The A mating type and blue light regulate all known differentiation processes in the basidiomycete Coprinus cinereus

Monokaryons of Coprinus cinereus constitutively form small spores (oidia) in the aerial mycelium. Some strains also produce large, inflated single cells (chlamydospores) at the agar/air interface, and hyphal aggregates (hyphal knots) that can develop into sclerotia. Monokaryons show various reactions upon transformation with heterologous A mating type genes. Production of oidia in such A-activated transformants is repressed in the dark and induced by blue light. Five of six monokaryons tested following transformation with A genes showed induced production of hyphal knots and sclerotia in the dark, and at least three strains showed enhanced chlamydospore production in the dark. Continuous incubation under blue light inhibited formation of hyphal knots, sclerotia and chlamydospores in both competent monokaryons and in A-activated transformants. On artificial medium and on a 12 h light/12 h dark regime, A-activated transformants of one distinct monokaryon (218) formed fruit-body primordia that were arrested in development before karyogamy. Our studies show that A mating type genes control all major differentiation processes in Coprinus, but whether developmental processes can proceed depends on the genetic background of the strain. Received: 11 May 1998 / Accepted: 15 July 1998     

Influence of activated<Emphasis Type="Italic"> A</Emphasis> and<Emphasis Type="Italic"> B</Emphasis> mating-type pathways on developmental processes in the basidiomycete<Emphasis Type="Italic"> Coprinus cinereus</Emphasis>

The A and B mating type pathways in Coprinus cinereus monokaryons can be activated by transformation with cloned genes from strains of compatible mating types. The presence of heterologous A mating-type genes (Aon) induces production of submerged chlamydospores, hyphal knots and sclerotia in cultures kept in the dark. Upon illumination of transformants of certain strains (218), fruiting body primordia may develop that arrest before karyogamy. Furthermore, formation of aerial spores (oidia) is repressed by the action of A mating type genes in the dark, but light overrides this repression. Heterologous B mating type genes enhance the effects of the A genes on developmental processes, and partially repress the negative action of light on A-mediated regulation of development. Most notably, A-induced fruiting occurs more efficiently and earlier when the B mating type pathway is also active (Bon). However, activation of the B pathway alone is not sufficient to induce fruiting. Unlike A-activated transformants, A+ B-activated transformants of monokaryon 218 form mature fruiting bodies. Therefore, the B genes control fruiting body maturation at the stage of karyogamy. Basidia within the fruiting bodies that were analysed contained four spores in a typical post-meiotic arrangement. In the absence of an activated A mating type pathway, B mating type genes cause deformation and hyperbranching of vegetative hyphae, a reduction in aerial mycelium, and invasion of the agar substrate - a phenotype resembling the "flat" phenotype known from B-activated Schizophyllum commune strains. B-activated transformants usually show enhanced production of chlamydospores and hyphal knots, but maturation of sclerotia is variably efficient. Activation of the B mating type pathway in monokaryons blocked acceptance of nuclei, but not activation of the A mating type pathway.     

Life history and developmental processes in the basidiomycete Coprinus cinereus.

Coprinus cinereus has two main types of mycelia, the asexual monokaryon and the sexual dikaryon, formed by fusion of compatible monokaryons. Syngamy (plasmogamy) and karyogamy are spatially and temporally separated, which is typical for basidiomycetous fungi. This property of the dikaryon enables an easy exchange of nuclear partners in further dikaryotic-monokaryotic and dikaryotic-dikaryotic mycelial fusions. Fruiting bodies normally develop on the dikaryon, and the cytological process of fruiting-body development has been described in its principles. Within the specialized basidia, present within the gills of the fruiting bodies, karyogamy occurs in a synchronized manner. It is directly followed by meiosis and by the production of the meiotic basidiospores. The synchrony of karyogamy and meiosis has made the fungus a classical object to study meiotic cytology and recombination. Several genes involved in these processes have been identified. Both monokaryons and dikaryons can form multicellular resting bodies (sclerotia) and different types of mitotic spores, the small uninucleate aerial oidia, and, within submerged mycelium, the large thick-walled chlamydospores. The decision about whether a structure will be formed is made on the basis of environmental signals (light, temperature, humidity, and nutrients). Of the intrinsic factors that control development, the products of the two mating type loci are most important. Mutant complementation and PCR approaches identified further genes which possibly link the two mating-type pathways with each other and with nutritional regulation, for example with the cAMP signaling pathway. Among genes specifically expressed within the fruiting body are those for two galectins, beta-galactoside binding lectins that probably act in hyphal aggregation. These genes serve as molecular markers to study development in wild-type and mutant strains. The isolation of genes for potential non-DNA methyltransferases, needed for tissue formation within the fruiting body, promises the discovery of new signaling pathways, possibly involving secondary fungal metabolites.     

