Lect27: Fungi and insect symbioses

Fungi and Insect Symbiosis

Introduction

We have previously described examples of symbiosis between fungi and photosynthetic organisms, i.e. mycorrhizae and lichens. Today, we will cover examples of symbiosis involving fungi and animals, specifically insects. We hear much less of fungus-animal symbiosis and usually think of such relationship only as one of host-parasitic relationship. However, the true relationship between fungi and animals are often not known.

One of the most important drives that may lead to such a relationship is the inability of animals to digest cellulose. When you think of herbivores, such as horses, sheep, cows, goats, etc., they do not actually have the ability to digest the cellulose from the plant material that they consume. Instead, they have symbiotic bacteria, in their stomach, that have the enzymes that digest the cellulose in the plant material for them. Other animals do not carry microorganisms in their gut, but rather consume mycelium in well-decomposed plant material as their source of food. Thus, symbiotic relationships between animals and various microorganisms are probably commonplace.

Some examples of those relationships that require more studies include: the Laboulbendiomycetes, Septobasidium and unrelated fungi that are known to grow on insects.

Laboulbeniomycetes

The Laboulbeniomycetes is a class of Ascomycota that is known mostly from the five-volume work by Thaxter (1896, 1908, 1924, 1926, 1931), which was updated by Benjamin (1971, 1973) and Tavarez (1979, 1985). For a summary of the most recent research on the phylogeny of the Laboulbeniomycetes, the following link by Weir and Blackwell (2005) may be consulted. They are best known as obligate parasites of various groups of arthropods, mostly insects, but also include mites, and millipedes. Infection of insects is wide spread and all species of insects may carry a species of Laboulbeniomycetes. Infection occurs by direct contact from infected insects. An unusual characteristic of this class is the lack of vegetative mycelium in most species. Other members of the mycelial ascomycetes have extensive mycelial growth that is derived from the germination of the ascospore. However, in the Laboulbeniomycetes, when ascospores germinate, the smaller of the two celled ascospore will penetrate the host by sending a haustorium through the exoskeleton that will reach the epidermal tissue where it will absorb nutrients. Although the haustorium does contact the epidermal layer of the host, there does not appear to be noticeable harm done to the host and infected hosts appear to live as long as those that are not infected. It was for the latter reasons that this relationship was thought to be one of commensalism. However, with the entry of haustoria into the host, there can be no doubt that this is a parasite/host relationship. This development is illustrated in Stigmatomyces cerotophorus below.

Development in the Pyxidiophoraceae, is the exception to the rule. Members of this family will produce extensive mycelial growth and is also not parasitic on arthropods. Instead the beetles on which they are found have a phoretic relationship and the extensive mycelial growth is found associated with another fungus on which it is parasitic.

Each species is known to have a high degree of host specificity even when compared to other groups of obligate parasites of arthropods. Loboulbeniomycete species are known to occur only on hosts of a single species, species within the same genus or at least closely related genera. However, some species of Loboulbeniomycetes have distribution ranges that extend from temperate to tropical latitudes, a distribution range that does not occur with the host arthropod species that are host to these fungi. Thus, there may be much greater host diversity than is believed.

The most interesting aspect of the host specificity exhibited by the Laboulbeniomycetes is the sex-host, position specificity that some species apparently exhibit. For example, in Stigmatomyces baeri, a species that can be found on house flies, the ascocarps are typically observed on the upper surface of the female fly and on the lower surface of the male. This phenomenon has most frequently been explained by the mating habit of the insects, i.e. how they have sex, and can be thought of in terms of a sexually transmitted disease. However, there are also species in which both sexes of host are equally heavily infested, and the ascocarps are observed in the same position on the integument. Thus, there is probably a great deal that is not understood of the Laboulbeniomycetes, with respect to their host, position preference.

Septobasidium

The genus is mostly composed of fungi that form symbiotic relationships with various scale insects. The genus is a member of the Basidiomycota and forms irregular, flattened, colonies that adhere closely to the bark of leaves of living trees, much like the growth of lichens. The colony may be flattened and only a few millimeters in diameter or may grow around the circumference of a branch. Based on this picture, Septobasidium appears to be nothing more than just another wood rotting fungus. However, there is a great deal more to this fungus than meets the eye.

The scale insects can be found in the middle layer of the fungus, in chambers that are only slightly larger than the insects, which are connected with numerous tunnels. Some of these insects are parasitized by the fungus, which have inserted haustoria, specialized feeding hyphae, into the insect body. These parasitized individuals are immobile and usually occupy a chamber where they are attached to the plant by their sucking apparatus the suctorial tube. The insect nourishes itself with the plant sap that it obtains through its sucking apparatus, and because the fungus is attached to the insect by haustoria, it is indirectly being nourished by the plant. Although this is a parasite/host relationship and seems to only favors the fungus, there are also benefits for the scale insect as well? A number of scale insects live within the so-call colony of the fungus and are sheltered from the environment and have their food supply beneath them. In addition, they are also protected from predators. Thus, the fungus colony offers the insects shelter, resulting in them living longer than their free-living counter parts.

