Editorial

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Open AccessEditorial

Special Issue: Formation and Function of Fungal Ascospores

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Aaron M. Neiman

J. Fungi 2021, 7(8), 618; https://doi.org/10.3390/jof7080618 - 29 Jul 2021

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Abstract

I wish to thank all of the authors who contributed papers to this Special Issue on the Formation and Function of Ascospores [...] Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

Research

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Open AccessCommunication

Role of RIM101 for Sporulation at Alkaline pH in Ashbya gossypii

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Lisa Wasserstrom

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Jürgen Wendland

J. Fungi 2021, 7(7), 527; https://doi.org/10.3390/jof7070527 - 30 Jun 2021

Cited by 3 | Viewed by 1363

Abstract

Microorganisms need to sense and adapt to fluctuations in the environmental pH. In fungal species, this response is mediated by the conserved pacC/RIM101 pathway. In Aspergillus nidulans, PacC activates alkaline-expressed genes and represses acid-controlled genes in response to alkaline pH [...] Read more.

Microorganisms need to sense and adapt to fluctuations in the environmental pH. In fungal species, this response is mediated by the conserved pacC/RIM101 pathway. In Aspergillus nidulans, PacC activates alkaline-expressed genes and represses acid-controlled genes in response to alkaline pH and has important functions in regulating growth and conidia formation. In Saccharomyces cerevisiae, the PacC homolog Rim101 is required for adaptation to extracellular pH and to regulate transcription of IME1, the Initiator of MEiosis. S. cerevisiae rim101 mutants are defective in sporulation. In Ashbya gossypii, a filamentous fungus belonging to the family of Saccharomycetaceae, little is known about the role of pH in regulating growth and sporulation. Here, we deleted the AgRIM101 homolog (AFR190C). Our analyses show that Rim101 is important for growth and essential for sporulation at alkaline pH in A. gossypii. Acidic liquid sporulation media were alkalinized by sporulating strains, while the high pH of alkaline media (starting pH = 8.6) was reduced to a pH ~ 7.5 by these strains. However, Agrim101 mutants were unable to sporulate in alkaline media and failed to reduce the initial high pH, while they were capable of sporulation in acidic liquid media in which they increased the pH like the wild type. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

Abundant Small Protein ICARUS Inside the Cell Wall of Stress-Resistant Ascospores of Talaromyces macrosporus Suggests a Novel Mechanism of Constitutive Dormancy

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J. Fungi 2021, 7(3), 216; https://doi.org/10.3390/jof7030216 - 17 Mar 2021

Cited by 3 | Viewed by 1495

Abstract

Ascospores of Talaromyces.macrosporus belong to the most stress resistant eukaryotic cells and show a constitutive dormancy, i.e., no germination occurs in the presence of rich growth medium. Only an extreme trigger as very high temperature or pressure is able to evoke synchronized germination. [...] Read more.

Ascospores of Talaromyces.macrosporus belong to the most stress resistant eukaryotic cells and show a constitutive dormancy, i.e., no germination occurs in the presence of rich growth medium. Only an extreme trigger as very high temperature or pressure is able to evoke synchronized germination. In this study, several changes within the thick cell wall of these cells are observed after a heat treatment: (i.) a change in its structure as shown with EPR and X-ray diffraction; (ii.) a release of an abundant protein into the supernatant, which is proportional to the extent of heat activation; (iii.) a change in the permeability of the cell wall as judged by fluorescence studies in which staining of the interior of the cell wall correlates with germination of individual ascospores. The gene encoding the protein, dubbed ICARUS, was studied in detail and was expressed under growth conditions that showed intense ascomata (fruit body) and ascospore formation. It encodes a small 7–14 kD protein. Blast search exhibits that different Talaromyces species show a similar sequence, indicating that the protein also occurs in other species of the genus. Deletion strains show delayed ascomata formation, release of pigments into the growth medium, higher permeability of the cell wall and a markedly shorter heat activation needed for activation. Further, wild type ascospores are more heat-resistant. All these observations suggest that the protein plays a role in dormancy and is related to the structure and permeability of the ascospore cell wall. However, more research on this topic is needed to study constitutive dormancy in other fungal species that form stress-resistant ascospores. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

The Smk1 MAPK and Its Activator, Ssp2, Are Required for Late Prospore Membrane Development in Sporulating Saccharomyces cerevisiae

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Matthew Durant

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Joseph M. Roesner

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Xheni Mucelli

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Christian J. Slubowski

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J. Fungi 2021, 7(1), 53; https://doi.org/10.3390/jof7010053 - 14 Jan 2021

Cited by 1 | Viewed by 1887

Abstract

During sporulation in the budding yeast Saccharomyces cerevisiae, proper development of the prospore membrane is necessary for the formation of viable spores. The prospore membrane will eventually become the plasma membrane of the newly formed haploid spore and also serves as the [...] Read more.

