A Neurospora crassa genomic DNA library was screened with a cDNA probe enriched in sequences expressed in conidiating cultures. Clones were isolated that preferentially hybridized to this probe versus a second cDNA probe complementary to polyadenylated RNA isolated from mycelia. Twelve clones contained unique sequences that hybridized to 22 transcripts, 19 of which accumulated preferentially in conidiating cultures. Eight transcripts were present in higher levels in conidiating cultures than in mycelia. Eleven transcripts were detected only in conidiating cultures and first appeared at different times during the asexual cycle. We mapped genomic sequences homologous to the 11 clones by conventional crosses using restriction fragment-length polymorphisms as genetic markers. The sequences homologous to genes expressed preferentially in conidiating cultures are distributed on six of the seven chromosomes. Clones that map to the same chromosome are linked. No recombination occurred between genomic sequences homologous to three clones, suggesting that the genes contained in these clones may constitute a gene cluster.Asexual reproduction in Neurospora crassa involves the differentiation of three cell structures-mycelia or vegetative hyphae, aerial hyphae, and conidia, the asexual spores. N. crassa can be grown under conditions that either promote vegetative growth or induce conidiation. In liquid medium, under continuous agitation, only vegetative growth occurs. However, on a solid surface or when mycelia are harvested onto filter paper and incubated under aerobic conditions, the vegetative hyphae undergo a number of morphological changes, possibly in response to aerobiosis and desiccation or nutrient limitation. The abrupt change in growth conditions upon filtration triggers the steps leading to conidiation, which then proceed in a synchronous fashion. Within 2 h vegetative hyphae begin to grow upward, perpendicular to the surface of the substrate, and differentiate into aerial hyphae. The aerial hyphae elongate, branch, and by 12 h produce large numbers of conidia by repeated apical budding. By manipulating the growth conditions in this way, homogeneous populations of cells at different stages of differentiation can be readily isolated and analyzed (1).Studies of asexual differentiation in Aspergillus nidulans have shown that approximately 1,000 polyadenylated [poly(A)+] RNA sequences, representing 6% of the genome, preferentially accumulate in cultures induced to conidiate (27). Several lines of evidence suggest that differential gene expression also occurs during the formation of aerial hyphae and conidia in N. crassa. First, a number of "phase-specific" mutants have been isolated that are defective in the differentiation of aerial hyphae or conidia, but are unaffected in vegetative growth or sexual reproduction (13,22). Second, on two-dimensional gels of proteins synthesized during the asexual cycle, 12 polypeptides were detected in extracts of aerial hyphae or conidia that were not readily detected in extracts of...