Arbuscular mycorrhizal (AM) fungi are hypothesized to have been key players in facilitating thetransition from aquatic to terrestrial plants and continue to benefit plants through their symbioticassociation after 450 million years. These fungi form mycelia that can contain hundreds of nucleiwithin one aseptate cytoplasm, which leads to the ongoing debate on whether thesemultinucleated fungi are homokaryotic or heterokaryotic. Not only is there evidence to supportthe hypothesis of the nuclei as genetically identical, but also the other hypothesis of divergentnuclei within a single strain. There has been no evidence of sexual reproduction, howeverspecialized genomic regions specific to meiosis and a putative mating-type (MAT) locus haverecently been identified and may help answer the ongoing debate between homokaryosis andheterokaryosis.

In this study I applied de novo genome assembly and annotation of 24 individual nuclei from asingle spore of Claroideoglomus candidum. The full length of the de novo genome assembly was87.6 Mb with 17,542 genes. Estimated polymorphism between the nuclei was very low. Iidentified the MAT locus in C. candidum, using a previously sequenced MAT locus fromanother congeneric species. Only one of the MAT locus alleles was found in the examined spore.The evidence points towards homokaryosis as the genetic organization of Claroideoglomuscandidum.