The mechanism of ascus firing – Merging biophysical and mycological viewpoints

The actively discharging ascus is the unique spore-bearing cell that is responsible to dispatch spores into the atmosphere. From a physical perspective, this type of ascus is a sophisticated pressure gun that reliably discharges the spores at an extremely high velocity, without breaking apart. We identify four essential steps in discharge of spores whose order and timing may vary across species. First, asci that fire are mature, so a cue must be present that prevents discharge of immature spores and signals maturity. Second, pressure within the ascus serves to propel the spores forward; therefore a mechanism should be present to pressurize the ascus. Third, in ostiolate fruiting bodies (e.g. perithecia), the ascus extends through an opening to fire spores into the air. The extension process is a relatively unique aspect of the ascus and must be structurally facilitated. Fourth, the ascus must open at its tip for spore release in a controlled rupture. Here we discuss each of these aspects in the context of understanding the process of ascus and fruiting body function. While there is great diversity among fungi, we focus on discharge in a few model species, and then discuss how this framework may vary in other fungi. Our goal is to tie the physiological and molecular studies of ascus function with concepts in engineering that dictate structure.     

New information on the mechanism of forcible ascospore discharge from Ascobolus immersus

Many ascomycete fungi spurt their spores from asci pressurized by osmosis. This paper explores the details of this process in the coprophilous species Ascobolus immersus, through a combination of biomechanical and biochemical experiments, and mathematical modeling. A. immersus forms large asci that expel 8 spores as a single, mucilage-embedded projectile. Measurements of ascus turgor using a microprobe attached to a strain gauge showed a pressure of 0.3 MPa or 3 atm. Analysis of ascus sap using GC/MS identified glycerol as a major osmolyte, accounting for 0.1 MPa of the osmotic pressure within the ascus sap. A mathematical model indicated that a pressure of 0.2 MPa would be sufficient to propel the cluster of ascospores over the distance measured in previous studies. The difference between the measured and predicted pressures is ascribed to loss of pressure as the spores are forced through the tip of the open ascus.     

紫红曲有性阶段的研讨

紫红曲因在其菌丝体及孢子生长过程中,具有合成某些药用及色素物质功能而被重视。这些物质在东方国家中早被用于医疗及食物染色方面。本文在阐明该菌闭襄壳内数目众多的子囊孢子系来自多数子囊,其子囊壁在发育早期即已消失。具8个子囊孢子的单一子囊的确时有发生,所以红曲属名拉丁文原意“单一子囊”,虽然有时极易被置凝,但基本是正确的。此菌之另一显著特性为一次性结合可产生多数产囊丝钩及发育后的闭囊壳。此现象导致孢子形成以子囊孢子数目远超过分生孢子。     

Ascospore formation and morphology in two species of the genusErysiphe remaining immature on the living host plant

Erysiphe cumminsiana andE. galeopsidis, which have immature asci in the current season, have been recorded from Japan, but the ascospores of both fungi have not been described. In the present experiments, some observations before and after overwintering were made on the cleistothecia ofE. cumminsiana on three species ofCacalia and two species ofLigularia, andE. galeopsidis onGeranium thunbergii. After overwintering, the former fungus developed six to eight, rarely four spores in an ascus and the latter fungus always four spores in an ascus. Their teleomorphic characteristics including those of ascospores are also described.     

