus microsporus.
Fig. 2: Quantification of RmNV-20S and RmNV-23S along development and reproduction of R. microsporus.
Fig. 3: Generation and characterization of narnaviruses-free (nv−), bacteria-free (b−), and bacteria-reinfected (b*) fungal strains.
Fig. 4: Effect of narnaviruses in the asexual and sexual reproduction of R. microsporus.

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Acknowledgements

Authors acknowledge María Nélida Vázquez Sánchez for technical support; Luis Delaye for advice on the phylogenetic analyses, as well as Karin Grothe, Robert Winkler, and the three anonymous referees for their useful comments. LPPM is thankful to Consejo Nacional de Ciencia y Tecnología (CONACyT) in Mexico, which financed most of this research with grant FOINS-2015-01-006.

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LPPM, ANEV, JRBB, and CAG: designed and planned research; ANEV, JRBB, JFCR, GCL, FMC, AMV, DACP, LPPM: performed experiments and analyzed the data; SJM and TEP generated RNA-Seq libraries and provided R. microsporus wild-type strains; LPPM secured funding; LPPM and ANEV wrote the paper; all authors read, improved, and approved the final version of this document.

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Correspondence to Laila P. Partida-Martínez.

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Espino-Vázquez, A.N., Bermúdez-Barrientos, J.R., Cabrera-Rangel, J.F. et al. Narnaviruses: novel players in fungal–bacterial symbioses. ISME J 14, 1743–1754 (2020). https://doi.org/10.1038/s41396-020-0638-y

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