Evolutionary Stability Of Fungal-Bacterial Endosymbioses

Abstract

Many eukaryotes interact with heritable endobacteria to satisfy diverse metabolic needs. Of the characterized fungal-bacterial symbioses, endobacterial associations with the Gigasporaceae (Glomeromycota) and Rhizopus microsporus (Mucoromycotina) are the best described. Both fungal hosts associate with closely related bacterial endosymbionts from the Burkholderia lineage of [beta]-proteobacteria. Through investigating patterns of co-divergence between partners, we have shown that the Glomeribacter-Glomeromycota symbiosis is at least 400 million years old, while still remaining non-essential for the host. To further explore what adaptations have taken place to allow for the persistence of this association, we created a computational pipeline which utilizes patterns of adaptation to infer microbial lifestyle. We show that this pipeline is effective at inferring microbial lifestyle, and that genes involved in DNA regulation, energy metabolism, and pathogenicity are likely important for survival of Ca. Glomeribacter within their fungal hosts. Additionally, we identified that non-essential endosymbionts are as effective at purging slightly deleterious mutations from their genomes as free-living organisms. Unlike Glomeribacter, Burkholderia rhizoxinica, the endosymbiont of Rhizopus microsporus is capable of free living yet is simultaneously of great importance to host survival. Our work has revealed that endosymbionts are required for sexual reproduction of the fungal host. Through phenotypic observation and transcriptome profiling, we found that endosymbionts control fungal reproduction through hijacking of host reproductive machinery. Specifically, bacteria control expression levels of Ras2, a signaling protein important for reproductive development as well as filamentous growth. We also exploited endosymbiont control over reproduction to explore conservation of sexually relevant genes across Fungi, including the Mucoromycotina. This approach identified several genes that appear core to all fungal reproduction, as well as reproduction related genes which are specific to members of the Mucoromycotina. In particular, we found two candidate class C seven transmembrane G-protein coupled receptors (GPCRs), TriR1 and TriR2, which may be responsible for perception of trisporic acid during mating in Mucoromycotina. These receptors are closely related to the retinoic acid GPCRs present in animal systems.