Hiding in a crowd—does diversity facilitate persistence of a low-quality fungal partner in the mycorrhizal symbiosis? Miranda M. Hart & Jennifer Forsythe & Brian Oshowski & Heike Bücking & Jan Jansa & E. Toby Kiers Received: 1 October 2012 / Accepted: 25 October 2012 / Published online: 10 November 2012 # Springer Science+Business Media Dordrecht 2012 Abstract Given that arbuscular mycorrhizal (AM) fungi are not consistently beneficial to their host plants, it is difficult to explain the evolutionary persistence of this relationship. We tested the hypothesis that increasing either fungal or host biodiversity allows an AM fungus to persist on a host where it shows little benefit. We found that growing such a fungus (an isolate of Glomus custos associating with Plantago laceolata) in combination with certain fungi improved its success as measured by mtLSU DNA abundance. Increasing plant species richness facilitated the spread of this fungus as measured by spore density and fungal colonization; the role of host species richness was not as clear when looking at measures of root abundance. These results indicate that diversity in the AM symbiosis, both plant and fungal, can promote the persistence of low-quality fungi. By existing within a complex mycelial network fungal strains that show little growth benefit to their hosts have a better chance of persisting on that same host. This has the potential to pro- mote selection for heterogeneous AM fungal communities on a small spatial scale. Keywords Mutualism . Host detection . Cheaters . Arbuscular mycorrhizal fungi . Biodiversity . Common mycelial network 1 Introduction The relationship between arbuscular mycorrhizal (AM) fungi of the phylum Glomeromycota and their plant hosts is consid- ered as a classic example of a reciprocally beneficial mutual- ism; both partners benefit from the symbiosis, with plants providing carbohydrates to their fungal partners and fungi providing mineral nutrients, such as nitrogen (N) and phos- phorus (P) to their host plants. However, the benefits do not always outweigh the costs of the symbiosis; and not all AM fungi are equally mutualistic under all conditions (Johnson 1993). Many confer few benefits to plant hosts (Kiers et al. 2011; Smith and Smith 2011). As a result, there are numerous examples in which negative effects of AM fungi on host plant growth have been described (Johnson 1993; Klironomos 2003; Jones and Smith 2004; Johnson et al. 1997; Li et al. 2008). Plants can be colonized on a very small spatial scale (1 cm or smaller) by multiple fungal species differing in mutualistic benefit (Vandenkoornhuyse et al. 2003; Helgason and Fitter 2009). Simultaneously, each fungal individual can interact with multiple host plants or species that differ in the amounts of carbohydrates that they provide (Lekberg et al. 2010; Electronic supplementary material The online version of this article (doi:10.1007/s13199-012-0197-8) contains supplementary material, which is available to authorized users. MMH and ETK designed the experiment, BO collected data, JJ designed the molecular probes, JF and MMH performed the molecular and statistical analyses, MMH wrote the first draft and HB, JJ and ETK contributed substantially to revisions. M. M. Hart (*) : J. Forsythe : B. Oshowski Biology Department, University of British Columbia Okanagan, Kelowna, BC V1V 1V7, Canada e-mail: miranda.hart@ubc.ca H. Bücking Department of Biology and Microbiology, South Dakota State University, Brookings, SD 57007, USA J. Jansa Institute of Microbiology, Academy of Sciences of the Czech Republic, Vídeňská 1083, Praha 4 – Krč, Czech Republic E. T. Kiers Institute of Ecological Science, Vrije Universiteit, De Boelelaan 1085, Amsterdam 1081 HV, The Netherlands Symbiosis (2013) 59:47–56 DOI 10.1007/s13199-012-0197-8