Septal ultrastructure in Sirobasidium magnum and its taxonomic implications

The parenthesomes of Sirobasidium magnum (Sirobasidiaceae) are composed of arrays of ampulliform vesicles from whose bases microfibrils connect to bands of striated material in the dolipore orifices. This septal morphology and the associated character of a yeast haplophase are comparable to that found in species of the Tremellaceae and Filobasidiaceae. The similarity of these basic characteristics in these three families supports their separation into the suborder Tremellineae (species of the Tremellales with regular dolipore/parenthesome septa and mycelial monokaryons are grouped in the suborder Exidiineae).     

An ultrastructural study of septal development in hyphae of Polyporus biennis

A method was developed which allowed the ultrastructural study of septal formation in the basidiomycete Polyporus biennis. The method involved fixing and embedding single clamp connections. Clamp connections with septa at desired developmental stages were located by light microscopy. The septum grew by the incorporation of vesicles of wall material precursors. The rim of the developing septum was drawn centripetally inwards by a contracting collar of microfilaments within a darkly staining matrix. The inflation of the central pore swelling was governed by realigned microfilaments. The parenthesomes were formed, starting at the apex, by the utilization of the microfilament/matrix material lying along the septum. On completion of the parenthesomes a transient striated structure, governed by microfilaments, was formed in the pore channel and the areas enclosed by the parenthesomes. The maturation of the septum involved the laying down of ER along the septum and the occlusion of each end of the pore channel.     

An ultrastructural study of septal dissolution inSchizophyllum commune

A strain ofSchizophyllum commune carrying a mutation in theB-mating factor (B-mut) shows septal dissolution when grown at 30° C for 2 to 3 days. The septa are intact if the organism is grown at 25° C for the same time, but begin to break down within 1 h after transfer to 30° C. At the ultrastructural level the dolipore swelling is the first part of the septal apparatus to be degraded, closely followed by the disorganization of the parenthesomes. A progressive thinning of the septal cross-wall produces an enlargement of the septal aperture sufficient to allow the passage of nuclei. It appears that degradative enzymes are probably carried to the site of septal dissolution in vesicles derived from endoplasmic reticulum in the area of the septal apparatus.     

A frequently occurring mutation that blocks the expression of fruiting genes in Schizophyllum commune

Summary Fruiting in the basidiomycete Schizophyllum commune readily occurs in a homokaryon with constitutive mutations in both the A and B mating-type genes. Such a homokaryon frequently expresses a mutation, fbf, which completely blocks fruiting and leads to a somewhat faster growth rate. The mutation is unlinked to the A and B genes, frequently reverts to its wild-type allele, and is recessive with respect to fruiting in matings with wild-type homokaryons. The mutation suppresses the accumulation of a number of mRNAs which are regulated by the mating-types genes and are specific for fruiting. The expression of the Sc-3 gene, structurally related to two of the fruiting genes (Sc-1 and Sc-4) but not regulated by the mating-type genes, is unaffected.     

Mise en évidence de la dissolution des cloisons dolipores préalable à la migration nucléaire chez Dacrymyces stillatus Nees : Fries (Basidiomycete)

In the process of dicaryotisation of haploïd mycelium in Dacrymyces stillatus, the complex dolipore septum/parenthesomes desintegrates to open a way to the migrant nucleus. Ultrastructure images show that this disintegration begins first simultaneously at the level of parenthesomes and septal swellings, then at the level of the septum itself with the enlargement of its pore to enable the passage of the migrant nucleus, the role of which is to dicaryotize the cells of the haploïd mycelium.     

A note on Wallemia sebi

A review of the available information on the monotypic genus Wallemia supports the interpretation that W. sebi is probably a basidiomycete and may be a teleomorph. It has dolipore/parenthesome septa similar to those found in the suborder Tremellineae, except that the associated parenthesome vesicles are very small and composed of a single membrane. A conjectural interpretation is made that the spore chains, which are composed of repeating sets of four, are meiospores generated from a permanently diploid mother cell that divides repeatedly to generate meiocyte daughter cells.     

SEPTAL PORES IN THE HETEROBASIDIOMYCETIDAE,PUCCINIA GRAMINIS AND P. RECONDITA

The septal pores in uredial mycelium of Puccinia graminis and P. recondita lack the septal swelling and septal pore cap (dolipore-parenthosome configuration) typically associated with the pores of previously investigated Homobasidiomycetidae and the Tremellales among the Heterobasidiomycetidae. The pores in young hyphae of these two species of Puccinia are characterized by the presence of a cytoplasmic matrix which apparently occludes the pore and acts as a plug, thus preventing the migration of organelles from cell to cell. Large vesicles are typically present at the periphery of the pore matrix and the matrix may be very incompletely bounded by a membrane. Nuclei and other cytoplasmic structures migrate from cell to cell through an opening in the septum lateral to the pore. The available evidence indicates that this peripheral gap in the septum results from a breakdown of a portion of an initially complete septum rather than from incomplete septum formation. In addition to the centripetally formed septa, the hyphae of P. graminis and P. recondita are further compartmentalized by shallow infoldings of the lateral wall and limited unilateral septum formation. There is apparent free passage of cellular material between adjacent compartments.     

