Diverse interactions mediate asymmetric incompatibility by the het-6 supergene complex in Neurospora crassa

Fungal Cell Death: The Beginning of the End

Death is an important part of an organism's existence and also marks the end of life. On a cellular level, death involves the execution of complex processes, which can be classified into different types depending on their characteristics. Despite their "simple" lifestyle, fungi carry out highly specialized and sophisticated mechanisms to regulate the way their cells die, and the pathways underlying these mechanisms are comparable with those of plants and metazoans. This review focuses on regulated cell death in fungi and discusses the evidence for the occurrence of apoptotic-like, necroptosis-like, pyroptosis-like death, and the role of the NLR proteins in fungal cell death. We also describe recent data on meiotic drive elements involved in "spore killing" and the molecular basis of allorecognition-related cell death during cell fusion of genetically dissimilar cells. Finally, we discuss how fungal regulated cell death can be relevant in developing strategies to avoid resistance and tolerance to antifungal agents.

Molecular Mechanisms Regulating Cell Fusion and Heterokaryon Formation in Filamentous Fungi

For the majority of fungal species, the somatic body of an individual is a network of interconnected cells sharing a common cytoplasm and organelles. This syncytial organization contributes to an efficient distribution of resources, energy, and biochemical signals. Cell fusion is a fundamental process for fungal development, colony establishment, and habitat exploitation and can occur between hyphal cells of an individual colony or between colonies of genetically distinct individuals. One outcome of cell fusion is the establishment of a stable heterokaryon, culminating in benefits for each individual via shared resources or being of critical importance for the sexual or parasexual cycle of many fungal species. However, a second outcome of cell fusion between genetically distinct strains is formation of unstable heterokaryons and the induction of a programmed cell death reaction in the heterokaryotic cells. This reaction of nonself rejection, which is termed heterokaryon (or vegetative) incompatibility, is widespread in the fungal kingdom and acts as a defense mechanism against genome exploitation and mycoparasitism. Here, we review the currently identified molecular players involved in the process of somatic cell fusion and its regulation in filamentous fungi. Thereafter, we summarize the knowledge of the molecular determinants and mechanism of heterokaryon incompatibility and place this phenomenon in the broader context of biotropic interactions and immunity.

Identification of Allorecognition Loci in Neurospora crassa by Genomics and Evolutionary Approaches

Understanding the genetic and molecular bases of the ability to distinguish self from nonself (allorecognition) and mechanisms underlying evolution of allorecognition systems is an important endeavor for understanding cases where it becomes dysfunctional, such as in autoimmune disorders. In filamentous fungi, allorecognition can result in vegetative or heterokaryon incompatibility, which is a type of programmed cell death that occurs following fusion of genetically different cells. Allorecognition is genetically controlled by het loci, with coexpression of any combination of incompatible alleles triggering vegetative incompatibility. Herein, we identified, characterized, and inferred the evolutionary history of candidate het loci in the filamentous fungus Neurospora crassa. As characterized het loci encode proteins carrying an HET domain, we annotated HET domain genes in 25 isolates from a natural population along with the N. crassa reference genome using resequencing data. Because allorecognition systems can be affected by frequency-dependent selection favoring rare alleles (i.e., balancing selection), we mined resequencing data for HET domain loci whose alleles displayed elevated levels of variability, excess of intermediate frequency alleles, and deep gene genealogies. From these analyses, 34 HET domain loci were identified as likely to be under balancing selection. Using transformation, incompatibility assays and genetic analyses, we determined that one of these candidates functioned as a het locus (het-e). The het-e locus has three divergent allelic groups that showed signatures of positive selection, intra- and intergroup recombination, and trans-species polymorphism. Our findings represent a compelling case of balancing selection functioning on multiple alleles across multiple loci potentially involved in allorecognition.

VibA, a homologue of a transcription factor for fungal heterokaryon incompatibility, is involved in antifungal compound production in the plant-symbiotic fungus Epichloë festucae

