Lack of global population genetic differentiation in the arbuscular mycorrhizal fungus Glomus mosseae suggests a recent range expansion which may have coincided with the spread of agriculture SØREN ROSENDAHL,* PETER MCGEE† and JOSEPH B MORTON‡ *Department of Biology, University of Copenhagen, Oster Farimagsgade 2D, DK-1353 Copenhagen K, Denmark, †School of Biological Sciences, University of Sydney, Sydney, NSW 2006, Australia, ‡Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV, 26506-6108 USA Abstract The arbuscular mycorrhizal fungus Glomus mosseae is commonly found in agricultural fields. The cosmopolitan species is found in Africa, Europe, America, Asia and Australia. Three hypotheses may explain this worldwide distribution: First, speciation occurred before the continents separated 120 Ma; second, the distribution is a result of human- mediated dispersal related to agriculture and finally, the morphologically defined species may encompass several local endemic species. To test these hypotheses, three genes were sequenced from 82 isolates of G. mosseae originating from six continents and the resulting sequences analysed for geographical subdivision and estimation of migration between continents. Coalescent analyses estimated divergence and age of mutations. Bayesian coalescent modelling was used to reveal important past population changes in the global population. The sequence data showed no geographical structure, with identical genotypes found on different continents. Coalescence analyses indicated a recent diversification in the species, and the data could be explained by a recent population expansion in G. mosseae. The results of this study suggest that speciation and the range expansion happened much later than continental spread and that human activity may have had a major impact on the dispersal and the population structure of the fungus. Keywords: agriculture, cosmopolitan, evolution, population structure Received 12 May 2009; revision received 2 August 2009; accepted 9 August 2009 Introduction Arbuscular mycorrhizal fungi (AMF) are among the most common organisms found on this planet, as they form a mutualistic symbiosis with most plants in almost all types of vegetation and habitats (Smith & Read 2008). The benefits of these fungi for plant nutrition have been recognized for more than four decades, and numerous recent studies have demonstrated their cru- cial role in ecological processes at all scales (Van der Heijden et al. 1998). Fossil AMF dating back to Ordovician (Redecker et al. 2000) and molecular data (Berbee & Taylor 1993) support the hypothesis first proposed by Pirozynski & Malloch (1975) that fungi, and AMF in particular, evolved with the first terrestrial plants, and it is possible that AMF facilitated the transition from aquatic to terrestrial life forms. Fossilized mycorrhizal structures and resting spores resemble contemporary structures, suggesting that the fungi may have under- gone few morphological changes during 400 Myr (Morton 1990; Remy et al. 1994). Sexual reproduction via meiosis or sexual reproductive structures has not been reported. All of this evidence suggests that these fungi may have evolved clonally over several Correspondence: Søren Rosendahl, Fax: +45 3532 2321; E-mail: soerenr@bio.ku.dk Ó 2009 Blackwell Publishing Ltd Molecular Ecology (2009) 18, 4316–4329 doi: 10.1111/j.1365-294X.2009.04359.x