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Ashbya gossypii: a model for fungal developmental biology

Key Points

  • Fungal cells have two basic cellular morphologies: either yeast-like, such as Saccharomyces cerevisiae, or filamentous, such as Ashbya gossypii. Some fungi can switch from the yeast to the hyphal phase, and in the human fungal pathogen Candida albicans this morphogenetic switching is important in virulence.

  • The filamentous fungus A. gossypii is closely related to yeasts, as shown by its genome organization. Based on the phylogeny of genes such as rDNA, A. gossypii belongs to the family of Saccharomycetaceae. This means that A. gossypii is a suitable model for comparing the cell biology of filamentous fungi and yeasts.

  • Rho-protein modules are important regulators of the actin cytoskeleton and are involved in cell polarity establishment and the maintenance of hyphal growth. They are part of signalling networks that activate adaptor proteins that stimulate actin filament assembly.

  • The A. gossypii Wiskott–Aldrich syndrome protein homologue Wal1 has dual functions for endocytosis and vacuolar motility, and the localization of cortical actin patches, which are required for fast polarized cell growth.

  • Septation in filamentous fungi is different from cytokinesis in yeast, as it produces a septal pore that interconnects hyphal segments and therefore does not result in cell separation. In contrast to the S. cerevisiae Bud3, a central function of Bud3 in A. gossypii is the localization of the IQGAP-homologue Cyk1 to septal sites.

  • In conclusion, functional analyses of morphogenetic genes in yeast and a closely related filamentous fungus such as A. gossypii will provide new details on the general mechanisms of growth and septation and might also contribute to understanding how these processes evolved.

Abstract

Ashbya gossypii is a riboflavin-overproducing filamentous fungus that is closely related to unicellular yeasts such as Saccharomyces cerevisiae. With its close ties to yeast and the ease of genetic manipulation in this fungal species, A. gossypii is well suited as a model to elucidate the regulatory networks that govern the functional differences between filamentous growth and yeast growth, especially now that the A. gossypii genome sequence has been completed. Understanding these networks could be relevant to related dimorphic yeasts such as the human fungal pathogen Candida albicans, in which a switch in morphology from the yeast to the filamentous form in response to specific environmental stimuli is important for virulence.

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Figure 1: Life cycle of Ashbya gossypii.
Figure 2: Potential signal cascades that regulate polarized hyphal growth and septation in Ashbya gossypii.
Figure 3: Actin cytoskeleton components in Ashbya gossypii.
Figure 4: Septation and septation mutants in Ashbya gossypii.
Figure 5: The mating-type MATA locus of Ashbya gossypii.

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Acknowledgements

The authors would like to thank the members of the Wendland and Philippsen labs for stimulating discussions. Research on A. gossypii in J.W.'s laboratory is supported by the Deutsche Forschungsgemeinschaft.

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Correspondence to Jürgen Wendland.

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FURTHER INFORMATION

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A. gossypii genome

A. gossypii Genome Database

Saccharomyces Genome Database

Glossary

STIGMATOMYCOSIS

A fungal infection resulting in wet, slimy kernels. It is transmitted by insect mouthparts that penetrate the kernel.

ISOTROPIC GROWTH PHASE

Non-polarized growth over the entire cell surface during spore germination.

SPORANGIUM

Cell or organ that produces or contains spores.

SYNNEMA/-TA

Erect hyphae that are grouped together. Conidia might be formed along the length of the synnema or just at its apex.

BIOTROPHIC PHASE

The pathogen invades host tissue without killing host cells and feeds on living cells.

SPITZENKÖRPER

A phase-dark, usually spherical body that is found in growing hyphal tips, and represents an accumulation of secretory vesicles — also known as the vesicle supply centre.

POLARISOME

Protein complex that consists, in S. cerevisiae, of the formin Bni1, Spa2, Bud6 and Pea2, and is involved in organization of the actin cytoskeleton and required for polarized cell growth.

CORTICAL CUES

Membrane-associated proteins that function as landmarks to direct protein complexes that are required for the establishment of cell polarity.

SYNTENY

Evolutionary conservation of gene order (including their transcriptional orientation) between two loci.

CONIDIOPHORE

Structure that bears and generates the conidiospores.

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Wendland, J., Walther, A. Ashbya gossypii: a model for fungal developmental biology. Nat Rev Microbiol 3, 421–429 (2005). https://doi.org/10.1038/nrmicro1148

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