MUTATIONS AFFECTING HETEROKARYOSIS IN SCHIZOPHYLLUM COMMUNE

Mutations that affect the basic characteristics of heterokaryons of S. commune occur spontaneously and are preferentially selected in the common-A heterokaryon and in its homokaryotic mimics, strains carrying a mutated B factor or strains disomic for heteroallelic B factors. Nine independent mutations were compared: all segregate independently of A and B incompatibility factors, and at least 3 distinct loci, of which 2 are linked, are involved. None of the mutations is phenotypically expressed in the homokaryon or in the common-AB heterokaryon. All 9 mutations increase vegetative vigor of the common-A the effects of all the mutations are additive in both heteroallelic and homoallelic combinations. At least 1 type-II mutation also affects nuclear distribution in common-B heterokaryons. Type-II mutations appear to reduce common-A, common-B, and compatible heterokaryons to a single type unlike any of the normal heterokaryons. Pseudoclamping often persists for extended periods in modified homokaryons isolated from modified heterokaryons. Several cases of somatic recombination have been observed among components of modified heterokaryons.     

Trametes fumoso-avellanea (Aphyllophorales): a taxonomic study

The morphology, cultural features, sexuality, nuclear behavior and dolipore/parenthesome apparatus of Trametes fumoso-avellanea were studied in an attempt to settle its taxonomic position. Data provided by the micromorphology, biological (non-morphological) characters and the non-perforate parenthesomes sustain its redisposition in the genus Trichaptum Murr.     

ULTRASTRUCTURE OF THE METABASIDIUM OF AURICULARIA FUSCOSUCCINEA

Maturation of the metabasidium of Auricularia fuscosuccinea was followed with light microscopy and transmission electron microscopy. The basidium was divided into four compartments by septa which developed centripetally as in hyphae. Each septum formed a septal pore apparatus with imperforate pore caps. A band of electron-dense material was situated in the middle of the septal pore. There was a large increase in the volume of cytoplasm, excluding vacuoles, in each compartment during sterigmal outgrowth. Compartments were evacuated in basipetal sequence and vacuole enlargement began at the base of a compartment only when sterigmal formation was well advanced. The septal pore apparatus was intact until late in maturation of a compartment when septal swellings occluded the pore. The metabasidial wall was differentiated from those of other hymenial and subhymenial cells. The pattern of basidial maturation is compared with that in other phragmobasidiate and holobasidiate fungi. Use of the septal pore apparatus for phylogenetic and taxonomic purposes is discussed, as is the concept of primary and adventitious septa.     

The genome of the basal agaricomycete Xanthophyllomyces dendrorhous provides insights into the organization of its acetyl-CoA derived pathways and the evolution of Agaricomycotina

Background

Xanthophyllomyces dendrorhous is a basal agaricomycete with uncertain taxonomic placement, known for its unique ability to produce astaxanthin, a carotenoid with antioxidant properties. It was the aim of this study to elucidate the organization of its CoA-derived pathways and to use the genomic information of X. dendrorhous for a phylogenomic investigation of the Basidiomycota.

Results

The genome assembly of a haploid strain of Xanthophyllomyces dendrorhous revealed a genome of 19.50 Megabases with 6385 protein coding genes. Phylogenetic analyses were conducted including 48 fungal genomes. These revealed Ustilaginomycotina and Agaricomycotina as sister groups. In the latter a well-supported sister-group relationship of two major orders, Polyporales and Russulales, was inferred. Wallemia occupies a basal position within the Agaricomycotina and X. dendrorhous represents the basal lineage of the Tremellomycetes, highlighting that the typical tremelloid parenthesomes have either convergently evolved in Wallemia and the Tremellomycetes, or were lost in the Cystofilobasidiales lineage. A detailed characterization of the CoA-related pathways was done and all genes for fatty acid, sterol and carotenoid synthesis have been assigned.

Conclusions

The current study ascertains that Wallemia with tremelloid parenthesomes is the most basal agaricomycotinous lineage and that Cystofilobasidiales without tremelloid parenthesomes are deeply rooted within Tremellomycetes, suggesting that parenthesomes at septal pores might be the core synapomorphy for the Agaricomycotina. Apart from evolutionary insights the genome sequence of X. dendrorhous will facilitate genetic pathway engineering for optimized astaxanthin or oxidative alcohol production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1380-0) contains supplementary material, which is available to authorized users.     

