The Predicted Mannosyltransferase Gt69-2 Antagonizes Rfw-1 To Regulate Cell Fusion In Neurospora Crassa

Filamentous fungi undergo somatic cell fusion to create a syncytial, interconnected hyphal network which confers a fitness benefit during colony establishment. However, barriers to somatic cell fusion between genetically different cells have evolved that reduce invasion by parasites or exploitation by maladapted genetic entities (cheaters). Here, we identified a predicted mannosyltransferase, glycosyltransferase family 69 protein (GT69-2) that was required for somatic cell fusion in Neurospora crassa. Cells lacking GT69-2 prematurely ceased chemotropic signaling and failed to complete cell wall dissolution and membrane merger in pairings with wild-type cells or between Delta gt69-2 cells (self fusion). However, loss-of-function mutations in the linked regulator of cell fusion and cell wall remodeling-1 (rfw-1) locus suppressed the self-cell-fusion defects of Delta gt69-2 cells, although Delta gt69-2 Delta rfw-1 double mutants still failed to undergo fusion with wild-type cells. Both GT69-2 and RFW-1 localized to the Golgi apparatus. Genetic analyses indicated that RFW-1 negatively regulates cell wall remodeling-dependent processes, including cell wall dissolution during cell fusion, separation of conidia during asexual sporulation, and conidial germination. GT69-2 acts as an antagonizer to relieve or prevent negative functions on cell fusion by RFW-1. In Neurospora species and N. crassa populations, alleles of gt692 were highly polymorphic and fell into two discrete haplogroups. In all isolates within haplogroup I, rfw-1 was conserved and linked to gt69-2. All isolates within haplogroup II lacked rfw-1. These data indicated that gt69-2/rfw-1 are under balancing selection and provide new mechanisms regulating cell wall remodeling during cell fusion and conidial separation.IMPORTANCE Cell wall remodeling is a dynamic process that balances cell wall integrity versus cell wall dissolution. In filamentous fungi, cell wall dissolution is required for somatic cell fusion and conidial separation during asexual sporulation. In the filamentous fungus Neurospora crassa, allorecognition checkpoints regulate the cell fusion process between genetically different cells. Our study revealed two linked loci with transspecies polymorphisms and under coevolution, rfw-1 and gt69-2, which form a coordinated system to regulate cell wall remodeling during somatic cell fusion, conidial separation, and asexual spore germination. RFW-1 acts as a negative regulator of these three processes, while GT69-2 functions antagonistically to RFW-1. Our findings provide new insight into the mechanisms involved in regulation of fungal cell wall remodeling during growth and development.