The silencing of gene expression by segments of DNA present in excess of the normal number is called cosuppression in plants and quelling in fungi. We describe a related process, meiotic silencing by unpaired DNA (MSUD). DNA unpaired in meiosis causes silencing of all DNA homologous to it, including genes that are themselves paired. A semidominant Neurospora mutant, Sad-1, fails to perform MSUD. Sad-1 suppresses the sexual phenotypes of many ascus-dominant mutants. MSUD may provide insights into the function of genes necessary for meiosis, including genes for which ablation in vegetative life would be lethal. It may also contribute to reproductive isolation of species within the genus Neurospora. The wild-type allele, sad-1(+), encodes a putative RNA-directed RNA polymerase.

Filamentous fungi grow as a multicellular, multinuclear network of filament-shaped cells called hyphae. A fungal individual can be viewed as a fluid, dynamic system that is characterized by hyphal tip growth, branching, and hyphal fusion (anastomosis). Hyphal anastomosis is especially important in such nonlinear systems for the purposes of communication and homeostasis. Filamentous fungi can also undergo hyphal fusion with different individuals to form heterokaryons. However, the viability of such heterokaryons is dependent upon genetic constitution at heterokaryon incompatibility (het) loci. If hyphal fusion occurs between strains that differ in allelic specificity at het loci, vegetative incompatibility, which is characterized by hyphal compartmentation and cell lysis, is induced. This review covers microscopic and genetic analysis of hyphal fusion and the molecular and genetic analysis of the consequence of hyphal fusion between individuals that differ in specificity at het loci in filamentous ascomycetes.

A gene unpaired during the meiotic homolog pairing stage in Neurospora generates a sequence-specific signal that silences the expression of all copies of that gene. This process is called Meiotic Silencing by Unpaired DNA (MSUD). Previously, we have shown that SAD-1, an RNA-directed RNA polymerase (RdRP), is required for MSUD. We isolated a second gene involved in this process, sad-2. Mutated Sad-2 RIP alleles, like those of Sad-1, are dominant and suppress MSUD. Crosses homozygous for Sad-2 are blocked at meiotic prophase. SAD-2 colocalizes with SAD-1 in the perinuclear region, where small interfering RNAs have been shown to reside in mammalian cells. A functional sad-2 ؉ gene is necessary for SAD-1 localization, but the converse is not true. The data suggest that SAD-2 may function to recruit SAD-1 to the perinuclear region, and that the proper localization of SAD-1 is important for its activity.epigenetics ͉ meiosis ͉ MSUD ͉ Neurospora ͉ RNA interference C oenocytic organisms, in which nuclei coexist in a common cytoplasm, are probably at especially high risk from proliferation of detrimental retrotransposons. In the haploid ascomycete Neurospora crassa, several gene-silencing mechanisms exist to maintain its genome integrity. Quelling, which defends the organism during the vegetative phase, is an RNA interference (RNAi) system that suppresses the expression of transgenes occurring in more than one copy (1, 2). Another surveillance system, known as repeat-induced point mutation (RIP), is a premeiotic process that scans the genome for duplicated sequences and targets them for C to T mutations (3). A third silencing mechanism, named meiotic silencing by unpaired DNA, scans the genome and monitors the pairing of DNA segments with their homologs during meiotic prophase (4-6). This mechanism probably prevents the expression and transposition of invasive sequences, and serves the organism in its need to counter exogenous elements and perhaps to regulate endogenous elements. Deletion or extensive mutation in an RdRPencoding gene, sad-1 (suppressor of ascus dominance), reduces meiotic silencing to a low level. RdRP plays an important role in some RNAi systems (2). For example, if foreign nucleic acids trigger the production of aberrant RNA (aRNA), the singlestranded aRNA can be replicated into a double-stranded species (dsRNA) via the activity of an RdRP. The dsRNA is then processed into small interfering RNA (siRNA) duplexes by Dicer. The siRNAs subsequently guide the cleavage of mRNA via the RNA-induced silencing complex. The fact that an RdRP is required for meiotic silencing suggests that the synthesis of dsRNA, its amplification, or both, are essential for the process.We have now identified an additional gene, sad-2, which is also required for meiotic silencing. Dominant mutations (Sad-2) can suppress the meiotic silencing of unpaired loci with efficiency comparable to that of Sad-1. A Sad-2 mutation does not give any obvious abnormal phenotype during vegetative growth and, correspondingly, sad-2 ϩ mRNA can only...