Evolutionary trends in root-microbe symbioses

Abstract Some of the various symbiotic associations that occur between plant roots and microbes are found on a very wide range of plants, others on very few. By far the most widespread association is the arbuscular mycorrhizal symbiosis, which appears to be an ancestral characteristic; other mycorrhizas and all bacterial nodule symbioses have much more restricted distributions. It has recently been shown that the ability to form nitrogen-fixing symbioses is a trait that occurs in only one clade of the angiosperms. Here we analyse the phylogenetic pattern of mycorrhizal associations, and show that the ectomycorrhizal association has almost certainly arisen more than once, although other types are more concentrated phylogenetically. A detailed com parative analysis of these symbioses awaits a more secure dataset, but it has been possible to undertake such an analysis for arbuscular mycorrhizas in the British flora, which has revealed that the non-mycorrhizal state is a derived one representing habitat and other forms of specialization.