Blue Light Overrides Repression of Asexual Sporulation by Mating Type Genes in the Basidiomycete Coprinus cinereus

Monokaryotic mycelia of the homobasidiomycete Coprinus cinereus form asexual spores (oidia) constitutively in abundant numbers. Mycelia with mutations in both mating type loci (Amut Bmut homokaryons) also produce copious oidia but only when exposed to blue light. We used such an Amut Bmut homokaryon to define environmental and inherent factors that influence the light-induced oidiation process. We show that the Amut function causes repression of oidiation in the dark and that light overrides this effect. Similarly, compatible genes from different haplotypes of the A mating type locus repress sporulation in the dark and not in the light. Compatible products of the B mating type locus reduce the outcome of light on A-mediated repression but the mutated B function present in the Amut Bmut homokaryons is not effective. In dikaryons, the coordinated regulation of asexual sporulation by compatible A and B mating type genes results in moderate oidia production in light. Copyright 1998 Academic Press.     

Isolation and characterization of developmental variants in fruiting using a homokaryotic fruiting strain ofCoprinus cinereus

Developmental variants in fruiting ofCoprinus cinereus were induced by mutagenizing oidia of the homokaryotic fruiting strain CopD5-12 with UV light. Through screening of 2,696 isolates, 1,018 strains exhibited defects in fruiting and were classified into 8 groups: (1) knotless variants, which fail to form hyphal knots, the first visible sign of fruiting; (2) primordiumless variants, which form hyphal knots but fail to develop fruit-body primordia; (3) maturationless variants, which form fruit-body primordia but do not form mature fruit bodies; (4) elongationless variants, which form mature fruit bodies with short stipes; (5) expansionless variants, which form mature fruit bodies with unexpanded pilei; (6) sporeless variants, which fail to produce black basidiospores, resulting in fruit bodies with white pilei after maturation; (7) compound type, which includes variants exhibiting several of the phenotypes described above; (8) others, including variants that produce a “dark stipe” even under in light/dark conditions, which is formed under continuous darkness in the wild-type. Two elongationless variants were characterized histologically.     

Sexual Reproduction in Aspergillus flavus Sclerotia: Acquisition of Novel Alleles from Soil Populations and Uniparental Mitochondrial Inheritance

Aspergillus flavus colonizes agricultural commodities worldwide and contaminates them with carcinogenic aflatoxins. The high genetic diversity of A. flavus populations is largely due to sexual reproduction characterized by the formation of ascospore-bearing ascocarps embedded within sclerotia. A. flavus is heterothallic and laboratory crosses between strains of the opposite mating type produce progeny showing genetic recombination. Sclerotia formed in crops are dispersed onto the soil surface at harvest and are predominantly produced by single strains of one mating type. Less commonly, sclerotia may be fertilized during co-infection of crops with sexually compatible strains. In this study, laboratory and field experiments were performed to examine sexual reproduction in single-strain and fertilized sclerotia following exposure of sclerotia to natural fungal populations in soil. Female and male roles and mitochondrial inheritance in A. flavus were also examined through reciprocal crosses between sclerotia and conidia. Single-strain sclerotia produced ascospores on soil and progeny showed biparental inheritance that included novel alleles originating from fertilization by native soil strains. Sclerotia fertilized in the laboratory and applied to soil before ascocarp formation also produced ascospores with evidence of recombination in progeny, but only known parental alleles were detected. In reciprocal crosses, sclerotia and conidia from both strains functioned as female and male, respectively, indicating A. flavus is hermaphroditic, although the degree of fertility depended upon the parental sources of sclerotia and conidia. All progeny showed maternal inheritance of mitochondria from the sclerotia. Compared to A. flavus populations in crops, soil populations would provide a higher likelihood of exposure of sclerotia to sexually compatible strains and a more diverse source of genetic material for outcrossing.     

Development of SCAR Markers to Determine the Mating Types of Lepista nuda Protoplast Monokaryons

Lepista nuda (Bull. ex Fr.) Cooke belongs to Tricholomataceae and is an edible fungus with both economic and medical value. Mycelia were isolated from the fruiting bodies of L. nuda and were used to prepare the protoplast monokaryons. One hundred and fifteen monokaryons were obtained and their mating types were determined using somatic incompatibility tests. Protoplast monokaryons segregated into either the A₁B₁ or the A₂B₂ mating types. Inter-simple sequence repeats and sequence-related amplified polymorphism fingerprinting were used to analyse the mating types of these protoplast monokaryons and 16 sequence-characterised amplified region primers were developed to efficiently differentiate between the monokaryon mating types. Multiplex PCR analyses were also established. The data presented here outline a method for the precise and rapid identification of protoplast monokaryon mating types, which has the promise to shorten the period required for conventional crossbreeding.     

