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Nat Microbiol. 2016 Mar 21;1(6):16033. doi: 10.1038/nmicrobiol.2016.33.

Evidence for the sexual origin of heterokaryosis in arbuscular mycorrhizal fungi.

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Canadian Institute for Advanced Research, Department of Biology, University of Ottawa, Ottawa, Ontario, Canada.
LMU Munich, Faculty of Biology, Genetics, D-82152 Planegg-Martinsried, Germany.
FASTERIS S.A., Ch. du Pont-du-Centenaire 109, PO Box 28, CH-1228 Plan-les-Ouates, Switzerland.
Institute of Botany, Academy of Sciences of the Czech Republic, Zámek 1, Průhonice, CZ-25243, Czech Republic.
Leibniz Institute of Plant Genetics and Crop Plant Research, D-06466 Gatersleben, Germany.


Sexual reproduction is ubiquitous among eukaryotes, and fully asexual lineages are extremely rare. Prominent among ancient asexual lineages are the arbuscular mycorrhizal fungi (AMF), a group of plant symbionts with a multinucleate cytoplasm. Genomic divergence among co-existing nuclei was proposed to drive the evolutionary success of AMF in the absence of sex(1), but this hypothesis has been contradicted by recent genome analyses that failed to find significant genetic diversity within an AMF isolate(2,3). Here, we set out to resolve issues surrounding the genome organization and sexual potential of AMF by exploring the genomes of five isolates of Rhizophagus irregularis, a model AMF. We find that genetic diversity in this species varies among isolates and is structured in a homo-dikaryon-like manner usually linked with the existence of a sexual life cycle. We also identify a putative AMF mating-type locus, containing two genes with structural and evolutionary similarities with the mating-type locus of some Dikarya. Our analyses suggest that this locus may be multi-allelic and that AMF could be heterothallic and bipolar. These findings reconcile opposing views on the genome organization of these ubiquitous plant symbionts and open avenues for strain improvement and environmental application of these organisms.

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