Specificity in host-fungus associations: Do mutualists differ from antagonists?

Summary

Physically intimate interactions between organisms are assumed to be highly specific, yet intimate mutualisms exhibiting little specificity are common and important in many communities. We compare host records for ectomycorrhizal fungi (mutualists) to those for biotrophic shoot fungi and necrotrophic root fungi (both antagonists) in order to test two alternative predictions: (1) intimate physical associations (biotrophy) are more specific than less intimate ones (necrotrophy); (2) antagonisms are more specific than mutualisms. Specificity of fungi for hosts supports prediction (1): ectomycorrhizal fungi and shoot biotrophs are more host specific than root necrotrophs. Fungal symbiont ranges of hosts supports prediction (2): woody hosts are associated with a greater number of mutualistic fungi than antagonistic fungi. The numbers of fungi in the three groups infecting hosts are all significantly positively correlated. This result suggests that some hosts are resistant to fungal invasion and others are quite susceptible. Thus, plants may not be able to erect selective barriers to only antagonistic fungi. The marked asymmetry of specificity from the perspectives of hosts vs fungi suggests that evolutionary and ecological processes act differently on partners in symbioses.