Allorecognition upon Fungal Cell-Cell Contact Determines Social Cooperation and Impacts the Acquisition of Multicellularity.

Somatic cell fusion and conspecific cooperation are crucial social traits for microbial unicellular-to-multicellular transitions, colony expansion, and substrate foraging but are also associated with risks of parasitism. We identified a cell wall remodeling (cwr) checkpoint that acts upon cell contact to assess genetic compatibility and regulate cell wall dissolution during somatic cell fusion in a wild population of the filamentous fungus Neurospora crassa. Non-allelic interactions between two linked loci, cwr-1 and cwr-2, were necessary and sufficient to block cell fusion: cwr-1 encodes a polysaccharide monooxygenase (PMO), a class of enzymes associated with extracellular degradative capacities, and cwr-2 encodes a predicted transmembrane protein. Mutations of sites in CWR-1 essential for PMO catalytic activity abolished the block in cell fusion between formerly incompatible strains. In Neurospora, alleles cwr-1 and cwr-2 were highly polymorphic, fell into distinct haplogroups, and showed trans-species polymorphisms. Distinct haplogroups and trans-species polymorphisms at cwr-1 and cwr-2 were also identified in the distantly related genus Fusarium, suggesting convergent evolution. Proteins involved in chemotropic processes showed extended localization at contact sites, suggesting that cwr regulates the transition between chemotropic growth and cell wall dissolution. Our work revealed an allorecognition surveillance system based on kind discrimination that inhibits cooperative behavior in fungi by blocking cell fusion upon contact, contributing to fungal immunity by preventing formation of chimeras between genetically non-identical colonies.