Programmed ascospore death in the homothallic ascomycete Coniochaeta tetraspora

Immature asci of Coniochaeta tetraspora originally contain eight uninucleate ascospores. Two ascospore pairs in each ascus survive and mature, and two die and degenerate. Arrangement of the two ascospore types in individual linear asci is what would be expected if death is controlled by a chromosomal gene segregating at the second meiotic division in about 50% of asci. Cultures originating from single homokaryotic ascospores or from single uninucleate conidia are self-fertile, again producing eight-spored asci in which four spores disintegrate, generation after generation. These observations indicate that differentiation of two nuclear types occurs de novo in each sexual generation, that it involves alteration of a specific chromosome locus, and that the change occurs early in the sexual phase. One, and only one, of the two haploid nuclei entering each functional zygote must carry the altered element, which is segregated into two of the four meiotic products and is eliminated when ascospores that contain it disintegrate. Fusion of nuclei cannot be random-a recognition mechanism must exist. More study will be needed to determine whether the change that is responsible for ascospore death is genetic or epigenetic, whether it occurs just before the formation of each ascus or originates only once in the ascogonium prior to proliferation of ascogenous hyphae, and whether it reflects developmentally triggered alteration at a locus other than mating type or the activation of a silent mating-type gene that has pleiotropic effects. Similar considerations apply to species such as Sclerotinia trifoliorum and Chromocrea spinulosa, in which all ascospores survive but half the spores in each ascus are small and self-sterile. Unlike C. tetraspora, another four-spored species, Coniochaetidium savoryi, is pseudohomothallic, with ascus development resembling that of Podospora anserina.     

Phylogenetic origins of two cleistothecial fungi, Orbicula parietina and Lasiobolidium orbiculoides, within the operculate discomycetes

Parsimony, maximum-likelihood and Bayesian analyses of SSU rDNA sequences of representative taxa of Pezizomycetes, Eurotiomycetes, Dothideomycetes, Leotiomycetes and Sordariomycetes, all strongly support the cleistothecial fungi Orbicula parietina and Lasiobolidium orbiculoides to be of pezizalean origin. Previous hypotheses of close affinities with cleistothecial or highly reduced fungi now placed in the Thelebolales, Eurotiales or Onygenales are rejected. Orbicula parietina and L. orbiculoides are deeply nested within Pyronemataceae (which subsumes the families Ascodesmidaceae, Glaziellaceae and Otideaceae). LSU rDNA sequences suggest that Orbicula is nested within the apothecia-forming genus Pseudombrophila (including Nannfeldtiella and Fimaria) and that L. orbiculoides is closely related. Ascodesmis and Lasiobolus, which have been suggested as closely related to Orbicula and Lasiobolidium, are identified as a sister lineage to the Pseudombrophila lineage. Cleistothecial forms that have lost the ascus operculum and ability to discharge spores actively have evolved at least once in the Pseudombrophila lineage. Some species of Pseudombrophila produce subglobular ascomata initials that are closed early in development and open only in the mid-mesohymenial phase. We hypothesize that, in the Pseudombrophila lineage, ascomata forms that never open are derived from ascomata that open late in development. The placement of O. parietina and L. orbiculoides within Pseudombrophila is supported by morphological characters, ecology and temperature optima for fruiting.     

Sexual process in eight-spored ascus-forming yeast, Schizosaccharomyces japonicus I. Culture medium for the synchronous sexual process

The sexual process of Schizosaccharomyces japonicus consistsof sexual flocculation, zygote formation, eight-spored ascusformation and liberation of spores from the ascus. The culturemedium in which this sexual process took place synchronouslywas prepared. For the completion of the sexual process, glucosewas essential and inorganic salts and vitamins were also required.Elimination of the nitrogen source stimulated the rate of sporeformation. The temporal relationship among the sexual eventswas also elucidated: sexual flocculation, zygote formation,ascus formation and spore liberation occurred at 4, 7, 13 and20 hr, respectively, after transfer to the medium for the synchronoussexual process. (Received May 11, 1978; )     

Convergence in ascospore discharge mechanism among pyrenomycete fungi based on 18S ribosomal RNA gene sequence.

