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, a rbuscular mycorrhizal fungi are thought to have been asexual for approximat ely 400 million years(3,4). In the absence of sex, highly divergent descend ants of formerly allelic nucleotide sequences are thought to evolve in a ge nome(2). In mycorrhizal fungi, where individual offspring receive hundreds of nuclei from the parent, it has been hypothesized that a population of ge netically different nuclei should evolve within one individual(5,6). Here w e use DNA-DNA fluorescent in situ hybridization to show that genetically di fferent nuclei co-exist in individual arbuscular mycorrhizal fungi. We also show that the population genetics techniques(4) used in other organisms ar e unsuitable for detecting recombination because the assumptions and underl ying processes do not rt the fungal genomic structure shown here. Instead w e used a phylogenetic approach to show that the within-individual genetic v ariation that occurs in arbuscular mycorrhizal fungi probably evolved throu gh accumulation of mutations in an essentially clonal genome, with some inf requent recombination events. We conclude that mycorrhizal fungi have evolv ed to be multi-genomic.