During sporulation in the budding yeast Saccharomyces cerevisiae, proper development of the prospore membrane is necessary for the formation of viable spores. The prospore membrane will eventually become the plasma membrane of the newly formed haploid spore and also serves as the template for the deposition of the spore wall. The prospore membrane is generated de novo during meiosis II and the growing edge of the prospore membrane is associated with the Leading Edge Protein (LEP) complex. We find that the Smk1 MAP kinase, along with its activator Ssp2, transiently localizes with the LEP during late meiosis II. SSP2 is required for the leading edge localization of Smk1; this localization is independent of the activation state of Smk1. Like other LEP components, the localization of Smk1 at the leading edge also depends on Ady3. Although prospore membrane development begins normally in smk1 and ssp2 mutants, late prospore membrane formation is disrupted, with the formation of ectopic membrane compartments. Thus, MAP kinase signaling plays an important role in the formation of the prospore membrane. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

Quick-Freeze, Deep-Etch Electron Microscopy Reveals the Characteristic Architecture of the Fission Yeast Spore

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Yuhei O. Tahara

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Makoto Miyata

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Taro Nakamura

J. Fungi 2021, 7(1), 7; https://doi.org/10.3390/jof7010007 - 26 Dec 2020

Cited by 6 | Viewed by 3109 | Correction

Abstract

The spore of the fission yeast Schizosaccharomyces pombe is a dormant cell that is resistant to a variety of environmental stresses. The S. pombe spore is coated by a proteinaceous surface layer, termed the Isp3 layer because it comprises mainly Isp3 protein. Although [...] Read more.

The spore of the fission yeast Schizosaccharomyces pombe is a dormant cell that is resistant to a variety of environmental stresses. The S. pombe spore is coated by a proteinaceous surface layer, termed the Isp3 layer because it comprises mainly Isp3 protein. Although thin-section electron microscopy and scanning electron microscopy have revealed the fundamental structure of the spore, its architecture remains unclear. Here we visualized S. pombe spores by using a quick-freeze replica electron microscopy (QFDE-EM) at nanometer resolution, which revealed novel characteristic structures. QFDE-EM revealed that the Isp3 layer exists as an interwoven fibrillar layer. On the spore cell membrane, many deep invaginations, which are longer than those on the vegetative cell membrane, are aligned in parallel. We also observed that during spore germination, the cell surface changes from a smooth to a dendritic filamentous structure, the latter being characteristic of vegetative cells. These findings provide significant insight into not only the structural composition of the spore, but also the mechanism underlying the stress response of the cell. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

Unconventional Constituents and Shared Molecular Architecture of the Melanized Cell Wall of C. neoformans and Spore Wall of S. cerevisiae

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J. Fungi 2020, 6(4), 329; https://doi.org/10.3390/jof6040329 - 01 Dec 2020

Cited by 12 | Viewed by 2020

Abstract

The fungal cell wall serves as the interface between the cell and the environment. Fungal cell walls are composed largely of polysaccharides, primarily glucans and chitin, though in many fungi stress-resistant cell types elaborate additional cell wall structures. Here, we use solid-state nuclear [...] Read more.

The fungal cell wall serves as the interface between the cell and the environment. Fungal cell walls are composed largely of polysaccharides, primarily glucans and chitin, though in many fungi stress-resistant cell types elaborate additional cell wall structures. Here, we use solid-state nuclear magnetic resonance spectroscopy to compare the architecture of cell wall fractions isolated from Saccharomyces cerevisiae spores and Cryptococcus neoformans melanized cells. The specialized cell walls of these two divergent fungi are highly similar in composition. Both use chitosan, the deacetylated derivative of chitin, as a scaffold on which a polyaromatic polymer, dityrosine and melanin, respectively, is assembled. Additionally, we demonstrate that a previously identified but uncharacterized component of the S. cerevisiae spore wall is composed of triglycerides, which are also present in the C. neoformans melanized cell wall. Moreover, we identify a tyrosine-derived constituent in the C. neoformans wall that, although it is not dityrosine, is a non-pigment constituent of the cell wall. The similar composition of the walls of these two phylogenetically distant species suggests that triglycerides, polyaromatics, and chitosan are basic building blocks used to assemble highly stress-resistant cell walls and the use of these constituents may be broadly conserved in other fungal species. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