Isolation of the <Emphasis Type="Italic">B</Emphasis> incompatibility factor mutants in <Emphasis Type="Italic">Pleurotus ostreatus</Emphasis>

 B incompatibility factor mutants (Bmut) in Pleurotus ostreatus were recovered from common-B mating heterokaryons resulted from matings between wild-type monokaryons with different A but the same B factors (A1B2 and A2B2) after NTG mutagenesis. The mutant monokaryons such as A1B2mut and A2B2mut were observed to have regularly uninucleated hyphal cells and to be compatible with each other. Matings between A1B2mut and A2B2mut monokaryons produced stable heterokaryons (A1B2mut + A2B2mut) that had binucleated hyphal cells with true clamp connections and formed normal fruit-bodies. Mating tests using basidiospore progeny from each of these heterokaryons revealed the bipolar mating pattern. Genetic analysis suggested that the mutation of B factor in P. ostreatus might occur in the B incompatibility factor genes. Received: August 3, 2001 / Accepted: January 18, 2002     

Morphological and cytological aspects of oidium formation in a basidiomycete,Pholiota nameko

Pholiota nameko produced abundant oidia on aerial hyphae from monokaryotic and dikaryotic test stocks, but oidia were rare on submerged hyphae. The oidia from the former stocks had a layer of hydrophobic protein between the cell wall and the inner cell membrane which was absent in the oidia from the latter. The only remarkable differences in the morphological features of the oidia from monokaryotic and dikaryotic mycelia was the slightly larger size of the latter. Observation of various test stocks on slide cultures revealed that about 80% of oidia were produced from the secondary branched hypha, and about 20% from the terminal hyphal, cell of the main hypha. In the former, the secondary hyphae were segmented to form several oidium cells; in the latter, a single or several oidia were formed at the terminal end of the main hypha. Most oidia from monokaryons and dikaryons had only one haploid nucleus, while the remainders were multinucleate. Among the stocks tested, most oidia had a DNA content with a haploid amount at the G1 phase of the cell cycle, but a few contained twice that amount corresponding to the G2 phase     

Identification and Functional Analysis of Pheromone and Receptor Genes in the B3 Mating Locus of Pleurotus eryngii

Pleurotus eryngii has recently become a major cultivated mushroom; it uses tetrapolar heterothallism as a part of its reproductive process. Sexual development progresses only when the A and B mating types are compatible. Such mating incompatibility occasionally limits the efficiency of breeding programs in which crossing within loci-shared strains or backcrossing strategies are employed. Therefore, understanding the mating system in edible mushroom fungi will help provide a short cut in the development of new strains. We isolated and identified pheromone and receptor genes in the B3 locus of P. eryngii and performed a functional analysis of the genes in the mating process by transformation. A genomic DNA library was constructed to map the entire mating-type locus. The B3 locus was found to contain four pheromone precursor genes and four receptor genes. Remarkably, receptor PESTE3.3.1 has just 34 amino acid residues in its C-terminal cytoplasmic region; therefore, it seems likely to be a receptor-like gene. Real-time quantitative RT-PCR (real-time qRT-PCR) revealed that most pheromone and receptor genes showed significantly higher expression in monokaryotic cells than dikaryotic cells. The pheromone genes PEphb3.1 and PEphb3.3 and the receptor gene PESTE3.3.1 were transformed into P5 (A3B4). The transformants were mated with a tester strain (A4B4), and the progeny showed clamp connections and a normal fruiting body, which indicates the proposed role of these genes in mating and fruiting processes. This result also confirms that PESTE3.3.1 is a receptor gene. In this study, we identified pheromone and receptor genes in the B3 locus of P. eryngii and found that some of those genes appear to play a role in the mating and fruiting processes. These results might help elucidate the mechanism of fruiting differentiation and improve breeding efficiency.     