Fungi of the class Pyrenomycetes (Ascomycotina) form a morphological series ranging from those that shoot ascospores (sexual spores) forcibly from the ascus (spore sac) to fungi that ooze ascospores or have no obvious mechanism for ascospore release. Did forcible ascospore discharge evolve within these pyrenomycetes, or has it been lost in the group? We determined the sequences of the 18S ribosomal RNA gene from three fungi and used these, along with six sequences from our previous work and three sequences from GenBank, to infer the phylogeny of 12 ascomycetes with various ascospore discharge mechanisms. The 1720 base pairs of sequence data per fungus yielded 361 variable sites, 198 phylogenetically informative sites, and a single most parsimonious tree requiring 562 nucleotide changes. The tree shows that the capacity to shoot ascospores into the air has been lost or, less probably, gained repeatedly and independently. Species lacking forcible ascospore discharge are intercalated among three lineages of species with forcible discharge. In this tree, seven of the nine internal branches appeared in 95% or more of 500 bootstrap replicates. A tree uniting the fungi with forcible ascospore discharge into a monophyletic group required 45 additional steps and fit significantly less well with the data than the most parsimonious tree, based on a maximum likelihood test. Two of the fungi whose sequence we determined, Pseudallescheria boydii and Sporothrix schenckii, are not closely related to one another, even though both are human pathogens and both are from pyrenomycete lineages lacking forcible ascospore discharge. Using the well-resolved, most parsimonious tree, we inferred base substitution patterns in the 18S rRNA. The transition-to-transversion ratio was 1.9. Of all 12 possible substitutions, 29% were from U to C. At sites corresponding to yeast stem positions, A to G transitions were frequent, perhaps compensating for some of the U to C changes, and maintaining secondary structure base pairing (A to G:U to C = 3:4). In loop or bulge positions without secondary structure base pairing, U to C transitions were still frequent, but A to G transitions were rare (A to G:U to C = 1:5).     

Genetic variation in the orientation of nuclear spindles during the development of asci in Neurospora

The effect of a single mutant gene, peak-2, in Neurospora crassa is to alter the ascus from one in which the eight spores are linearly arranged to one in which the eight spores are arranged non-linearly. The same gene transferred to N. tetrasperma determines an ascus similar to that of the mutant N. crassa but in striking contrast to the normal ascus for N. tetrasperma, which includes four heterokaryotic spores, linearly arranged. A comparative cytological study has been made on asci from both species, including material from zygotes that are homozygous normal, heterozygous for peak-2, and homozygous for peak-2. The observed morphogenetic differences are closely related to differences in nuclear spindle orientation, which in turn are correlated with the geometry of the cell. The observations are shown to bear on the basis of pseudohomothallism in N. tetrasperma. Speculations on the importance of the cell wall in relation to spindle orientation are presented. Differences of expression of peak-2 in the different species are discussed.     

A sensitive method for examining whole-cell biochemical composition in single cells of filamentous fungi using synchrotron FTIR spectromicroscopy

Cell function is related to cell composition. The asexual state of filamentous fungi (molds and mildews) has two main life cycle stages: vegetative hyphae for substrate colonization and nutrient acquisition, and asexual spores for survival and dispersal. Hyphal composition changes over a few tens of microns during growth and maturation; spores are different from hyphae. Most biochemical analyses are restricted to studying a few components at high spatial resolution (e.g. histochemistry) or many compounds at low spatial resolution (e.g. GC-MS). Synchrotron FTIR spectromicroscopy can be used to study fungal cell biology by fingerprinting varieties of carbohydrates, proteins, and lipids at about 6 microm spatial resolution. FTIR can distinguish fungal species and changes during hyphal growth, and reveals that even fungi grown under optimal vs mildly stressed conditions exhibit dramatic biochemical changes without obvious morphological effects. Here we compare hypha and spore composition of two fungi, Neurospora and Rhizopus. There are clear biochemical changes when Neurospora hyphae commit to spore development, during spore maturation and following germination, many of which are consistent with results from molecular genetics, but have not been shown before at high spatial resolution. Rhizopus spores develop within a fluid-containing sporangium that becomes dry at maturity. Rhizopus spores had similar protein content and significantly more carbohydrate than the sporangial fluid, both of which are novel findings.     

