Ashbya gossypii

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History and Significance

Ashbya gossypii is a filamentous fungus or mold which was originally isolated from cotton as a pathogen of stigmatomycosis by Ashby and Novell in 1926. This disease affects the development of hair cells in cotton bolls and can be transmitted to citrus fruits, which thereupon dry out and collapse (dry rot disease). In the first part of the 20^th century, A. gossypii and two other related fungi causing stigmatomycosis (Eremothecium coryli, Aureobasidium pullulans) made it virtually impossible to grow cotton in certain regions of the subtropics, causing severe economical losses. Control of the spore transmitting insects - cotton stainer (Dysdercus suturellus) and Antestia - permitted to fully eradicate infections. It was recognized that A. gossypii is a natural overproducer of riboflavin, also known as vitamin B₂, which protects its spores against ultraviolet light. This made it an interesting organism for industries, where genetically modified strains are still used to produce this vitamin.

A. gossypii as a model organism

A few years ago, A. gossypii became recognized as an attractive model to study the growth of long and multinucleate fungal cells (hyphae) because of its small genome, haploid nuclei, and efficient gene targeting methods. It is generally assumed that a better understanding of filamentous fungal growth will greatly stimulate the development of novel fungicides. The use of Ashbya gossypii as a model organism is particularly promising because of the high level of gene order conservation (synteny) between the genomes of A. gossypii and the yeast Saccharomyces cerevisiae.

Genome

The complete sequencing and annotation of the entire A. gossypii genome, as published in 2004, was initiated when a significant degree of gene synteny was observed in preliminary studies in comparison to the genome of budding yeast, Saccharomyces cerevisiae. This not only helped to improve gene annotation of S. cerevisiae, but also allowed the reconstruction of the evolutionary history of both organisms. A. gossypii and S. cerevisiae originate from a common ancestor which carried about 5000 genes. Divergence of these two close relatives started some 100 million years ago. One branch of evolution involving up to 100 viable genome rearrangements (translocations and inversions), a few million base pair changes, and a limited number of gene deletions, duplications and additions lead to modern A. gossypii with its 4718 protein-coding genes and 9.2 million base pairs (smallest genome of a free-living eukaryote yet characterized) spread over seven chromosomes. The genome of S. cerevisiae underwent a more eventful evolution, which includes a whole-genome duplication.

Despite the long evolutionary history and of the two organisms and fundamentally different ways of growth and development, the complete synteny map of both genomes reveals that 95 % of A. gossypii genes are orthologs of S. cerevisiae genes and 90 % map within blocks of synteny (syntenic homologs).

Growth, Development and Morphology

  The A. gossypii life cycle starts with the only known phase of isotropic growth in wild type: germination of the haploid spore to form a germ bubble. This is followed by apical growth, extending two germ tubes in succession on opposing sites of the germ bubble. More axes of polarity are established with lateral branch formation in young mycelium. Maturation is characterized by apical branching (tip splitting) and a dramatic increase of growth speed (up to 200 μm/h at 30°C), which enables it to cover an 8 cm Petri dish of full medium in about 7 days. Sporulation is thought to be induced by nutrient deprivation, leading to contraction at the septa, cytokinesis and subsequent abscission of sporangia which contain up to 8 haploid spores. Hyphae are compartmentalized by septa, which in young parts appear as rings that allow transfer of nuclei and in older parts may appear as closed discs. Compartments typically contain around eight nuclei.

Research Interests

Mechanisms of cell polarity

Polar growth is essential for proper function of many cells. Without cell polarity many transport processes and signal perception of vision and sound would be impossible. Investigating how Ashbya gossypii hyphae maintain polarity and grow constantly into one direction will help to understand the basic processes of cell polarity.

Evolution of the cell cycle in multinucleated cells

How do cell shape, size and nuclear organization influence the function of conserved cell cycle networks? This work examines alternative modes of transcriptional control, spatial organization, and post-translational regulation that were adopted for nuclear division to accurately function in the framework of a multinucleated cell.

Spatial control of mitosis

It has been discovered that a conserved family of proteins called the septins may contribute to the location of mitoses. Thus septins seem to provide instructions that direct where a mitosis takes place. This project focuses on understanding how the septins signal to the cell cycle machinery.

Nutritional control of cyclin dependent kinase activity

It has been shown that the Cyclin dependent kinase (CDK) is phosphorylated and inhibited by the action of Swe1p (a wee1-like kinase) in response to low nutrients. This work focuses on understanding how external nutrient status is transmitted through the cell to regulate Swe1p kinase.

Basis for asynchronous mitoses in a common cytoplasm

There is some evidence proteins can be exchanged between neighboring nuclei in Ashbya cells, however, the nuclei still divide independently of their neighbors. This work involves identifying the basis for nuclear independence within a common cytoplasm including experiments evaluating how Spindle Pole Bodies, Nuclear Pore Complex components and/or the Endoplasmic reticulum may be involved in maintaining nuclear autonomy.

References

• Ashby SF and Nowell W (1926). "The Fungi of Stigmatomycosis.". Ann Bot os-40 (1): 69-84.
• Dietrich FS, Voegeli S, Brachat S, Lerch A, Gates K, Steiner S, Mohr C, Pohlmann R, Luedi P, Choi S, Wing RA, Flavier A, Gaffney TD and Philippsen P (2004). "The Ashbya gossypii genome as a tool for mapping the ancient Saccharomyces cerevisiae genome.". Science 304 (5668): 304-7. PMID 15001715.
• Philippsen P, Kaufmann A and Schmitz HP (2005). "Homologues of yeast polarity genes control the development of multinucleated hyphae in Ashbya gossypii.". Curr Opin Microbiol 8 (4): 370-7. PMID 16023404.
• Gladfelter AS (2006). "Nuclear anarchy: asynchronous mitosis in multinucleated fungal hyphae.". Curr Opin Microbiol 9 (6): 547-52. PMID 17045513.
• Kaufman A (2007). Polarized growth and septation in the filamentous ascomycete Ashbya gossypii analyzed by life cell imaging.