Ariel spores produced by higher fungi are easily the most used dispersal mechanism in the fungal kingdom. So many spores are released every day that these bio-aerosols actually have the ability to alter weather patterns on a huge scale. Spores with the help of rising air currents can travel vast distances, even traversing oceans! However, to maintain these immense dispersal ranges, the spore must remain small and largely unprotected. For quite some time, we knew how far these spores could disperse, but it wasn’t up until very recently that we tested their environmental longevity. Dispersal means next to nothing if the biological element is damaged and can’t carry out its function. This trade-off between size and dispersal is not only common, but riddled throughout the natural world.
In plants, there is a clear trade-off between seed size and dispersal. Small seeds get carried farther by wind, and larger seeds tend to stay around their parent plant. Smaller seeds, although they can disperse far, are limited by the nutrients allocated to the seed, and are almost always less protected from the elements. For these reasons, smaller seeds don’t germinate as well as hardier, larger seeds. With this idea in mind, Veera Norros and her team tested the germination of fungal spores subjected to different levels of sunlight and temperatures.
Using 17 different species of wood-decay fungi, these researchers subjected spores to a combination simulated solar radiation and freezing at -25°C, at four different durations of time. -25°C was used because it is a temperature spores experience in early spring and late autumn when they are transported to high altitudes in northern Europe; where this study was carried out. After implementing these treatments, spores where then placed in Petri dishes filled with malt nutrient agar. The presence of spore germination was affirmed by looking at the nutrient agar with a compound microscope, looking for signs of active hyphae.
This study showed that both freezing and solar radiation reduce fungal spore germination. Overall, solar radiation was more damaging than freezing. These researches paired these results with spore wall thickness and found that thicker walled spores were more protected from solar radiation. Similar to the trade-off seen in plant seed dispersal, thicker walled spores from species like Phellinus punctatus have a larger mass which limits their dispersal distance. Thinner walled spores from Antrodia serialis travel farther, but are more vulnerable to solar radiation damage. Interestingly, thicker spore walls didn’t help against freeze damage.
Freeze damage was lessened in species with elongated spores. This is a brand-new concept that needs more research because we really don’t know why this is. A thicker spore wall clearly reduces sun damage because it filters out more of the suns harmful radiation. How exactly spore shape reduces freeze damage is something I am still scratching my head over. Elongated spores have a greater surface to volume ratio, and should be less protected from the cold, but this research suggests that more elongated spores are better adapted to -25°C conditions.
The trade-offs between different spore structures doesn’t stop there. These researchers did a preliminary experiment before spores were subjected to radiation and freezing treatments. Here, they found that thick walled spores don’t germinate as fast as thin walled spores. Thin walled spores may better sense their environment and can initiate germination faster, or maybe germinating hyphae takes longer to penetrate the thicker spore wall. Regardless, this is another trade-off. Thicker walled spores are more protected from solar radiation, but disperse shorter distances, and take longer to germinate. If thinner walled spores make it to a suitable substrate without being damaged, they can germinate and outcompete their thicker walled counterpart.
In 1982, Kramer found that most saprotrophic fungi from the Basidiomycota release their spores at night. Up until now, this was thought to be strictly because of the internal environment of the fruiting body and its associated spore release mechanism of the Buller’s droplet. This adaptation could have evolved because spores that disperse at night are less damaged by harmful solar radiation.
This recent publication should change the way we understand the fungal realm. Because elongated spores are more protected from freeze damage, species occurring farthest from the equator, in high elevations, and/or fruit in late fall should produce spores that are more elongated. Species that occur closer to the equator, and/or fruit in spring where solar radiation is more of a factor should produce spores with thicker walls. Fungal spores can travel huge distances, but for the most part they are rather fragile. These species trade-offs are ever present in fungal spore dispersal, and has driven the diversity of spore shape and morphology over millennia.