Symbiotic association of epichloae endophytes (Epichloë/Neotyphodium species) with cool-season grasses of the subfamily Pooideae confers bioprotective benefits to the host plants against abiotic and biotic stresses. While the production of fungal bioprotective metabolites is a well-studied mechanism of host protection from insect herbivory, little is known about the antibiosis mechanism against grass pathogens by the mutualistic endophyte. In this study, an Epichloë festucae mutant defective in antimicrobial substance production was isolated by a mutagenesis approach. In an isolated mutant that had lost antifungal activity, the exogenous DNA fragment was integrated into the promoter region of the vibA gene, encoding a homologue of the transcription factor VIB-1. VIB-1 in Neurospora crassa is a regulator of genes essential in vegetative incompatibility and promotion of cell death. Here we show that deletion of the vibA gene severely affected the antifungal activity of the mutant against the test pathogen Drechslera erythrospila. Further analyses showed that overexpressing vibA enhanced the antifungal activity of the wild-type isolate against test pathogens. Transformants overexpressing vibA showed an inhibitory activity on test pathogens that the wild-type isolate could not. Moreover, overexpressing vibA in a nonantifungal E. festucae wild-type Fl1 isolate enabled the transformant to inhibit the mycelial and spore germination of D. erythrospila. These results demonstrate that enhanced expression of vibA is sufficient for a nonantifungal isolate to obtain antifungal activity, implicating the critical role of VibA in antifungal compound production by epichloae endophytes.

Vegetative incompatibility loci with dedicated roles in allorecognition restrict mycovirus transmission in chestnut blight fungus

Vegetative incompatibility (vic), a form of nonself allorecognition, operates widely in filamentous fungi and restricts transmission of virulence-attenuating hypoviruses in the chestnut blight fungus Cryphonectria parasitica. We report here the use of a polymorphism-based comparative genomics approach to complete the molecular identification of the genetically defined C. parasitica vic loci with the identification of vic1 and vic3. The vic1 locus in the C. parasitica reference strain EP155 consists of a polymorphic HET-domain-containing 771-aa ORF designated vic1a-2, which shares 91% identity with the corresponding vic1a-1 allele, and a small (172 aa) idiomorphic DUF1909-domain-containing ORF designated vic1b-2 that is absent at the vic1-1 locus. Gene disruption of either vic1a-2 or vic1b-2 in strain EP155 eliminated restrictions on virus transmission when paired with a vic1 heteroallelic strain; however, only disruption of vic1a-2 abolished the incompatible programmed cell death (PCD) reaction. The vic3 locus of strain EP155 contains two polymorphic ORFs of 599 aa (vic3a-1) and 102 aa (vic3b-1) that shared 46 and 85% aa identity with the corresponding vic3a-2 and vic3b-2 alleles, respectively. Disruption of either vic3a-1 or vic3b-1 resulted in increased virus transmission. However, elimination of PCD required disruption of both vic3a and vic3b. Additional allelic heterogeneity included a sequence inversion and a 8.5-kb insertion containing a LTR retrotransposon sequence and an adjacent HET-domain gene at the vic1 locus and a 7.7-kb sequence deletion associated with a nonfunctional, pseudo vic locus. Combined gene disruption studies formally confirmed restriction of mycovirus transmission by five C. parasitica vic loci and suggested dedicated roles in allorecognition. The relevance of these results to the acquisition and maintenance of vic genes and the potential for manipulation of vic alleles for enhanced mycovirus transmission are discussed.

Natural variation of heterokaryon incompatibility gene het-c in Podospora anserina reveals diversifying selection

In filamentous fungi, allorecognition takes the form of heterokaryon incompatibility, a cell death reaction triggered when genetically distinct hyphae fuse. Heterokaryon incompatibility is controlled by specific loci termed het-loci. In this article, we analyzed the natural variation in one such fungal allorecognition determinant, the het-c heterokaryon incompatibility locus of the filamentous ascomycete Podospora anserina. The het-c locus determines an allogenic incompatibility reaction together with two unlinked loci termed het-d and het-e. Each het-c allele is incompatible with a specific subset of the het-d and het-e alleles. We analyzed variability at the het-c locus in a population of 110 individuals, and in additional isolates from various localities. We identified a total of 11 het-c alleles, which define 7 distinct incompatibility specificity classes in combination with the known het-d and het-e alleles. We found that the het-c allorecognition gene of P. anserina is under diversifying selection. We find a highly unequal allele distribution of het-c in the population, which contrasts with the more balanced distribution of functional groups of het-c based on their allorecognition function. One explanation for the observed het-c diversity in the population is its function in allorecognition. However, alleles that are most efficient in allorecognition are rare. An alternative and not exclusive explanation for the observed diversity is that het-c is involved in pathogen recognition. In Arabidopsis thaliana, a homolog of het-c is a pathogen effector target, supporting this hypothesis. We hypothesize that the het-c diversity in P. anserina results from both its functions in pathogen-defense, and allorecognition.

