Heterokaryon incompatibility function of barrage-associated vegetative incompatibility genes (vic) in Cryphonectria parasitica

Transcriptome analysis implicates secondary metabolite production, redox reactions, and programmed cell death during allorecognition in Cryphonectria parasitica

The underlying molecular mechanisms of programmed cell death associated with fungal allorecognition, a form of innate immunity, remain largely unknown. In this study, transcriptome analysis was used to infer mechanisms activated during barrage formation in vic3-incompatible strains of Cryphonectria parasitica, the chestnut blight fungus. Pronounced differential expression occurred in barraging strains of genes involved in mating pheromone (mf2-1, mf2-2), secondary metabolite production, detoxification (including oxidative stress), apoptosis-related, RNA interference, and HET-domain genes. Evidence for secondary metabolite production and reactive oxygen species (ROS) accumulation is supported through UPLC-HRMS analysis and cytological staining, respectively. Differential expression of mating-related genes and HET-domain genes was further examined by RT-qPCR of incompatible interactions involving each of the six vegetative incompatibility (vic) loci in C. parasitica and revealed distinct recognition process networks. We infer that vegetative incompatibility in C. parasitica activates defence reactions that involve secondary metabolism, resulting in increased toxicity of the extra- and intracellular environment. Accumulation of ROS (and other potential toxins) may result in detoxification failure and activation of apoptosis, sporulation, and the expression of associated pheromone genes. The incompatible reaction leaves abundant traces of a process-specific metabolome as conidiation is initiated.

Histocompatibility as adaptive response to discriminatory within-organism conflict: a historical model

Multicellular tissue compatibility, or histocompatibility, restricts fusion to close kin. Histocompatibility depends on hypervariable cue genes, which often have more than 100 alleles in a population. To explain the evolution of histocompatibility, I here take a historical approach. I focus on the specific example of marine invertebrate histocompatibility. I use simple game-theoretical models to show that histocompatibility can evolve through five steps. These steps include the evolution of indiscriminate fusion, the evolution of discriminatory within-organism conflict, the evolution of minor histocompatibility, the evolution of major histocompatibility, and the evolution of major histocompatibility cue polymorphism. Allowing for gradual evolution reveals discriminatory within-organism conflict as a selective pressure for histocompatibility and associated cue polymorphism. Existing data from marine invertebrates and other organisms are consistent with this hypothesis.

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.

[Cryphonectria parasitica as a host of fungal viruses: a tool useful to unravel the mycovirus world]

There appear to be over a million of fungal species including those that have been unidentified and unreported, where a variety of viruses make a world as well. Studies on a very small number of them conducted during the last two decades demonstrated the infectivity of fungal viruses that had previously been assumed to be inheritable, indigenus and non-infectious. Also, great technical advances were achieved. The chest blight fungus (Cryphonectria parasitica), a phytopathogenic ascomycetous fungus, has emerged as a model filamentous fungus for fungal virology. The genome sequence with annotations, albeit not thorough, many useful research tools, and gene manipulation technologies are available for this fungus. Importantly, C. parasitica can support replication of homologous viruses naturally infecting it, in addition to heterologous viruses infecting another plant pathogenic fungus, Rosellinia necatrix taxonomically belonging to a different order. In this article, I overview general properties of fungal viruses and advantages of the chestnut blight fungus as a mycovirus host. Furthermore, I introduce two recent studies carried out using this fungal host:''Defective interfering RNA and RNA silencing that regulate the replication of a partitivirus'' and'' RNA silencing and RNA recombination''.

Viruses of the white root rot fungus, Rosellinia necatrix

Rosellinia necatrix is a filamentous ascomycete that is pathogenic to a wide range of perennial plants worldwide. An extensive search for double-stranded RNA of a large collection of field isolates led to the detection of a variety of viruses. Since the first identification of a reovirus in this fungus in 2002, several novel viruses have been molecularly characterized that include members of at least five virus families. While some cause phenotypic alterations, many others show latent infections. Viruses attenuating the virulence of a host fungus to its plant hosts attract much attention as agents for virocontrol (biological control using viruses) of the fungus, one of which is currently being tested in experimental fields. Like the Cryphonectria parasitica/viruses, the R. necatrix/viruses have emerged as an amenable system for studying virus/host and virus/virus interactions. Several techniques have recently been developed that enhance the investigation of virus etiology, replication, and symptom induction in this mycovirus/fungal host system.

