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Artillery fungus

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Posted August 28, 2014

The species in the kingdom of Fungi are the most important decomposers of organic material among all life forms. They can easily degrade almost any carbon-containing substrate, ranging from the cellulose and lignin of plant cell walls to the jet fuel and house paint.

The fungi in the genus of Pilobolus grow on herbivore dung. The black sporangia at the tip of the transparent fluid-filled stalks are often called "hats". Image source: Wikimedia Commons.

The fungi in the genus of Pilobolus grow on herbivore dung. The black sporangia at the tip of the transparent fluid-filled stalks are often called “hats”. Image source: Wikimedia Commons.

Though the majority of people are only familiar with edible mushrooms, moulds and yeast that have long-lasting history in alcoholic beverages and bread production, so far there have been described more than 100,000 species of fungi. However, it is estimated that there are actually as many as 1.5 million species of fungi. The genus of Pilobolus in the phylum of Zygomycota consists of quite widespread species of fungi that can be described as “Hat Throwers”. They are the fungal artillery equivalent to the bombardier beetle in the kingdom of Animalia.

The survival of the fungal species is completely dependent on the ability to disperse the spores, similarly to the plants that are dependent on seed dispersal. Though most fungi simply disperse their spores by wind or water, some species have evolved spore discharge processes, also known as “squirt guns”. In these fungi, fluid-filled stalks that support spore-filled sporangia are pressurized by osmosis and after some time the sporangia are discharged from the stalk. The genus of Pilobolus consists of species that have evolved such spore dispersal mechanism.

P. kleinii sporangium is being discharged at 20,000 g acceleration. The video was captured by the research team, led by Levi Yafetto, using ultra-high speed video camera. Image source: PLoS ONE

P. kleinii sporangium is being discharged at 20,000 g acceleration. The video was captured by the research team, led by Levi Yafetto, using ultra-high speed video camera. Image source: PLoS ONE.

The species of fungi in the genus of Pilobolus grow on herbivore dung. The fruiting structure of Pilobolus species consists of a transparent stalk, which ends up with fluid-filled subsporangial vesicle. On top of this, a black sporangium, informally known as “hat”, develops. The fruiting structure has the remarkable ability of orienting itself to point directly towards a light source, an ability known as phototropism that is predominantly associated with plants. The subsporangial vesicle plays crucial role in orientation by focusing the light via carotenoid pigments at the base of the vesicle. Thus, the vesicle acts as a lens.

Sporangium, oriented towards the light source, is discharged onto a plant substrate such as grass. A herbivorous animal such as a horse then eats the substrate, unknowingly consuming the sporangium as well. The Pilobolus sporangium survives the passage through the gastrointestinal tract without germinating, and emerges with the excrement. Once outside its host, spores within the sporangium germinate and grow as a mycelium within the excrement, where it is a primary colonizer. Later, the fungus fruits to produce more spores. The life cycle repeats.

The spore discharge is the most important step in the life of Pilobolus genus fungi. The research team, led by Levi Yafetto from Miami University, USA, set out to investigate the launch processes of Pilobolus kleinii and other fungi to gather more information about the speeds and accelerations taking action during the discharge. Scientists used ultra-high speed video cameras running at maximum frame rates of 250,000 fps to analyze the entire launch process.The researchers found out that sporangia of Pilobolus kleinii are propelled by a stream of ejected fluid with the acceleration of over 20,000 g over distances of up to 2.5 meters.

“The launch speeds of the species in this study are likely to be among the fastest among any fungi because their coprophilous ecology has demanded much longer ranges than those necessary for the dispersal of species that need only escape boundary layers”, the scientists conclude in the research paper titled “The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi”.

Written by Edvinas Stankūnas

Reference: PLoS ONE

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