Trans-species activity of a nonself recognition domain

Background

The ability to distinguish nonself from self is a fundamental characteristic of biological systems. In the filamentous fungus Neurospora crassa, multiple incompatibility genes mediate nonself recognition during vegetative growth. One of these genes, un-24, encodes both nonself recognition function and the large subunit of a type I ribonucleotide reductase, an evolutionarily conserved enzyme that is essential for the conversion of NDP precursors into dNDPs for use in DNA synthesis. Previous studies have shown that co-expression of the two allelic forms of un-24, Oakridge (OR) and Panama (PA), in the same cell results in cell death.

Results

We identify a 135 amino acid nonself recognition domain in the C-terminus region of UN-24 that confers an incompatibility-like phenotype when expressed in the yeast, Saccharomyces cerevisiae. Low-level expression of this domain results in several cytological and phenotypic characteristics consistent with an incompatibility reaction in filamentous fungi. These incompatibility phenotypes are correlated with the presence of a non-reducible complex consisting of the PA incompatibility domain and Rnr1p, a large subunit of ribonucleotide reductase in yeast. When the PA incompatibility domain is switched to high-level expression, the incompatibility phenotype transitions to wild-type concomitant with the appearance of a complex containing the PA incompatibility domain and Ssa1p, an Hsp70 homolog.

Conclusions

Results from this study provide insights into the mechanism and control of vegetative nonself recognition mediated by ribonucleotide reductase in N. crassa, thus establishing the yeast system as a powerful tool to study fungal nonself recognition. Our work shows that heat shock proteins may function to deactivate vegetative incompatibility systems, as required for entry into the sexual cycle. Finally, our results suggest that variations on the PA incompatibility domain may serve as novel and specific antimicrobial peptides.

Nonself recognition through intermolecular disulfide bond formation of ribonucleotide reductase in neurospora

Type I ribonucleotide reductases (RNRs) are conserved across diverse taxa and are essential for the conversion of RNA into DNA precursors. In Neurospora crassa, the large subunit of RNR (UN-24) is unusual in that it also has a nonself recognition function, whereby coexpression of Oak Ridge (OR) and Panama (PA) alleles of un-24 in the same cell leads to growth inhibition and cell death. We show that coexpressing these incompatible alleles of un-24 in N. crassa results in a high molecular weight UN-24 protein complex. A 63-amino-acid portion of the C terminus was sufficient for un-24(PA) incompatibility activity. Redox active cysteines that are conserved in type I RNRs and essential for their catalytic function were found to be required for incompatibility activity of both UN-24(OR) and UN-24(PA). Our results suggest a plausible model of un-24 incompatibility activity in which the formation of a complex between the incompatible RNR proteins is potentiated by intermolecular disulfide bond formation.

Diversification of a protein kinase cascade: IME-2 is involved in nonself recognition and programmed cell death in Neurospora crassa

Kinase cascades and the modification of proteins by phosphorylation are major mechanisms for cell signaling and communication, and evolution of these signaling pathways can contribute to new developmental or environmental response pathways. The Saccharomyces cerevisiae kinase Ime2 has been well characterized for its role in meiosis. However, recent studies have revealed alternative functions for Ime2 in both S. cerevisiae and other fungi. In the filamentous fungus Neurospora crassa, the IME2 homolog (ime-2) is not required for meiosis. Here we determine that ime-2 interacts genetically with a transcription factor vib-1 during nonself recognition and programmed cell death (PCD). Mutations in vib-1 (Δvib-1) suppress PCD due to nonself recognition events; however, a Δvib-1 Δime-2 mutant restored wild-type levels of cell death. A role for ime-2 in the post-translational processing and localization of a mitochondrial matrix protein was identified, which may implicate mitochondria in N. crassa nonself recognition and PCD. Further, Δvib-1 strains do not produce extracellular proteases, but protease secretion reverted to near wild-type levels in a Δvib-1 Δime-2 strain. Mass spectrometry analysis revealed that the VIB-1 protein is phosphorylated at several sites, including a site that matches the IME-2 consensus. The genetic and biochemical data for ime-2 and vib-1 indicate that IME-2 is a negative regulator of VIB-1 and suggest parallel negative regulation by IME-2 of a cell death pathway in N. crassa that functions in concert with the VIB-1 cell death pathway. Thus, IME2 kinase function has evolved following the divergence of S. cerevisiae and N. crassa and provides insight into the evolution of kinases and their regulatory targets.