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Spore Dispersal

Introduction

Fungi disperse beyond their initial colony. Dispersal takes place after spore release. The modes of dispersal are simple yet they tell us a lot about the nature of the fungi themselves. Some modes of movement are obviously "accidental" and probably shared by many different microbes. Others are highly complex and closely integrated with the life history of the individual species. The details of dispersal remain to be clarified in many cases, but in principle they probably all involve an agent carrying the spore from the source.

Agents of Dispersal

The most common agent of dispersal is wind. Spores also travel in and on water, and with animal vectors.

Air

Wind may carry spores quite long distances, though the proportion that is carried any distance is remarkably small. First the spores must pass through the boundary layer which surrounds all stationary objects. The boundary layer is a thin layer of air which remains still because of friction against the surface. An agaric fruitbody whose cap is above the surface of the litter avoids the boundary layer by dropping spores into the turbid layer. Other fungi use active or passive release mechanisms which place spores in moving air. LINK

One consequence of spores being released in still air is that most dry spores are deposited close to the place of formation and release. In the case of asexual spores, lack of dispersal may result in the proliferation of the fungi in conditions that suit the genotype.

"Still air" is a relative term. Small spores can be carried in air that moves more slowly than is necessary to carry larger, heavier spores. However, small spores have fewer reserves than large spores so can remain viable in air for shorter periods. Further, light spores take longer than heavier spores to be removed from air simply because slower winds lift and carry them. Understanding the entry to and settle from the air stream is a critically important aspect of the dispersal of air-borne fungi.

Water

A range of fungi sporulate in aquatic conditions. Aquatic fungi form a diverse array of asexual spores with long appendages, or coiled structures. The effect of these shapes is that spores are carried on the water surface, due to the high surface tension, or because air is trapped within the conidium. Splash lands spores on dry surfaces. Germination may take place through the attachments to the surface. LINK

Drops of water are also responsible for transferring spores from one surface to another. The impact of a raindrop lifts spores from a colonised surface and carries them in the fragments of the drops that are formed. The fragments of the raindrop may carry the spores to fresh surfaces. The effective distance of splash dispersal is quite small, but significant in some plant pathogens, where splash may spread the pathogen extremely rapidly within a susceptible crop.

A significant group of fungi disperse via zoospores which are carried in water. The fungi are found in water films in soil, plant surfaces and in aquatic and marine environments. While the zoospores are flagellate, they are light and carried passively in water currents. Because the spores are so small, they are essentially held in the water and move with the water. Thus the water is the vector, in much the same way as for other fungi. The motile stage is of most value for finding sites for attachment: zoospores are attracted by chemotaxis, and swim towards sites which release chemoattractants. In most cases, the zoospores are in water films and so do not move great distances from their source. In the case of some chytrids, this means that colonies are formed by the periodic deposition of a few more zoospores into the rhizoids of parent sporangium/a.

Vectors

Spores can be carried on or in a vector. By accidentally brushing against a sticky mass of spores, insects can move away with a large inoculum load on their bodies. Spores will be lost from the body following contact with another object, or breaking away at the spore mass dries. These spores are highly likely to be taken to environments suitable for fungal colonisation because many insects visit similar locations one after the other. However, because insects are mostly active during dry conditions, immediate germination of the spores at the new location may be limited.

Mycophagy is the ingestion of fungi. Several animals deliberately ingest fruiting bodies. LINK The spores of the sporocarp are rarely completely degraded, and many that pass through the gastrointestinal tract are able to germinate in the faeces. Some dung fungi require passage through herbivores to complete their lifecycle. Indeed, the interaction with dung beatles may ensure that spores in faeces are buried almost immediately following deposition. Animal dispersal is important for some fungi. For instance, sequestrate ectomycorrhizal and arbuscular mycorrhizal fungi cannot disperse unless the fruit body is eaten by a mycophagous animal.

Dung fungi eject their spores onto surrounding (plant) surfaces. The herbivore eats the contaminated plant material and ingest the fungal spores with the plant material. LINK Animals may carry the spores considerable distances from the source. Indeed, the relationship may be very close: the spores of some dung fungi require fatty acids only found within the dung to initiate germination.

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Dispersal Distances

Fungi can move laterally and vertically in air. The distance of movement is related to the wind speed, and size and shape of the spore. Typically, fungi disperse in a log reduction with distance from the source. The concentration of spores is highest at the source and more than 99% have been deposited within 10 m.

