Saprotrophic fungi must degrade dead organic matter for survival, whereas other fungi have evolved a mycorrhizal symbiotic lifestyle that enables them to obtain carbon compounds from their plant partners. Kohler et al. analysed the evolution of mycorrhizal fungi by comparing 49 fungal genomes and showed that ectomycorrhizal (ECM) species, which do not penetrate the plant cell wall, have evolved repeatedly from diverse saprotrophic species owing to the convergent loss of several genes encoding plant cell wall-degrading enzymes (PCWDEs). By comparing mRNA expression in free-living and root-associated ECM species, they identified genes that are upregulated during symbiosis. A large proportion of these were orphan genes that are restricted to a single ECM species and have no homologues in other ECM species. Together, these data suggest that convergent evolution of ECM fungi involved not only the loss of PCWDEs but also the acquisition of a species-specific toolkit of symbiosis-associated genes.