Fungal Dispersal Mechanisms
Ballistospores are forcibly discharged basidiospores that are common in the phylum Basidiomycota. Hydroscopic material called the hilar appendix is secreted by the fungus at the base of each balistospore. The presence of the hilar appendix causes differential wettability of the developing spores and thus condensation droplets are preferentially attracted to the appendix. When a condensation droplet merges with this appendix, there is significant reduction in surface energy and thus this change in energy becomes impetus for discharge (energy release) by changing the center of gravity. Although this method of dispersal is common in Basidiomycota, many other dispersal mechanisms have evolved within this phylum and other fungal phyla to cope with their particular environmental conditions. Spore discharge mechanisms can be either active or passive and may even exploit other organisms to disperse their progeny
Other active dispersal mechanisms
Fungi within the phylum Basidiomycota are not the only group that can actively discharge their spores. The zoospores of Chytridiomycota also exhibit active dispersal since they have motile spores. The flagellated cells utilize chemical signals (chemotaxis) to find and encyst upon a new host. In addition, Pilobolus of Zygomycota has also achieved active discharge. The spores of these fungi are growing out of dung and must be eaten by a cow to complete their lifecycle. Therefore, they must escape the dung heap to complete their life cycle. These fungi exhibit phototaxisis as the sporiangiophore tracks the sun in order to set the trajectory for spore discharge. When the appropriate trajectory is made, osmotically active compounds begin to generate within the sporagiophore, and the structure continues to swell until eventually it bursts sending the entire sticky sporangium about 4 feet away (hopefully onto some blades of grass!!) In Ascomycota, the asci of some fungi absorb water when the spores become mature. Internal pressure builds are the insoluble materials within the ascus are converted to soluble material. These asci, typically exhibit a weak point (or lid) at the tip, which is responsible for the subsequent directed explosive spore release.
Passive discharge
Passive discharged spores are usually released by the drying action of the wind or development of a sticky mucilage (to “hitchhike”); however, the ultimate vectors of transport for these spores are highly variable. Further, the lack of an “active” discharge mechanism does not suggest that the fungus is primitive or poorly adapted to dispersal. Many of these fungal spores exhibit complex and unusual adaptations that allow them to effectively exploit their particular dispersal vector.
Wind dispersal
Generally wind-dispersed fungi produce a large number of small spores because the probability of success for any given spore to establish itself in a suitable habitat is quite small. As such, this type of dispersal is often referred to as the “sweepstakes dispersal” However, just because their odds of success are so low, does not mean these spores are not adapted to this mode of dispersal. In fact, many wind-dispersed spores exhibit complex morphologies that facilitate their wind dispersal.
Rain dispersal
Bird’s nest fungi develop cup-shaped basidiocarps that form clusters on dead wood. The basidia are found within structures are a formed within a thick-walled peridiole. Peridoles are usually attached to the basidiocarp by a string-like structure called a funiculus. These spores are splashed out of the basidiocarp “cups” by rainfall and the funiculus can act as “grabber” that attaches the spore to a new substrate
Insect dispersal
Finally, fungi can disperse their spores by utilizing insect vectors. Insect dispersed spores are often sticky in order to facilitate their “hitchhicking” efforts. These insect/fungal associations can evolve as mutualistic symbioses in which both partners benefit or by the fungus exploiting the insect’s innate behavioral responses.
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