Identification of new mutant alleles of <Emphasis Type="Italic">pcc1</Emphasis> in the homobasidiomycete <Emphasis Type="Italic">Coprinopsis cinerea</Emphasis>

It is known that a mutation in the pcc1 gene of the homobasidiomycete Coprinopsis cinerea leads to pseudoclamp development and fruit-body formation in a homokaryon without mating. In this study we characterize two strains that were reported previously to exhibit pseudoclamp development and fruiting without mating, together with six new mutants exhibiting the same phenotype. A frame-shift, nonsense, or intron splice site mutation was present within pcc1 in each of the eight mutants. The results suggest that the Pcc1 protein is a key element in a pathway(s) leading to pseudoclamp development and fruiting.     

Bulleromyces genus novum (Tremellales), a teleomorph for Bullera alba,and the occurrence of mating in Bullera variabilis

Mating is observed in Bullera alba and B. variabilis, resulting in the formation of dikaryotic mycelium with clamps, haustorial branches, and lateral and terminal dikaryotic, clavate, lageniform or subglobose cells. These cells develop in B. alba into tremellaceous phragmobasidia. Karyogamy has been observed in young non-divided basidia. Germination of the phragmobasidia occurred by acropetal chains of yeast cells, ballistospores or hyphae. Septal pores are dolipores with parenthesomes made up of U-shaped vesicles (Tremellales type). For the teleomorph of B. alba a new genus, Bulleromyces, is proposed, with only one species, viz. Bulleromyces albus.     

Interspecific competitive ability of homokaryotic and heterokaryotic wood decay basidiomycetes

The role of the homokaryotic life stage in the dynamics of fungal communities is relatively unknown. However, homokaryons are thought to be only a temporary stage and are therefore not generally used in ecological experiments with fungi. In this study, the relative competitive ability and growth rates of homokaryons and heterokaryons of wood decay fungi were tested to assess the potential role of homokaryons in community dynamics. A homokaryon and a heterokaryon of each of four species (Aleurodiscus lividocoeruleus, Peniophora sp. 1, Peniophora sp. 2 and Pereniporia medulla‐panis) were assessed for their competitive abilities on an agar medium. The relationship between nuclear status and competitive ability varied between species. The homokaryon of Peniophora sp. 2 was competitively superior to its heterokaryon, whereas the homokaryon of Peniophora sp. 1 was inferior to its heterokaryon. A hierarchy of competitive abilities of each isolate revealed that Pereniporia medulla‐panis homokaryon = P. medulla‐panis heterokaryon > Peniophora sp. 1 heterokaryon > Peniophora sp. 2 homokaryon > Peniophora sp. 2 heterokaryon > A. lividocoeruleus heterokaryon = A. lividocoeruleus homokaryon. This experiment indicates that homokaryons as well as heterokaryons have the potential to influence community structure through competitive effects.     

A new basidiomycetous anamorph genus with cruciform conidia

An undescribed hyphomycete with clamped septa, conidiophores aggregated in sporodochia, and cruciform, aseptate conidia apically borne on clamps was found on a branch of a deciduous tree lying on the ground. The septal pores are dolipores with perforate parenthesomes indicating a homobasidiomycetous relationship. The morphology and ecology of the fungus are discussed in relation to morphologically similar anamorphic basidiomycetes. Because the fungus could not be classified in any known genus, the new genus and new speciesCruciger lignatilis is proposed to accommodate this species.     

SEPTAL PORE STRUCTURE IN TRICHODERMA SATURNISPORUM

Electron microscopic observations of a previously undescribed ascomycetous septal pore structure are presented and discussed. Hyphal septa and septa at the base of phialides in the hyphomycete, Trichoderma saturnisporum Hammill, developed a membrane-bounded, electron-opaque septal pore body which was fine-structurally similar to Woronin bodies. Within a septal pore body, several electron-transparent layers appeared to develop centripetally from the septal pore rim. The number of layers observed varied from two to about five, with lower numbers being more frequently observed. The electron-transparent layers perhaps functioned as a vinculum, binding the septal pore body in place. Questions about the origin and function of septal pore bodies are discussed.     