Thus while we might predict limited dispersal in most cases, the potential for a few spores to disperse enormous distances has economic significance. Spores of highly pathogenic rust fungi are believed to have travelled from South Africa to Australia (13,000 km), from Western Australia to eastern Australia and from Australia to New Zealand (2,000 km). The fungi survived UV radiation, desiccation and adverse temperatures. Species with large thick walled brown spores would presumably better survive long distance dispersal than those with small hyaline thin walled spores. Note that large spores are more readily deposited than small spores and that stronger winds are required to lift and carry heavier spores.

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Deposition

After travelling long distances a variety of factors aide the deposition of spores from air. Of the factors that aid deposition, impaction, sedimentation and rain deposit appear to be most important.

Impaction is caused by the spore being too heavy to be carried in the air flow as it passes around an object. Most airborne spores will bypass an immobile object, but a proportion impact on the object. Efficiency of impaction increases with wind speed, increasing mass of spores, small diameter of the object, and by the spore being sticky. Most spores would not impact on a tree trunk, but many plant pathogens have heavy spores that enable impaction on narrower objects such as stems and leaves.

Sedimentation takes place when the air current is too slow to carry the spores. Gravity causes the spores to drop. Usually sedimentation only takes place in the boundary layer, which is normally up to 1 mm above a surface. However, on still nights, the boundary layer may be 1 m above ground layer.

Rain drops form around nuclei such as dust and spores. The drops of water reach a critical weight and sediment from the air. On the way down, they may also impact on airborne spores. Further, drops may carry an electrical charge which would attract spores of opposite charge. Rain is usually responsible for clearing spores from the air.

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Conclusion

Huge numbers of spores are dispersed. Concentrations of up to 10,000 per m3 are measured around crops at harvest. Most spores normally deposit within a short distance from the source. However, a small number may be successfully dispersed significant distances from the source. A very small number of these usually establishes a new colony. The loss of propagules in normal circumstances is balanced with the huge potential increase in colonies when conditions are suitable for dispersal and establishment.

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References

Carlile MJ & Watkinson SC 1994 The Fungi, Academic Press pp183 – 189.

Ingold CT & Hudson HJ 1993 The Biology of Fungi, 6 th Ed, Chapman Hall pp 119 – 132.

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Cropping and Spore Dispersal

Humans unwittingly assist in the dispersal of some fungi. Crop (cereals and cotton in NSW, Australia) harvesting usually involves beating the crop remains over a grid at the end of the harvester, and blowing the plant (and fungal) debris away from the machine. Extremely high concentrations of spores of common endophytic fungi such as Alternaria and Cladosporium are elevated, and become airborne. Similarly, handling the harvest may elevate spores: grain handling at silos and cotton ginning are associated with the release to the air of massive densities of spores. Once air-borne, the spores may travel considerable distances.

One of the consequences of crop harvest and handling is that asthma, triggered by these two fungi, is higher in cropping areas than on the coast where cropping is rare. LINK This condition is critically important in country NSW where childhood incidence of asthma is extremely high.

Harvesting also influences dispersal of plant pathogens. Spores of rust fungi may be elevated during harvest, dispersing spores and increasing the subsequent incidence of disease in distant crops.

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Biocontrol of Phytoplankton

Chytrids live in sea and fresh water. They are easy to trap, simple suspension of sterile cellophane in natural water will become colonised by cellulolytic Chytrids within a few days. However, their identification is difficult. For the most part, the thallus is eucarpic and monocentric. Zoospores of a range of species are morphologically identical. The fungi are of considerable economic importance, being decomposers of plant remains, and parasites of a diversity of planktonic algae.

Classic studies of Chytrid colonisation of diatoms in the British lakes district indicate that the size of the population of Chytrids follows the size of the population of the host. This feature may become useful in biocontrol of dangerous picoplankton such as toxic cyanobacteria which respond to eutrophic waters. Little information on the distribution and abundance is available. Recent studies in fresh water lakes indicate that chytrids, not Protists, constitute the most common eukaryotic component in the water column, suggesting that Chytrids have a role in regulating populations of plankton. However, few studies of plankton, and even fewer of Chytrids, exist to test the regulatory hypothesis.

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