CpBir1 is required for conidiation, virulence and anti-apoptotic effects and influences hypovirus transmission in Cryphonectria parasitica

Inhibitors of apoptosis proteins (IAPs) are critically important in the regulation of unicellular yeast and metazoan apoptosis. All IAPs contain one to three baculovirus IAP repeat (BIR) domains, which are essential for the anti-apoptotic activity of the IAPs. A homolog of IAPs, CpBir1, which bears two BIR domains, was recently identified from the chestnut blight fungus Cryphonectria parasitica genome. CpIAP was deleted by gene replacement, and the phenotypes of ΔIAP were characterized. CpBir1 was significantly down-regulated by hypovirus infection but up-regulated by H(2)O(2). Similar to Saccharomyces cerevisiae Bir1p, the Cpbir1 mutant was sensitive to H(2)O(2), and constitutive overexpression of CpBir1 increased resistance to H(2)O(2). The Cpbir1 mutant also showed defects in aerial hyphal formation, colony growth, mycelial morphology, conidiogenesis, pigmentation, resistance to stress conditions and virulence. Genetic complementation with native Cpbir1 fully recovered all these defective phenotypes. The CpBir1-eGFP fusion protein was localized to the nucleus in juvenile cultures, while it was found in the cytoplasm in old cultures, suggesting that the localization pattern of CpBir1 may correlate with the process of anti-apoptosis. Increased accumulation of reactive oxygen species (ROS) in the Cpbir1 deletion mutant further supports the anti-apoptotic function of CpBir1. Among five selected vegetative compatible (vc) types of C. parasitica, Cpbir1 deletion was found to block virus from transferring between Cpbir1 mutants. However, hypovirus infected Cpbir1 mutants showed a similar ability to transmit virus to other virus-free isolates compared with the infected wild-type strain. In summary, Cpbir1 encodes an IAP CpBir1 that is down-regulated by hypovirus infection and required for conidiation, virulence and anti-apoptosis, as well as affects hypovirus transmission in chestnut blight fungus C. parasitica.

Self/nonself recognition in Tuber melanosporum is not mediated by a heterokaryon incompatibility system

Vegetative incompatibility is a widespread phenomenon in filamentous ascomycetes, which limits formation of viable heterokaryons. Whether this phenomenon plays a role in maintaining the homokaryotic state of the hyphae during the vegetative growth of Tuber spp. Gene expression, polymorphism analysis as well as targeted in vitro experiments allowed us to test whether a heterokaryon incompatibility (HI) system operates in Tuber melanosporum. HI is controlled by different genetic systems, often involving HET domain genes and their partners whose interaction can trigger a cell death reaction. Putative homologues to HI-related genes previously characterized in Neurospora crassa and Podospora anserina were identified in the T. melanosporum genome. However, only two HET domain genes were found. In many other ascomycetes HET domains have been found within different genes including some members of the NWD (NACHT and WD-repeat associated domains) gene family of P. anserina. More than 50 NWD homologues were found in T. melanosporum but none of these contain a HET domain. All these T. melanosporum paralogs showed a conserved gene organization similar to the microexon genes only recently characterized in Schistosoma mansoni. Expression data of the annotated HI-like genes along with low allelic polymorphism suggest that they have cellular functions unrelated to HI. Moreover, morphological analyses did not provide evidence for HI reactions between pairs of genetically different T. melanosporum strains. Thus, the maintenance of the genetic integrity during the vegetative growth of this species likely depends on mechanisms that act before hyphal fusion.

Molecular characterization of vegetative incompatibility genes that restrict hypovirus transmission in the chestnut blight fungus Cryphonectria parasitica

Genetic nonself recognition systems such as vegetative incompatibility operate in many filamentous fungi to regulate hyphal fusion between genetically dissimilar individuals and to restrict the spread of virulence-attenuating mycoviruses that have potential for biological control of pathogenic fungi. We report here the use of a comparative genomics approach to identify seven candidate polymorphic genes associated with four vegetative incompatibility (vic) loci of the chestnut blight fungus Cryphonectria parasitica. Disruption of candidate alleles in one of two strains that were heteroallelic at vic2, vic6, or vic7 resulted in enhanced virus transmission, but did not prevent barrage formation associated with mycelial incompatibility. Detailed characterization of the vic6 locus revealed the involvement of nonallelic interactions between two tightly linked genes in barrage formation, heterokaryon formation, and asymmetric, gene-specific influences on virus transmission. The combined results establish molecular identities of genes associated with four C. parasitica vic loci and provide insights into how these recognition factors interact to trigger incompatibility and restrict virus transmission.

Genetic analysis of barrage line formation during mycelial incompatibility in Rosellinia necatrix

When mycelia of Rosellinia necatrix encounter mycelia of a different genetic strain, distinct barrage lines are formed between the two. These barrages have variable features such as pigmented pseudosclerotia structures, a clear zone, fuzzy hair-like mycelia, or tuft-like mycelia, suggesting that mycelial incompatibility triggers a number of cellular reactions. In this study, to evaluate cellular reactions we performed genetic analysis of mycelial incompatibility of R. nectarix, using 20 single ascospore isolates from single perithecia. Mycelial interaction zones were removed by spatula and cellular reactions studied on oatmeal agar media. The interaction zones were categorized into types such as sharp or wide lines, with or without melanin, and combinations of these. Although various reaction types were observed, we were able to identify a single genetic factor that appears to be responsible for the barrage line formation within oatmeal agar medium. DNA polymorphism analysis identified parental isolates and revealed that R. necatrix has a heterothallic life cycle.