Nuclear cooperation and conflict across symbiotic fungal networks

Arbuscular mycorrhizal fungi (AMF) form one of the most widespread terrestrial symbiosis on earth with the roots of plants, known as mycorrhizal symbiosis. Multiple AMF connect with multiple plants simultaneously forming large underground common mycorrhizal networks (CMN) where carbon and nutrients are exchanged. In contrast to most organisms where a single cell carries a single nucleus, a single AMF cell holds thousands of nuclei. This mysterious multinucleate and clonal biology has generated open questions in evolutionary biology. First, AMF have been called evolutionary scandals for supposedly surviving millions of years without sex, contradicting the theory that lack of sex in eukaryotes should lead to extinction. Second, their permanent multinucleate nature has generated debates about what defines an AMF individual.My work, and others, recently demonstrated that AMF have a unique genetic organization, with some AMF containing - not one - but two genomes (nucleotypes). While we now know that variation in this genetic system exists, it is unknow: (i) how this genetic diversity is produced, (ii) whether this variation exists in absence of sexual reproduction, and (iii) whether this variation affects nutrient trade across mycorrhizal networks. By using high resolution molecular techniques, advanced microscopy, image analysis, and DNA/RNA fluorescence in situ hybridization, I will ask: What are the costs and benefits of nuclei mixing from single cells to across connected CMN networks? Working across three scales, I will test whether co-existence of two genetic nuclei groups result in sexual reproduction (WP1) how interactions affect the mycelia fitness of interconnected strains across CMNs (WP2) and how nuclei mixing affects plant response and plant community assembly (WP3). This work will challenge long held dogmas (AMF asexuality and network connectivity) and can help maximize the benefits of the mycorrhizal mutualism for agriculture.