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Arbuscular mycorrhizal (AM) fungi (Glomeromycota) are thought to be the oldest group of asexual multicellular organisms. They colonize the roots of most land plants, where they facilitate mineral uptake from the soil in exchange for plant-assimilated carbon1. Cells of AM fungi contain hundreds of nuclei. Unusual polymorphism of ribosomal DNA observed in individual spores of AM fungi inspired a hypothesis that heterokaryosis—that is, the coexistence of many dissimilar nuclei in cells—occurs throughout the AM fungal life history2,3. Here we report a genetic approach to test the hypothesis of heterokaryosis in AM fungi. Our study of the transmission of polymorphic genetic markers in natural isolates of Glomus etunicatum, coupled with direct amplification of rDNA from microdissected nuclei by polymerase chain reaction, supports the alternative hypothesis of homokaryosis, in which nuclei populating AM fungal individuals are genetically uniform. Intrasporal rDNA polymorphism contained in each nucleus signals a relaxation of concerted evolution4, a recombination-driven process that is responsible for homogenizing rDNA repeats5. Polyploid organization of glomeromycotan genomes could accommodate intranuclear rDNA polymorphism and buffer these apparently asexual organisms against the effects of accumulating mutations.

Pawlowska and Taylor reply - To challenge the hypothesis of multigenomic structure of arbuscular mycorrhizal (AM) fungi1,2, we presented three lines of evidence consistent with the homokaryotic organization of within-individual genetic variation, including distribution of polymorphic genetic markers among and within field isolates of an AM fungus, and distribution of ribosomal DNA variants among individually microdissected nuclei3. Bever and Wang suggest4 that our data can be explained equally well by heterokaryosis, proposing a model that relies on the assumption that fusions of hyphae of genetically non-identical individuals contribute to the creation and maintenance of a multigenomic status of AM fungal cells. However, we do not believe that this assumption is supported by existing biological evidence.