The genome of Gigaspora margarita offers hints on ancient plant-fungal-bacterial interactions

Despite their importance in establishing mutualistic symbiosis with 72% of land plants, Arbuscular Mycorrhizal Fungi (AMF, Glomeromycotina) are still enigmatic from a biological point of view, since our knowledge on the genetic mechanisms regulating AM symbiosis mostly mirrors a plant-centric point of view. In addition to being members of the plant microbiota, some AMF also possess their own microbiota, harboring uncultivable endobacteria inside their cytoplasm and thus interacting with plant and bacterial kingdoms. Differently from the AMF so far sequenced, Gigaspora margarita may host different endobacterial populations, as well as viral sequences, representing a good model to investigate such interkingdom interactions. The genome of G. margarita is the largest fungal genome so far annotated (773.104 Mbp) with a relevant presence of transposable elements (more than 60%). It possesses several unique traits, such as the presence of secondary metabolites gene clusters, the expansion of some carbohydrate active enzymes families (i.e. chitinsynthases and secreted multicopper oxidases), the expression of a high number of genes involved in Pi metabolism, and potential horizontal gene transfer events. In the context of a comparative genome analysis, the sequencing of G. margarita reveals evolutionary pathways, which may have allowed early-diverging fungi to interact with both plants and bacteria. We provide insights into G. margarita biotrophic life style, reveal the importance of its immunity system, which has to guarantee interactions with both bacterial and plant cells, and advance hypothesis on the biological meaning of its expanded genome.