A NEW SPECIES OFCATILLARIAFROM COASTAL MEDITERRANEAN REGIONS

A revision of several specimens ofCatillaria mediterraneaHafellner revealed that two species can be recognized:C. mediterraneas. str., which has 8–16 spores per ascus, grows on foliose and fruticose lichens, and has a Mediterranean-Macaronesian, montane distribution, andC. praedictaTretiach & Hafellner sp. nov., which has (16–)24–32(–48) spores per ascus, occurs on shrubs in coastal Mediterranean maquis, and has a Mediterranean, maritime distribution. The systematic affinities of the two species with the 8-sporedC. nigroclavata(Nyl.) Schuler are briefly discussed.     

Expansion of the Stictidaceae by the addition of the saxicolous lichen-forming genus Ingvariella

The monotypic, lichen-forming genus Ingvariella originally was segregated from Diploschistes and placed within the Thelotremataceae (Ostropales) based on aspects of exciple morphology. However, the I+ hymenium and amyloid ascus wall suggest affinities to families other than the Thelotremataceae. To assess the identity of Ingvariella and to investigate its placement within the Ostropales, we inferred phylogenetic relationships of I. bispora by comparison of mtSSU rDNA and nuLSU rDNA sequences for 59 species encompassing a broad array of ostropalean fungi by means of Bayesian, maximum likelihood and weighted maximum parsimony methods. Here we report that Ingvariella is a member of the Stictidaceae, sister to the mainly saprotrophic genus Cryptodiscus. The inclusion of the first saxicolous lichen-forming fungus within this family expands the broad ecological diversity of the Stictidaceae, where saprotrophic fungi, corticicolous lichen-forming fungi and lichenized and non-lichenized conspecific taxa have been described previously. We also present new insights into the relationships among other families within the Ostropales.     

Influences of environmental factors on fruiting body induction,development and maturation in mushroom-forming fungi

Mushroom-forming fungi (restricted to basidiomycetous fungi in this review) differentiate by sensing several environmental factors for fruiting body formation. For fruiting body induction, nutrient, temperature and light conditions are critical environmental factors. Higher nitrogen and carbon sources in the media will suppress fruiting body induction in many mushroom-forming fungi, with induction being triggered by lower nitrogen and carbon concentrations. Low temperature or temperature downshift is another critical influencing factor for fruiting body induction in many cultivated mushrooms, such as Flammulina velutipes, Lentinula edodes, and Volvariella volvacea. Fungal response toward starvation and cold involves the production of sexual spores as the next generation. Species like F. velutipes and Coprinopsis cinerea can form fruiting bodies in the dark; however, light accelerates fruiting body induction in some mushroom-forming fungi. Remarkably, fruiting bodies formed in the dark have tiny or no pileus on heads (called dark stipe, pinhead fruiting body, or etiolated stipe). Light is essential for pileus differentiation in many, but not all mushroom species; one exception is Agaricus bisporus. Mushrooms have positive phototropism and negative gravitropism for effective dispersal of spores. Carbon dioxide concentrations also affect fruiting body development; pileus differentiation is suppressed at a high concentration of carbon dioxide. Thus, the pileus differentiation system of mushrooms may allow the most effective diffusion of spores. Full expansion of the pileus is followed by pileus autolysis or senescence. In C. cinerea, pileus autolysis occurs during spore diffusion. Fruiting body senescence, browning of gill, and softening occur after harvesting in several mushroom species. Fruiting body induction, development, and maturation in mushroom-forming fungi are discussed in this review.     