The Conserved MAP Kinase MpkB Regulates Development and Sporulation without Affecting Aflatoxin Biosynthesis in Aspergillus flavus

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Sang-Cheol Jun

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Jong-Hwa Kim

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Kap-Hoon Han

J. Fungi 2020, 6(4), 289; https://doi.org/10.3390/jof6040289 - 16 Nov 2020

Cited by 5 | Viewed by 1756

Abstract

In eukaryotes, the MAP kinase signaling pathway plays pivotal roles in regulating the expression of genes required for growth, development, and stress response. Here, we deleted the mpkB gene (AFLA_034170), an ortholog of the Saccharomyces cerevisiae FUS3 gene, to characterize its function in [...] Read more.

In eukaryotes, the MAP kinase signaling pathway plays pivotal roles in regulating the expression of genes required for growth, development, and stress response. Here, we deleted the mpkB gene (AFLA_034170), an ortholog of the Saccharomyces cerevisiae FUS3 gene, to characterize its function in Aspergillus flavus, a cosmopolitan, pathogenic, and aflatoxin-producing fungus. Previous studies revealed that MpkB positively regulates sexual and asexual differentiation in Aspergillus nidulans. In A. flavus, mpkB deletion resulted in an approximately 60% reduction in conidia production compared to the wild type without mycelial growth defects. Moreover, the mutant produced immature and abnormal conidiophores exhibiting vesicular dome-immaturity in the conidiophore head, decreased phialide numbers, and very short stalks. Interestingly, the ΔmpkB mutant could not produce sclerotia but produced aflatoxin B1 normally. Taken together, these results suggest that the A. flavus MpkB MAP kinase positively regulates conidiation and sclerotia formation but is not involved in the production of secondary metabolites such as aflatoxin B1. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

The Fission Yeast RNA-Binding Protein Meu5 Is Involved in Outer Forespore Membrane Breakdown during Spore Formation

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J. Fungi 2020, 6(4), 284; https://doi.org/10.3390/jof6040284 - 13 Nov 2020

Cited by 3 | Viewed by 2735

Abstract

In Schizosaccharomyces pombe, the spore wall confers strong resistance against external stress. During meiosis II, the double-layered intracellular forespore membrane (FSM) forms de novo and encapsulates the nucleus. Eventually, the inner FSM layer becomes the plasma membrane of the spore, while the [...] Read more.

In Schizosaccharomyces pombe, the spore wall confers strong resistance against external stress. During meiosis II, the double-layered intracellular forespore membrane (FSM) forms de novo and encapsulates the nucleus. Eventually, the inner FSM layer becomes the plasma membrane of the spore, while the outer layer breaks down. However, the molecular mechanism and biological significance of this membrane breakdown remain unknown. Here, by genetic investigation of an S. pombe mutant (E22) with normal prespore formation but abnormal spores, we showed that Meu5, an RNA-binding protein known to bind to and stabilize more than 80 transcripts, is involved in this process. We confirmed that the E22 mutant does not produce Meu5 protein, while overexpression of meu5⁺ in E22 restores the sporulation defect. Furthermore, electron microscopy revealed that the outer membrane of the FSM persisted in meu5∆ spores. Investigation of the target genes of meu5⁺ showed that a mutant of cyc1⁺ encoding cytochrome c also showed a severe defect in outer FSM breakdown. Lastly, we determined that outer FSM breakdown occurs coincident with or after formation of the outermost Isp3 layer of the spore wall. Collectively, our data provide novel insights into the molecular mechanism of spore formation. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

Transient Breakage of the Nucleocytoplasmic Barrier Controls Spore Maturation via Mobilizing the Proteasome Subunit Rpn11 in the Fission Yeast Schizosaccharomyces pombe

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J. Fungi 2020, 6(4), 242; https://doi.org/10.3390/jof6040242 - 23 Oct 2020

Cited by 3 | Viewed by 2136

Abstract

Forespore membrane (FSM) closure is a process of specialized cytokinesis in yeast meiosis. FSM closure begins with the contraction of the FSM opening and finishes with the disassembly of the leading-edge proteins (LEPs) from the FSM opening. Here, we show that the FSM [...] Read more.

