EVOLUTIONARY STABILITY IN A 400-MILLION-YEAR-OLD HERITABLE FACULTATIVE MUTUALISM
Article first published online: 27 MAR 2012
© 2012 The Author(s). Evolution© 2012 The Society for the Study of Evolution.
Volume 66, Issue 8, pages 2564–2576, August 2012
How to Cite
Mondo, S. J., Toomer, K. H., Morton, J. B., Lekberg, Y. and Pawlowska, T. E. (2012), EVOLUTIONARY STABILITY IN A 400-MILLION-YEAR-OLD HERITABLE FACULTATIVE MUTUALISM. Evolution, 66: 2564–2576. doi: 10.1111/j.1558-5646.2012.01611.x
- Issue published online: 26 JUL 2012
- Article first published online: 27 MAR 2012
- Accepted manuscript online: 2 MAR 2012 02:37PM EST
- Received September 27, 2011 , Accepted February 10, 2012
- Arbuscular mycorrhizal fungi;
- Ca. Glomeribacter gigasporarum;
- host switching;
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.