The number of nuclei in basidiospores of 63 species of ectomycorrhizal Homobasidiomycetes

The production of even a limited number of heterokaryotic spores would be advantageous for establishing new individuals after long distance dispersal. While Suillus and Laccaria species are known to produce binucleate, heterokaryotic spores, this condition is poorly studied for most ectomycorrhizal fungi. To begin addressing this matter the number of nuclei in basidiospores was recorded from 142 sporocarps in 63 species and 20 genera of ectomycorrhizal (EM) fungi. The mean proportion of binucleate basidiospores produced by sporocarps within a species ranged from 0.00 to 1.00, with most genera within a family showing similar patterns. Basidiospores from fungi in Amanita, Cortinariaceae and Laccaria were primarily binucleate but were likely still homokaryotic. Basidiospores from fungi in Boletaceae, Cantharellus, Rhizopogonaceae, Russulaceae, Thelephorales and Tricholoma were primarily uninucleate, but binucleate basidiospores were observed in many genera and in high levels in Boletus. Further research is needed to relate basidiospore nuclear number to reproductive potential in ectomycorrhizal species.     

Nuclear and genome dynamics in multinucleate ascomycete fungi

Genetic variation between individuals is essential to evolution and adaptation. However, intra-organismic genetic variation also shapes the life histories of many organisms, including filamentous fungi. A single fungal syncytium can harbor thousands or millions of mobile and potentially genotypically different nuclei, each having the capacity to regenerate a new organism. Because the dispersal of asexual or sexual spores propagates individual nuclei in many of these species, selection acting at the level of nuclei creates the potential for competitive and cooperative genome dynamics. Recent work in Neurospora crassa and Sclerotinia sclerotiorum has illuminated how nuclear populations are coordinated for fungal growth and other behaviors and has revealed both molecular and physical mechanisms for preventing and policing inter-genomic conflict. Recent results from population-level genomic studies in a variety of filamentous fungi suggest that nuclear exchange between mycelia and recombination between heterospecific nuclei may be of more importance to fungal evolution, diversity and the emergence of newly virulent strains than has previously been recognized.     

木材腐朽菌在森林生态系统中的功能

木材腐朽菌是森林生态系统的重要组成部分,在森林生态系统中起着极为重要的降解还原作用,主要包括担子菌门非褶菌目、子囊菌门盘菌纲和半知菌类的部分真菌,能全部或部分降解木材中的木质素、纤维素和半纤维素,其降解机制有3种：白色腐朽、褐色腐朽和软腐朽．木材腐朽菌与生态系统中其它生物关系密切,为很多昆虫、鸟类提供营养,有些昆虫也能使木腐菌得到传播．保护木材腐朽菌的生物多样性是保护森林生态系统、维护生态系统健康的重要因素．     

子囊菌交配型位点与交配型基因研究进展

有性生殖是真菌的生殖方式之一,是真菌遗传重组的重要驱动力。交配型(mating-type,MAT)位点控制真菌性别,在有性生殖过程中起决定性作用。不同类型真菌MAT位点的基因组成、排列方式和编码蛋白不尽相同。近年来,MAT位点和MAT基因的功能与调控网络研究进展较快。本文对子囊菌交配型位点的基因组成及分布、MAT基因的功能、MAT位点与有性生殖调控通路的关系等进行了综述。     

Mycopathogens of Mites in Poland - A Review

Studies on diseases of mites caused by entomopathogenic fungi have been undertaken in Poland about a half a century later than in West European countries. Nevertheless, during the last 30 years almost 40 species of arthropod pathogenic fungi have been identified, among them 23 species of Entomophthorales, 12 species of Hyphomycetales and four species of ectoparasitic Laboulbeniales. The most common are representatives of the genus Hirsutella , found both on phytophagous and parasitic or predaceous mites. Percentage infection by H. thompsonii in populations of some tarsonemid and eriophyid mites increases slowly from the end of spring reaching a maximum of 30-60% in August-September. Neozygites floridana is a common entomophthoralean species causing epizootics in spider mite populations; infection also peaks in late summer and autumn. The most numerous fungal taxa have been identified from the resting spores produced internally within the body of the host and show features of the genus Tarichium . From the total number of the taxa recorded within these studies, 22 were described as species new to science. Current research on invertebrate pathology in Poland includes many groups of mites and among their pathogens fungal diseases seem to be the most common.