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Low gene copy number shows that arbuscular mycorrhizal fungi inherit genetically different nuclei

Abstract

Arbuscular mycorrhizal fungi (AMF) are ancient asexually reproducing organisms that form symbioses with the majority of plant species, improving plant nutrition and promoting plant diversity1,2. Little is known about the evolution or organization of the genomes of any eukaryotic symbiont or ancient asexual organism. Direct evidence shows that one AMF species is heterokaryotic; that is, containing populations of genetically different nuclei3. It has been suggested, however, that the genetic variation passed from generation to generation in AMF is simply due to multiple chromosome sets (that is, high ploidy)4. Here we show that previously documented genetic variation in Pol-like sequences, which are passed from generation to generation, cannot be due to either high ploidy or repeated gene duplications. Our results provide the clearest evidence so far for substantial genetic differences among nuclei in AMF. We also show that even AMF with a very large nuclear DNA content are haploid. An underlying principle of evolutionary theory is that an individual passes on one or half of its genome to each of its progeny. The coexistence of a population of many genomes in AMF and their transfer to subsequent generations, therefore, has far-reaching consequences for understanding genome evolution.

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Figure 1: Three possibilities for how the genetic variation of PLS1 is organized in G. etunicatum.
Figure 2: Measurements of fluorescence intensity for estimating the nuclear DNA content of G. etunicatum.
Figure 3: Amplification plots and regressions obtained by real-time PCR.

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Acknowledgements

We thank the Swiss National Science Foundation for supporting this work with a fellowship awarded to I.R.S. under the professorial fellowship programme. The authors thank J. C. Dodd and A. Rodriguez for providing fungal material, and H. Dulieu for comments on the manuscript. We also wish to acknowledge the work on re-association kinetics by M. Hosny, who sadly passed away before his work could be published.

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Correspondence to Ian R. Sanders.

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Supplementary information

Supplementary Methods

This contains details of Supplementary Methods, including real-time PCR for quantification of PLS copy number, analysis of re-association kinetics data and additional references. (DOC 38 kb)

Supplementary Figure S1

The original re-association curve (ECot curve) obtained for Scutellospora castanea genomic DNA. (JPG 40 kb)

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Hijri, M., Sanders, I. Low gene copy number shows that arbuscular mycorrhizal fungi inherit genetically different nuclei. Nature 433, 160–163 (2005). https://doi.org/10.1038/nature03069

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