Evidence for the evolution of multiple genomes in arbuscular mycorrhizal fungi

Ancient asexuals directly contradict the evolutionary theories that explain why organisms should evolve a sexual life history(1,2). The mutualistic, arbuscular mycorrhizal fungi are thought to have been asexual for approximately 400 million years(3,4). In the absence of sex, highly divergent descendants of formerly allelic nucleotide sequences are thought to evolve in a genome(2). In mycorrhizal fungi, where individual offspring receive hundreds of nuclei from the parent, it has been hypothesized that a population of genetically different nuclei should evolve within one individual(5,6). Here we use DNA-DNA fluorescent in situ hybridization to show that genetically different nuclei co-exist in individual arbuscular mycorrhizal fungi. We also show that the population genetics techniques(4) used in other organisms are unsuitable for detecting recombination because the assumptions and underlying processes do not rt the fungal genomic structure shown here. Instead we used a phylogenetic approach to show that the within-individual genetic variation that occurs in arbuscular mycorrhizal fungi probably evolved through accumulation of mutations in an essentially clonal genome, with some infrequent recombination events. We conclude that mycorrhizal fungi have evolved to be multi-genomic.