Control of yeast cell type by the mating type locus: I. Identification and control of expression of the a-specific gene BAR1

The MATα allele of the yeast mating type locus confers the α mating phenotype and contains two complementation groups, MATα1 and MATα2. The α1–α2 hypothesis proposes that MATα1 is a positive regulator of α-specific genes and that MATα2 is a negative regulator of a-specific genes. According to this hypothesis, matα2 mutants, which are defective in mating and in production of extracellular α-factor, express both a-specific functions (because they lack MATα2 product) and α-specific functions (because they contain MATα1 product). Failure to produce extracellular α-factor results from antagonism between these functions; in particular, because α-factor (an α-specific function) is degraded by an a-specific function. If this view is correct, matα2 mutants should acquire the ability to produce α-factor if they also carry a defect in the gene(s) responsible for α-factor degradation. We have isolated a derivative of a matα2 mutant that produces α-factor and have characterized the suppressor mutation in this strain. (1) This strain carries a mutation (bar1-1) tightly linked to HIS6 (on chromosome IX) that allows matα2 mutants to produce α-factor. (2) It does not allow matα1 mutants to produce α-factor. (3) Haploids of the a mating type bearing the bar1-1 mutation still mate, but are unable to act as a barrier to the diffusion of α-factor. MATa bar1-1 cells display increased sensitivity to α-factor. (4) A mutation (sst1?2) that causes increased sensitivity to α-factor is allelic to bar1-1 and also allows α-factor synthesis by matα2 mutants. The ability of matα2 bar1 double mutants to produce extracellular α-factor indicates that matα2 mutants do produce α-factor but that it is degraded by the Barrier function. These results suggest that BAR1 is normally expressed only in a cells, and is negatively regulated in α cells by the MATα2 product.     

A comparison of the mutation spectra of Menkes disease and Wilson disease

The genes for two copper-transporting ATPases, ATP7A and ATP7B, are defective in the heritable disorders of copper imbalance, Menkes disease (MNK) and Wilson disease (WND), respectively. A comparison of the two proteins shows extensive conservation in the signature domains, with amino acid identities outside of the conserved domains being limited. The mutation spectra of MNK and WND were compared to confirm and refine further regions critical for normal function. Mutations were found to be relatively widespread; however, the majority was concentrated within defined functional domains and membrane-spanning segments, reinforcing the importance of these regions for protein function. Of the total published point mutations in ATP7A, 23.0% are splice-site, 20.7% nonsense, 17.2% missense, and 39.1% small insertions/deletions. There is a high prevalence (58.2%) of missense mutations in ATP7B. For the other mutations in ATP7B, 7.4% are splice-site, 7.4% nonsense, and 27.0% small insertions/deletions. A region of possible importance is the intervening sequence between the last copper-binding domain and the first transmembrane helix, as this region has a high percentage of MNK mutations. Similarly, the region containing the ATP-binding domain has 24.6% of all WND mutations. The study of mutation locations is useful for defining critical regions or residues and for efficient molecular diagnosis.Electronic Supplementary Material Supplementary material is available for this article if you access the article at .     

Direct studies of nuclear movements in Schizophyllum commune

Summary Measurements of nuclear positions in apical cells of homokaryotic mycelia and dikaryotic mycelium of Schizophyllum commune showed that nuclei occupied a near central position in most cases. Forward nuclear movements observed in living hyphal apices occurred at rates within the range of hyphal growth and could account for the maintenance of centrally located nuclei. Opposed nuclear movements followed mitosis and greatly exceeded the rate of hyphal growth. Septum disruption and rapid nuclear movements characterized an A _xB_mut homokaryon. Neither cytoplasmic streaming nor actively participating granules or filaments could account for any of these nuclear movements.     

Characterisation of fission yeast alp11 mutants defines three functional domains within tubulin-folding cofactor B

The proper folding of tubulins prior to their incorporation into microtubules requires a group of conserved proteins called cofactors A to E. In fission yeast, homologues of these cofactors (at least B, D and E) are necessary for the biogenesis of microtubules and for cell viability. Here we show that the temperature-sensitive alp11-924 mutant, which is defective in the cofactor B homologue, contains an opal nonsense mutation, which results in the production of a truncated Alp11^B protein (Alp11_1–118). We isolated a tRNA^Trp gene as a multicopy suppressor of this mutation, which rescues alp11-924 by read-through of the nonsense codon. The truncated Alp11_1–118 protein lacks the C-terminal half of Alp11^B, consisting of a central coiled-coil region and the distal CLIP-170 domain found in a number of proteins involved in microtubule functions. Both of these domains are required for the maintenance of microtubule architecture in vivo. Detailed functional analyses lead us to propose that Alp11^B comprises three functional domains: the N-terminal half executes the essential function, the central coiled-coil region is necessary for satisfactory maintenance of cellular α-tubulin levels, and the C-terminal CLIP-170 domain is required for efficient binding to α-tubulin. Received: 29 November 1999 / Accepted: 18 April 2000