The so Locus Is Required for Vegetative Cell Fusion and Postfertilization Events in Neurospora crassa

Abstract

The process of cell fusion is a basic developmental feature found in most eukaryotic organisms. In filamentous fungi, cell fusion events play an important role during both vegetative growth and sexual reproduction. We employ the model organism Neurospora crassa to dissect the mechanisms of cell fusion and cell-cell communication involved in fusion processes. In this study, we characterized a mutant with a mutation in the gene so, which exhibits defects in cell fusion. The so mutant has a pleiotropic phenotype, including shortened aerial hyphae, an altered conidiation pattern, and female sterility. Using light microscopy and heterokaryon tests, the so mutant was shown to possess defects in germling and hyphal fusion. Although so produces conidial anastomosis tubes, so germlings did not home toward wild-type germlings nor were wild-type germlings attracted to so germlings. We employed a trichogyne attraction and fusion assay to determine whether the female sterility of the so mutant is caused by impaired communication or fusion failure between mating partners. so showed no defects in attraction or fusion between mating partners, indicating that so is specific for vegetative hyphal fusion and/or associated communication events. The so gene encodes a protein of unknown function, but which contains a WW domain; WW domains are predicted to be involved in protein-protein interactions. Database searches showed that so was conserved in the genomes of filamentous ascomycete fungi but was absent in ascomycete yeast and basidiomycete species.

TheSaccharomyces cerevisiae PRM1Homolog inNeurospora crassaIs Involved in Vegetative and Sexual Cell Fusion Events but Also Has Postfertilization Functions

“…In our trichogyne-fusion assay, microconidia containing a single H1-GFP-labeled nucleus were used as the male crossing partner (see materials and methods). Successful fusion between a microconidium and trichogyne of opposite mating type can be detected as a disappearance of the H1-GFP nucleus from the male cell as a consequence of its migration through the trichogyne and into the protoperithecium (Fleißner et al 2005).…”

“…However, not all N. crassa fusion mutants are affected in both germling/hyphal fusion and trichogyne-conidium fusion. For example, mutations in so, which encodes a filamentous ascomycetespecific protein, completely abolish germling and hyphal fusion in N. crassa, but trichogyne-conidium fusion is unaffected (Fleißner et al 2005); so mutants show no chemotropic interactions during germling fusion . These data indicate that N. crassa uses different molecular mechanisms for cell-cell communication and chemotropic growth that precede cell fusion during germling/hyphal fusion vs. mating during sexual reproduction.…”

Excitable behavior can explain the “ping‐pong” mode of communication between cells using the same chemoattractant

“…1B (see also Supporting information movies 1 ). Remarkably, another essential protein for CAT fusion, with unknown function, soft (SO) [17], exhibited a complementary oscillatory pattern, with a maximum intensity in one cell synchronized with the maximum of MAK-2 in the other. Fleißner et al [20] suggested that the MAK-2 peaks correspond to the receiving of a signal while the SO maxima to the sending of it (see Fig.…”

“…The CATs select their partners at a distance as evidenced by their purposeful homing toward each other that becomes apparent when their separation is less than $15 mm. Genetic analyses identified a number of candidate genes whose products were required for CAT formation, homing and fusion [14][15][16][17][18][19]. Deletion of the gene encoding the MAP kinase MAK-2 resulted in cells unable to chemotropically home and fuse with other, even normal, wild-type, cells.…”

Roles for Receptors, Pheromones, G Proteins, and Mating Type Genes During Sexual Reproduction in Neurospora crassa

“…Formation and fusion of conidial anastomosis tubes were assayed (Fleissner et al 2005;Kim and Borkovich 2006). The ability of two strains to undergo hyphal fusion and form a heterokaryotic colony was tested (Davis and Deserres 1970;Kim and Borkovich 2004).…”