Fungi-Animal Mutualism 
Fungi-Animal Mutualism One massive issue herbivorous animals face is an inability to digest certain plant molecules, most notably cellulose. Many animals simply focus on the more easily-edible parts of plants, such as berries or seeds, but other types of herbivores instead find help from other organisms, including fungi. Almost all of these symbiotic relationships occur with insects. This is because insects are awesome and everyone loves them. Additionally, insects are an ancient and incredibly diverse taxa, giving them many opportunities to develop unique interactions with plants and fungi alike. These factors allowed insect-fungi mutualism to evolve independently on many occasions, leading to a vast diversity of associations. This is only a brief overview.
Fungi often struggle with movement. Like plants, they can grow across and through substrate, and even launch spores incredible distances, but they lack the ability to move to new sources of food like animals can. On the other hand, insects excel at flight, but like other animals they struggle to digest certain foods, especially wood. That is the basis of this type of mutualism.
For specific species of wood-boring beetles, midges, and wasps, wood rotting fungi, specifically the biotrophic and necrotrophic types, provide the perfect solution to both their problems. These insects all share a specific structure called a mycangia, which is a small pouch that the adult fills with the spores (specifically conidia) of their symbiotic fungi. The mycangia not only contains the spores, but may have glands that secrete liquids to support the fungi while in transit. When the insect finds a suitable tree, it lays eggs within the wood, and places some spores alongside it. While the eggs quietly develop, the fungi wastes no time in growing. By the time the eggs hatch, the fungi will have spread deep into the wood and begun producing spores of their own. Since the larva are unable to eat wood, the fungi make up the entirety of their diet prior to adulthood. But after metamorphosis, the fully-grown bugs will stock their mycangia with spores and fly off to continue this cycle, ensuring the fungi will spread and that their children will be fed.
But wood-boring insects are diverse, and there are many differences between these groups. For one, the trees they target. Beetles of the families Scolytidae, Platypodidae, and Lymexylidae, as well as Sirex woodwasps all target weakened or fallen trees, while other insects are less picky. And in certain species, such as the Southern Pine Beetle, the insects eat the wood directly while the fungi serve to weaken the plant's defenses. Equally diverse are the fungi these insects interact with. The same insect species tend to carry the same fungi species, with little variation, but there is massive variation across taxa. They can be Ascomycota or Basidomycota, yeasts (single-celled form) or anamorphs (mold-like asexual reproductive stage). Many have entirely seperate life cycles outside of their symbiont, while others are highly reliant on insects. For example, the genus Amylostereum contains both symbiotic and independent species. But this relationship isn't just fungi and bugs teaming up to rot trees. Many types of plants have symbionts of their own to protect them, called endophytes. For more information, check out the page on plant-fungi interactions.
It's easy to understand why the fungi would benefit from this arrangement, but understanding why the insects need help requires a bit more background knowledge. After all, some species of insects can digest wood seemingly without any external help. But while animals can sometimes use symbionts to digest cellulose, they can't change that the wood itself lacks certain nutrients, specifically nitrogen. Animals require a lot of nitrogen to function, and wood is an extremely nitrogen-poor source of food. But fungi have a collection of abilities that allow them to acquire and accumulate many of the minerals and nutrients that animals struggle to get otherwise. Simply put, the fungi that consume wood provides a far richer source of food than the wood itself. For Anobiid beetles, the fungi are a necessary source of many vitamins and amino acids the insects cannot produce on their own. The larva will simply die without the fungus, regardless of their diet and the conditions around them. Their ability to enrich food, alongside the general difficulty of digesting cellulose, is one of the main factors that mutualism between animals and wood rotting fungi is so prevalent.
Image Source: John Tann from Sydney, Australia, CC BY 2.0, via Wikimedia Commons
Just as wood-eating fungi specifically can benefit from having help moving to new sources of food, almost all fungi can generally benefit from animals moving spores to new locations, either by carrying them on their shell or leaving them in their droppings. Because this type of mutualism requires much less commitment, nearly all fungivorous animals are known to assist in spore dispersal, and that includes mammals like us. Insects are still the most common, though.
While some would expect that flying animals would steal the show, soil-dwelling animals, such as earthworms and mites, are perhaps the most significant. These animals graze on the fruiting bodies and hyphae alike, and while this reduces the fitness of the fungi, these animals make up for it in their ability to travel to locations ideal for fungal growth. Because they have soil preferences, and those preferences tend to line up with whatever fungi they're eating, soil-dwelling animals are likely to deposit spores in similar locations to the parent fungus, and locations that have been recently disturbed. This process is key to certain fungi's ability to spread to sites of recent succession, allowing these immobile organisms to react to environmental changes that would benefit their offspring. Of soil-dwelling animals, mites and springtails are considered the most influental of all, and one research group once isolated over 100 species of fungi from the guts of springtails alone.
As for the fungi themselves, they tend to be saprotrophs and pathogens, rather than mycorrhizal fungi. But this doesn't mean that mycorrhizals don't benefit, and that soil-dwelling animals are enemies of trees. The presence of earthworms reduced the severity of Gaeumannomyces gramminis infections, despite increasing fungal and bacterial populations overall. Overall, passage through the gut increasing the viability of spores signficantly, indicating that while it does involve a greater cost (bugs eating them), there is also a greater benefit to traveling in the guts of insects rather than on the wind. And a few fungi in particular have evolved to take advantage of that.