Synchronous Activation of Cell Division by Light or Temperature Stimuli in the Dimorphic Yeast Schizosaccharomyces japonicus

Many fungi respond to light and regulate fungal development and behavior. A blue light-activated complex has been identified in Neurospora crassa as the product of the wc-1 and wc-2 genes. Orthologs of WC-1 and WC-2 have hitherto been found only in filamentous fungi and not in yeast, with the exception of the basidiomycete pathogenic yeast Cryptococcus. Here, we report that the fission yeast Schizosaccharomyces japonicus responds to blue light depending on Wcs1 and Wcs2, orthologs of components of the WC complex. Surprisingly, those of ascomycete S. japonicus are more closely related to those of the basidiomycete. S. japonicus reversibly changes from yeast to hyphae in response to environmental stresses. After incubation at 30°C, a colony of yeast was formed, and then hyphal cells extended from the periphery of the colony. When light cycles were applied, distinct dark- and bright-colored hyphal cell stripes were formed because the growing hyphal cells had synchronously activated cytokinesis. In addition, temperature cycles of 30°C for 12 h and 35°C for 12 h or of 25°C for 12 h and 30°C for 12 h during incubation in the dark induced a response in the hyphal cells similar to that of light. The stripe formation of the temperature cycles was independent of the wcs genes. Both light and temperature, which are daily external cues, have the same effect on growing hyphal cells. A dual sensing mechanism of external cues allows organisms to adapt to daily changes of environmental alteration.     

Trichoderma koningii as a potential parasite of sclerotia of Sclerotium rolfsii

Three different inoculum forms of Trichoderma koningii were tested in vitro for their ability to parasitize the sclerotia of Sclerotium rolfsii. Tests were conducted under two temperature regimes and three incubation periods. Wheat bran was proved to be the most potent inoculum form of the antagonist in reducing 0the viability of the sclerotia. Microscopical observations revealed the presence of hyphae, chlamydospores and conidia of T. koningii in the medullar tissues of the sclerotia. This is the first report of the effect of different inoculum forms of T. koningii on the sclerotia of S. rolfsii and of propagule (chlamydospores and conidia) formation of the antagonist inside the sclerotia of S. rolfsii.     

Diversity of <Emphasis Type="Italic">A</Emphasis> mating type in <Emphasis Type="Italic">Lentinula edodes</Emphasis> and mating type preference in the cultivated strains

Diversity of A mating type in Lentinula edodes has been assessed by analysis of A mating loci in 127 strains collected from East Asia. It was discovered that hypervariable sequence region with an approximate length of 1 kb in the A mating locus, spanning 5′ region of HD2-intergenic region-5′ region of HD1, could represent individual A mating type as evidenced by comprehensive mating analysis. The sequence analysis revealed 27 A mating type alleles from 96 cultivated strains and 48 alleles from 31 wild strains. Twelve of them commonly appeared, leaving 63 unique A mating type alleles. It was also revealed that only A few A mating type alleles such as A1, A4, A5, and A7 were prevalent in the cultivated strains, accounting for 62.5% of all A mating types. This implies preferred selection of certain A mating types in the process of strain development and suggests potential role of A mating genes in the expression of genes governing mushroom quality. Dominant expression of an A mating gene HD1 was observed from A1 mating locus, the most prevalent A allele, in A1-containing dikaryons. However, connections between HD1 expression and A1 preference in the cultivated strains remain to be verified. The A mating type was highly diverse in the wild strains. Thirty-six unique A alleles were discovered from relatively small and confined area of mountainous region in Korean peninsula. The number will further increase because no A allele has been recurrently observed in the wild strains and thus newly discovered strain will have good chances to contain new A allele. The high diversity in small area also suggests that the A mating locus has evolved rapidly and thus its diversity will further increase.     

The constitution of incompatibility factor and mating characteristics of spore isolates in a bipolar mushroom, <Emphasis Type="Italic">Pholiota nameko</Emphasis>

Pholiota nameko is a wood-rotting edible mushroom that carries a bipolar A incompatibility factor gene. The linkage analysis of the multiple allelomorphic A factor gene demonstrated that sexual reproduction produced a monospore isolate carrying a new A factor gene in addition to two parental mating types of isolates. However, 10%–30% of the modified monospore isolates could not produce a dikaryon with both of the parental monokaryons by crossing. It is concluded that the bipolar A incompatibility factor gene of P. nameko is constituted of two functional subunits, Aα and Aβ, which might be successively located beside each other with an apparent genetic distance of 0.3 centi-Morgan between them on the same chromosome. Further, some monospore isolates that did not conjugate with both parental monokaryons could produce dikaryons with different monokaryotic stocks with either one of the parental mating types. This result suggests that the crossing capability of these isolates were essentially those for one of the mating types of the parental monokaryons, but that their function for mating activity was made partially by unequal crossing-over in the process of sexual recombination. Received: May 1, 2001 / Accepted: December 5, 2001     

