Genome diversity in arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (Glomeromycota) are the most widespread and important symbionts of plants. They cannot be cultured without plants, are apparently asexual, and have multiple nuclei in a common cytoplasm. There is evidence for genetic variation among nuclei, and for segregation of this variation during growth, but these findings remain contentious. Recently, two papers have reported whole genome sequences for a strain of Rhizophagus irregularis; both suggest that genetic variation among nuclei is low. Genome assembly is very incomplete, though, so significant nuclear diversity cannot be excluded. While the diversity of nuclear genomes remains unresolved, multiple complete mitochondrial genomes are now available; there is virtually no variation within isolates, but significant variation between them.