The gene, which encodes a hydrophobin that coats aerial hyphae, is expressed in both monokaryons and dikaryons. The dikaryons were formed by mating two monokaryons with different A and B genes, leading to activation of the - and B-controlled pathways (A-on and B-on). In contrast to the monokaryons, the dikaryons also expressed other hydrophobin genes () as well as a gene () encoding a hydrophilic wall protein. None of these four genes was expressed in A-off B-on mycelia, indicating that B-on represses and that both A-on and B-on are required for activation of , and . In fruiting dikaryons, immunolabelling revealed that SC3p was produced by aerial hyphae but not by hyphae that constitute the fruit-body tissue. In contrast to aerial hyphae, the latter produced dikaryon-specific transcripts and secreted SC7p into the extracellular matrix of the tissue. This suggests that in the aerial hyphae of the dikaryon the B-on pathway was not effective (B-off). We observed that in these aerial hyphae the two nuclei were wider apart than in a typical dikaryon. Although other explanations are not ruled out, we tentatively propose that effective interaction of different B genes requires proximity of the two nuclei containing these genes, and that disruption of this binucleate state represents a novel mechanism of gene control for spatial cell differentiation in the secondary mycelium.


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