The A mating-type genes of the mushroom Coprinus cinereus are not differentially transcribed in monokaryons and dikaryons

Summary The A mating type factor of Coprinus cinereus regulates part of a developmental sequence that leads to the conversion of the asexual monokaryon into the fertile dikaryon. The A42 factor is a complex of seven genes, at least four of which are involved in determining the specificity of mating interactions. In this report we show that the A42 genes are constitutively expressed in both monokaryons and dikaryons. This has important implications with respect to intracellular recognition of a compatible mating, which requires an interaction between proteins already present within the cells of the mating partners, and for the subsequent maintenance of dikaryotic growth.     

Expression of A mating type genes of Coprinus cinereus in a heterologous basidiomycete host

The A mating factor of Coprinus cinereus determines compatibility in mating by regulating part of a developmental sequence that leads to dikaryon formation. The A genes that trigger development encode two different classes of homeodomain proteins, and for a successful mating, a protein of one class, HD 1, must interact with a protein of the other class, HD 2. In this report we show that C. cinereus A genes that encode HD 2 proteins, a2-1 and b2-1, can elicit A-regulated development in the heterologous host C. bilanatus. Transformation rates were very low, suggesting that the genes were poorly transcribed. The fact that the HD 2 genes are functionally expressed implies successful heteromultimeric association of putative DNA-binding proteins coded by the two Coprinus species. This interaction was sufficient to satisfy the need for different A factors in the formation of a fertile C. bilanatus dikaryon, but fertile dikaryons were more readily produced in matings with the a2-1 gene transformants. The C. cinereus A genes, b1-1 and d1-1, which encode HD1 proteins, were either not expressed or their proteins were non-functional in C. bilanatus. These experiments raise some interesting questions regarding HD1–HD2 protein interactions.     

Relationship between Monokaryotic Growth Rate and Mating Type in the Edible Basidiomycete Pleurotus ostreatus

The edible fungus Pleurotus ostreatus (oyster mushroom) is an industrially produced heterothallic homobasidiomycete whose mating is controlled by a bifactorial tetrapolar genetic system. Two mating loci (matA and matB) control different steps of hyphal fusion, nuclear migration, and nuclear sorting during the onset and progress of the dikaryotic growth. Previous studies have shown that the segregation of the alleles present at the matB locus differs from that expected for a single locus because (i) new nonparental B alleles appeared in the progeny and (ii) there was a distortion in the segregation of the genomic regions close to this mating locus. In this study, we pursued these observations by using a genetic approach based on the identification of molecular markers linked to the matB locus that allowed us to dissect it into two genetically linked subunits (matBα and matBβ) and to correlate the presence of specific matBα and matA alleles with differences in monokaryotic growth rate. The availability of these molecular markers and the mating type dependence of growth rate in monokaryons can be helpful for marker-assisted selection of fast-growing monokaryons to be used in the construction of dikaryons able to colonize the substrate faster than the competitors responsible for reductions in the industrial yield of this fungus.     

Light controls growth and development via a conserved pathway in the fungal kingdom

Light inhibits mating and haploid fruiting of the human fungal pathogen Cryptococcus neoformans, but the mechanisms involved were unknown. Two genes controlling light responses were discovered through candidate gene and insertional mutagenesis approaches. Deletion of candidate genes encoding a predicted opsin or phytochrome had no effect on mating, while strains mutated in the white collar 1 homolog gene BWC1 mated equally well in the light or the dark. The predicted Bwc1 protein shares identity with Neurospora crassa WC-1, but lacks the zinc finger DNA binding domain. BWC1 regulates cell fusion and repression of hyphal development after fusion in response to blue light. In addition, bwc1 mutant strains are hypersensitive to ultraviolet light. To identify other components required for responses to light, a novel self-fertile haploid strain was created and subjected to Agrobacterium-mediated insertional mutagenesis. One UV-sensitive mutant that filaments equally well in the light and the dark was identified and found to have an insertion in the BWC2 gene, whose product is structurally similar to N. crassa WC-2. The C. neoformans Bwc1 and Bwc2 proteins interact in the yeast two-hybrid assay. Deletion of BWC1 or BWC2 reduces the virulence of C. neoformans in a murine model of infection; the Bwc1-Bwc2 system thus represents a novel protein complex that influences both development and virulence in a pathogenic fungus. These results demonstrate that a role for blue/UV light in controlling development is an ancient process that predates the divergence of the fungi into the ascomycete and basidiomycete phyla.