While many mushrooms may incidentally benefit from flies and other insects carrying spores in their guts, the Stinkhorn fungi (order Phalleles) have turned it into a strategy. They produce famously foul-smelling fruiting bodies covered in a brownish slime, which helps create the impression of rotting meat. Insects such as bees, beetles, and especially flies, are all drawn in by the sight and smell of rot. The slime not only draws in insects, but is also full of spores, making it far less likely that spores will be spread by wind but far more likely to stick to the visiting insects. At the same time, the insects get to feast on the spore masses, getting a free meal in return for dispersing the fungi's spores. Keen readers may draw parallels between this form of mutualism and pollination, which has the same dynamic of an immobile symbiont providing food to insects in return for dispersal. Though there are differences, especially when comparing spores against pollen, it is a interesting connection worth thinking about.
But invertebrates aren't the only animals who eat fungi. Small mammals are an important influence, especially in colder regions with a lower density of flies. In glacial areas, mammals are considered signficant players in succession because they excell at bringing fungi to new habitats, helping shift a region from the bacterial stage of succession to one defined by fungi. Mammals are especially useful for dispersing hypogeal fungi, which produce underground fruiting bodies. Because many mammals have a strong sense of smell and can dig, they excell at locating and uprooting these hidden fungi. Voles and squirrels have been found to disperse arbuscular mycorrhizal fungi, and Gilbert's Potoroo, one of the most endangered mammals in the world, is believed to feed mostly on truffle-like fungi.
Image Source: Prof. Meike Piepenbring, CC BY-SA 3.0, via Wikimedia Commons
Termites and ants, while only distantly related, have evolved many otherwise unheard-of survival strategies that both groups share. One particularly impressive strategy, utilized by only certain species, is the ability to cultivate specific types of fungi in a controlled environment. Attini ants and fungus-growing termites, rather than eat plants themselves, carry plant matter from the surface into underground fungus gardens. Both of these insect taxa gather vegetation in organized groups, transport it into the nest, process it, and use the plant matter to construct elaborate, sponge-like combs on which the fungus will grow. They even weed and prune these gardens, pulling hyphae into different configurations to promote better growth and destroying spores from other fungi species. In ants, these fungal combs can grow to be as large as a human head, though they tend to make many smaller gardens instead. One four-year-old colony was found to have 1,027 chambers and 390 gardens. Termites create even larger combs, with one particular garden growing to be 50cm across and 25kg! And despite the complexity of this behavior, both taxa evolved this technique entirely independently.
There are some significant differences, however, both between and within the two groups. First, there's the fungi used. The species of fungus varies, but it tends to be the same within a species of insect, as the founding queen of a new colony will build the first fungal comb using chunks of hyphae brought from her previous home. These fungi are constantly producing large clusters of inflanted hyphae, which the ants eat. The fungi won't produce fruiting bodies while inside the nest, but spores can be cultivated in lab conditions, allowing scientists to identify them without DNA testing. One species, Attamyces bromatificus, is found exclusively within leafcutter ant colonies. Most of these mutualistic fungi are from Lepiotaceae, but there are a handful of species from other types of Basidiomycetes and Agaricales too, showing a vast amount of diversity across Attini ants.
Termites primarily cultivate species of Termitomyces while deliberately removing spores of other fungi. Their most common prominent competition is Aspergillus, a fungus found to be of lesser nutritional value to the termites. The fungi rarely fruit when inside the colony, but they may produce mushrooms if the colony dies off, or for other, more mysterious reasons. Some species of Termitomyces are not only edible, but culinarily valuable enough that there have been efforts to cultivate the fungus. The termites feed off of spherical conidiomata, a type of spore-dispersal structure which the fungi produce in large quantities. But the mutualistic relationships between termites and fungi go much further than these particular groups.
One difference between fungi-cultivating ants and termites is what they evolved from. Ants are extreme generalists, with some species mainly eating meat while others focus on seeds or other easily-digested plants. For fungus-growing ants, their symbiosis with fungi was necessary in order to take advantage of this valuable, but inaccessable, food source. But termites are different. Before the evolution of fungus cultivation, termites already had the ability to digest cellulose because of their mutualistic association with the bacteria, protists, and yeast in their gut. When the ancestors of these termites began growing fungal combs, they slowly lost most of these symbionts and, by extension, their ability to directly digest wood. For those termites, the fungi's ability to digest plant matter and make nitrogen available was more important than retaining their internal cellulose-digesting mutualists. It was a trade-off, but one that has been extraordinarily successful, judging by the population and diversity of fungi-cultivating termites.
But this isn't everything. There were many types of animal-fungi mutualism I wasn't able to cover, either because I struggled to find enough sources, had trouble fitting them in with the rest of the page, or simply never stumbled across them. The aphid species Meliarhizophagus fraxinifolii is one example, as are the wide array of insects with mutualistic yeast in their guts. This article should hopefully give you an idea of what's out there, and appreciate the complexity behind the things we take for granted.
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