Forespore membrane (FSM) closure is a process of specialized cytokinesis in yeast meiosis. FSM closure begins with the contraction of the FSM opening and finishes with the disassembly of the leading-edge proteins (LEPs) from the FSM opening. Here, we show that the FSM opening starts to contract when the event of virtual nuclear envelope breakdown (vNEBD) occurs in anaphase II of the fission yeast Schizosaccharomyces pombe. The occurrence of vNEBD controls the redistribution of the proteasomal subunit Rpn11 from the nucleus to the cytosol. To investigate the importance of Rpn11 re-localization during vNEBD, Rpn11 was sequestered at the inner nuclear membrane by fusion with the transmembrane region of Bqt4 (Rpn11-GFP-INM). Remarkably, in the absence of endogenous rpn11⁺, the cells carrying Rpn11-GFP-INM had abnormal or no spore formation. Live-cell imaging analysis further reveals that the FSM opening failed to contract when vNEBD occurred, and the LEP Meu14 was persistently present at the FSM in the rpn11-gfp-INM cells. The results suggest that the dynamic localization of Rpn11 during vNEBD is essential for spore development. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Open AccessArticle

Studies on the Properties of the Sporulation Specific Protein Dit1 and Its Product Formyl Tyrosine

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J. Fungi 2020, 6(2), 77; https://doi.org/10.3390/jof6020077 - 03 Jun 2020

Cited by 3 | Viewed by 2002

Abstract

The dityrosine layer is a unique structure present in the spore wall of the budding yeast Saccharomyces cerevisiae. The primary constituent of this layer is bisformyl dityrosine. A sporulation-specific protein, Dit1 is localized in the spore cytosol and produces a precursor of [...] Read more.

The dityrosine layer is a unique structure present in the spore wall of the budding yeast Saccharomyces cerevisiae. The primary constituent of this layer is bisformyl dityrosine. A sporulation-specific protein, Dit1 is localized in the spore cytosol and produces a precursor of bisformyl dityrosine. Although Dit1 is similar to isocyanide synthases, the loss of Dit1 is not rescued by heterologous expression of the Pseudomonas aeruginosa isocyanide synthase, PvcA, indicating that Dit1 does not mediate isocyanidation. The product of Dit1 is most likely formyl tyrosine. Dit1 can produce its product when it is expressed in vegetative cells; however, formyl tyrosine was not detected in the crude cell lysate. We reasoned that formyl tyrosine is unstable and reacts with some molecule to form formyl tyrosine-containing molecules in the cell lysate. In support of this hypothesis, formyl tyrosine was detected when the lysate was hydrolyzed with a mild acid. The same property was also found for bisformyl dityrosine. Bisformyl dityrosine molecules assemble to form the dityrosine layer by an unknown mechanism. Given that bisformyl dityrosine can be released from the spore wall by mild hydrolysis, the process of formyl tyrosine-containing molecule formation may resemble the assembly of the dityrosine layer. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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Review

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Open AccessReview

Sporulation in Ashbya gossypii

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Jürgen Wendland

J. Fungi 2020, 6(3), 157; https://doi.org/10.3390/jof6030157 - 29 Aug 2020

Cited by 7 | Viewed by 3273

Abstract

Ashbya gossypii is a filamentous ascomycete belonging to the yeast family of Saccharomycetaceae. At the end of its growth phase Ashbya generates abundant amounts of riboflavin and spores that form within sporangia derived from fragmented cellular compartments of hyphae. The length of [...] Read more.

Ashbya gossypii is a filamentous ascomycete belonging to the yeast family of Saccharomycetaceae. At the end of its growth phase Ashbya generates abundant amounts of riboflavin and spores that form within sporangia derived from fragmented cellular compartments of hyphae. The length of spores differs within species of the genus. Needle-shaped Ashbya spores aggregate via terminal filaments. A. gossypii is a homothallic fungus which may possess a and α mating types. However, the solo-MATa type strain is self-fertile and sporulates abundantly apparently without the need of prior mating. The central components required for the regulation of sporulation, encoded by IME1, IME2, IME4, KAR4, are conserved with Saccharomyces cerevisiae. Nutrient depletion generates a strong positive signal for sporulation via the cAMP-PKA pathway and SOK2, which is also essential for sporulation. Strong inhibitors of sporulation besides mutations in the central regulatory genes are the addition of exogenous cAMP or the overexpression of the mating type gene MATα2. Sporulation has been dissected using gene-function analyses and global RNA-seq transcriptomics. This revealed a role of Msn2/4, another potential PKA-target, for spore wall formation and a key dual role of the protein A kinase Tpk2 at the onset of sporulation as well as for breaking the dormancy of spores to initiate germination. Recent work has provided an overview of ascus development, regulation of sporulation and spore maturation. This will be summarized in the current review with a focus on the central regulatory genes. Current research and open questions will also be discussed. Full article

(This article belongs to the Special Issue Formation and Function of Fungal Ascospores)

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