The formation of the pileus in homobasidiomycete fungi is essential for sexual reproduction, because the pileus bears the hymenium, a layer of cells that includes the specialised basidia in which nuclear fusion, meiosis and sporulation occur. The developmental mutant ichijiku of Coprinus cinereus fails to develop a differentiated pileus at the apex of the primordial shaft, which is the basal part of the fruit-body primordia and formed in an early stage of fruit-body differentiation. Genetic analysis indicates that this phenotype is caused by a recessive mutation in a single gene (ich1). The ich1 gene was mapped to chromosome XII using restriction fragment length polymorphism markers and the marker chromosome method, and cloned by complementation using a chromosome-XII-specific cosmid library. The ich1 gene encodes a novel protein of 1,353 amino acids. The Ich1 amino-acid sequence contains nuclear targeting signals, suggesting that the Ich1 protein would function in the nucleus. Northern blot analysis indicates that the ich1 gene is specifically expressed in the pileus of the wild-type fruit-body. No ich1 mRNA was detected in the ichijiku mutant, consistent with loss of the promoter region of ich1 in the mutant genome. These data demonstrate that the ich1 gene product is essential for pileus formation.

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