Evolutionary stability in a 400-million-year-old heritable facultative mutualism |
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Authors: | Stephen J Mondo Kevin H Toomer Joseph B Morton Ylva Lekberg Teresa E Pawlowska |
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Institution: | Department of Plant Pathology & Plant Microbe-Biology, Cornell University, Ithaca, New York 14853-5904 E-mail: tep8@cornell.edu Division of Plant and Soil Sciences, West Virginia University, Morgantown, West Virginia 26506-6108 MPG Ranch, Missoula, Montana 59802 and Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, Montana 59812. |
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Abstract: | Many eukaryotes interact with heritable endobacteria to satisfy diverse metabolic needs. Some of these interactions are facultative symbioses, in which one partner is not essential to the other. Facultative symbioses are expected to be transitional stages along an evolutionary trajectory toward obligate relationships. We tested this evolutionary theory prediction in Ca. Glomeribacter gigasporarum, nonessential endosymbionts of arbuscular mycorrhizal fungi (Glomeromycota). We found that heritable facultative mutualisms can be both ancient and evolutionarily stable. We detected significant patterns of codivergence between the partners that we would only expect in obligate associations. Using codiverging partner pairs and the fungal fossil record, we established that the Glomeromycota-Glomeribacter symbiosis is at least 400 million years old. Despite clear signs of codivergence, we determined that the Glomeribacter endobacteria engage in recombination and host switching, which display patterns indicating that the association is not evolving toward reciprocal dependence. We postulate that low frequency of recombination in heritable endosymbionts together with host switching stabilize facultative mutualisms over extended evolutionary times. |
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Keywords: | Arbuscular mycorrhizal fungi Ca Glomeribacter gigasporarum cospeciation endosymbiosis host switching recombination |
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