In filamentous fungi, het loci (for heterokaryon incompatibility) are believed to regulate self͞nonself-recognition during vegetative growth. As filamentous fungi grow, hyphal fusion occurs within an individual colony to form a network. Hyphal fusion can occur also between different individuals to form a heterokaryon, in which genetically distinct nuclei occupy a common cytoplasm. However, heterokaryotic cells are viable only if the individuals involved have identical alleles at all het loci. One het locus, het-c, has been characterized at the molecular level in Neurospora crassa and encodes a glycine-rich protein. In an effort to understand the role of this locus in filamentous fungi, we chose to study its evolution by analyzing het-c sequence variability in species within Neurospora and related genera. We determined that the het-c locus was polymorphic in a field population of N. crassa with close to equal frequency of each of the three allelic types. Different species and even genera within the Sordariaceae shared het-c polymorphisms, indicating that these polymorphisms originated in an ancestral species. Finally, an analysis of the het-c specificity region shows a high occurrence of nonsynonymous substitution. The persistence of allelic lineages, the nearly equal allelic distribution within populations, and the high frequency of nonsynonymous substitutions in the het-c specificity region suggest that balancing selection has operated to maintain allelic diversity at het-c. Het-c shares this particular evolutionary characteristic of departing from neutrality with other self͞nonself-recognition systems such as major histocompatibility complex loci in mammals and the S (selfincompatibility) locus in angiosperms.
The mating-type alleles A and a of Neurospora crassa control mating in the sexual cycle and function in establishing heterokaryon incompatibility in the vegetative cycle. The A and a alleles were cloned, and they were shown to encode both the sexual functions and vegetative incompatibility. The mating-type clones contain nonhomologous DNA segments that are flanked by common DNA sequences. Neurospora crassa and all heterothallic and pseudohomothallic Neurospora species contain a single copy of one mating-type sequence or the other within each haploid genome. The six known self-fertile homothallic isolates contain an A homolog, but only one species also contains a homologous sequences. Homothallism in these species is not due to mating-type switching, as it is in Saccharomyces cerevisiae.
The colony of a filamentous ascomycete fungus typically grows as a multinucleate syncytium. While this syncytial organization has developmental advantages, it bears the risk of extensive damage caused by local injury of hyphae. Loss of cytoplasm in injured hyphae is restricted by the fast and efficient sealing of the central pores of hyphal crosswalls, or septa, by a peroxisome-derived organelle called the Woronin body. The formation of septal plugs is also associated with development and leads to separation of certain parts of the colony. Septal plugs associated with developmental processes or aging hyphae typically occur by the accumulation of sealing material. Here we report that in Neurospora crassa, a protein necessary for hyphal fusion and proper colony development called SO (SOFT) localizes to septal plugs. In response to injury, SO accumulates at the septal plug in a Woronin body-independent manner. However, the presence of the Woronin body affects the speed of accumulation of SO at the septal pore. We determined that SO contributes to, but is not essential for, septal plugging. SO accumulation was also observed at septal plugs formed during hyphal aging and during programmed cell death mediated by genetic differences at heterokaryon incompatibility (het) loci.Filamentous ascomycete fungi typically form mycelial colonies consisting of a network of interconnected multinucleate hyphae. Colonies grow by hyphal tip extension, branching, and fusion (4, 10). In filamentous ascomycete species, hyphal crosswalls or septa are incomplete and contain a single central pore. Septal pores allow cytoplasm and organelles, including nuclei, to move between hyphal compartments, thus making the fungal colony a syncytium. The syncytial, interconnected organization of a fungal colony enables translocation of cellular contents, such as organelles, metabolites, nutrients, or signaling compounds throughout the colony, presumably facilitating growth and reproduction. However, cytoplasmic continuity bears the risk of catastrophic loss of cellular contents as a result of hyphal injury. To prevent the loss of cytoplasm, septal pores become rapidly plugged in response to hyphal damage. In filamentous ascomycete species, plugging of septal pores is executed by a specialized organelle called the Woronin body (4,5,19,30,33). Woronin bodies are a specialized class of peroxisomal vesicles containing a crystalline proteinaceous core (34). In Neurospora crassa, the main component of the Woronin body core is the HEX-1 protein. Deletion of the hex-1 gene results in strains that lack Woronin bodies and results in the catastrophic loss of cytoplasm from hyphae after injury (13,29). Occlusion of septal pores in the hex-1 mutant eventually occurs but is significantly delayed. In a wild-type strain, the plugging of septal pores by the Woronin body initiates subsequent processes termed consolidation (6) in which the hyphal plug becomes permanently sealed by the deposition of additional material. Studies with N. crassa showed that electrondense mat...
The homothallic Neurospora species, N. africana, contains sequences that hybridize to the A but not to a mating-type sequences of the heterothallic species N. crassa. In this study, the N. africana mating-type gene, mt A-1, was cloned, sequenced and its function analyzed in N. crassa. Although N. africana does not mate in a heterothallic manner, its mt A-1 gene functions as a mating activator in N. crassa. In addition, the N. africana mt A-1 gene confers mating type-associated vegetative incompatibility in N. crassa. DNA sequence analysis shows that the N. africana mt A-1 open reading frame (ORF) is 93% identical to that of N. crassa mt A-1. The mt A-1 ORF of N. africana contains no stop codons and was detected as a cDNA which is processed in a similar manner to mt A-1 of N. crassa. By DNA blot and orthogonal field agarose gel electrophoretic analysis, it is shown that the composition and location of the mating-type locus and the organization of the mating-type chromosome of N. africana are similar to that of N. crassa.
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