Resolving the haplotypes of arbuscular mycorrhizal fungi highlights the role of two nuclear populations in host interactions

Arbuscular mycorrhizal fungi (AMF) are prominent root symbionts with a multinucleate cytoplasm that can carry thousands of nuclei deriving from two parental strains and varying in relative abundance in a large syncytium. Here, we set out to improve our understanding of such remarkable genetics by resolving the nuclear genomes of all publicly available AMF heterokaryons using PacBio HiFi and Hi-C sequencing. We find that all AMF heterokaryons carry two sets of homologous chromosomes, where genes associated with plant colonization reside in gene-sparse, repeat-rich compartments. The co-existing nuclear genomes are phylogenetically related but differ significantly in content and epigenetics, resulting in nucleus-specific regulatory programs during mycorrhizal interactions. AMF heterokaryons carry signatures of past genetic exchange indicative of sexual reproduction, followed by clonal haplotype evolution. This work uncovers the contribution and origin of nuclear genomes present in AMF heterokaryons and opens avenues for improvement and environmental application of these strains.