Both the fungus colony with the insects may also over-winter without any harm coming to either partners. The following spring, the fungus will continue growth and the female insects will be ready to lay its eggs. The larval stage that emerges will crawl about the colony, and if it should go to the surface, it will pick up the fungal spores, which will adhere to their bodies. Three things can happen when the young emerges from its colony of origin:

1. It may go back inside its colony of origin.

2. It may crawl to a neighboring colony and join that colony.

3. It may go to an area where there is not a preexisting colony and when the spores that are attached to its body germinates, a new colony of Septobasidium will form.

Although a number of species of Septobasidium are known, they are a poorly studied group and the relationship of the fungus and insect has not been that well studied. Also, the effects of the haustoria formed on the insect, by the fungus has not been well studied.

Fungi Symbionts With Colonial Insects

The most interesting of the fungus-insect symbiotic relationships are those involving colonial insects. One of the most important driving forces that result in this type of symbiotic relationships is the inability of insects to digest cellulose. When you think of herbivores, such as horses, sheep, cows, goats, etc., they do not actually have the ability to digest the cellulose from the plant material that they consume. Instead, they have symbiotic bacteria, in their stomach that have enzymes that digest the cellulose plant material for them. Other animals, such as detritivores, do not carry microorganisms in their gut, but rather consume mycelium in well decomposed plant material as their food source. Thus, symbiotic relationships between animals and various microorganisms are common. We will look at some examples of insect-fungi symbiosis, which I think are far more interesting than the above examples.

Ants, Termites and Mushrooms

Social insects have always been of interest because of their seemingly, well ordered societies. In some of these social insects, the mound-building termites of Africa and Asia, and the leaf-cutting ants of Central and South America, there has evolved a rather unique strategy in the utilization of cellulose-rich plant material. These insects cultivate fungi, in underground gardens, and the world "cultivate" is used here in the true sense of the word, because these insects are deliberately growing these fungi. They grow only one fungus in their garden, which is not easily done, since there are so many sources of contamination that can occur and prevent their gardens from being successful. However, these insects are able to keep their gardens pure by constantly weeding out foreign fungi. They also care for their garden by providing suitable a food source, i.e. cellulose (=plant material), and moisture. So the fungi obviously benefit from this arrangement, but the ants and termites also benefit from this relationship. These insects are exclusively mycophagous, i.e., they only eat fungi. The fungi that they cultivate decompose the wood and leaves brought in by the termites and ants, respectively, and provide them with digestible and nutritious mycelium.

Leaf-Cutting Ants, Leucoagaricus and Lepiota

These gardening ants are from the New World Tropics and are commonly referred to as the Attine ants. They represent hundreds of species of ants, from approximately fifty genera. Although you probably have never heard of these ants, to the people of South America they are an all too familiar sight. In their search for food, these ants will devastate the natural vegetation and crops that are in their path, as they search for plant material to feed their fungus. When the Spanish Conquistadors arrived, they conquered the Native Americans, but were unable to do anything about the ants. Their efforts in growing cassava and citrus fruits failed because of their inability to control these ants. At the base of their fruit trees could be seen the ant nests which were "white as snow," presumably from the mycelium that they were growing. Of all the known species, Atta sexdens is the most economically important and the one which is most intensely studied, and the species that we will look at in detail as representative of this group of ants.

Colonies of A. sexdens start from a single winged female, the future queen, carrying a small inoculum of the fungus, in a pocket, in the back of her mouth that apparently has evolved specifically for this purpose. The winged female by this time has already been fertilized and potentially can lay as many as 300 million eggs during her lifetime. When the winged female finds a suitable site for her future colony, she takes out the fungus from her mouth and finds suitable plant material on which to inoculate the fungus. Once the fungus is growing, the queen begins to lay her eggs on the fungus. At first she is laying approximately fifty eggs each day, but eats most of these in order to nourish herself until the worker population has become established, which normally takes approximately three months. So the queen has eaten a lot of eggs by that time! During this first year, there is only a single entrance to the ant colony, but by the end of the second year, another entrance is added. From there, entrances proliferate at a much greater rate and approximately 1,000 entrances may occur by the end of the third year. It is at this time that new winged females are produced each year that will establish colonies elsewhere. However, as is the case with many species, there are far more winged females produced than will ever establish successful colonies. It is estimated that as many as 99.7% of all new colonies established by winged females are destroyed within their first six months. Attine ants also have a strict caste system, with ants of different sizes, e.g. queen, king, works, soldiers, etc., as is the case with other social insects, as well, e.g. bees and termites.

In studies that have been carried out in excavated nest, it was found that one nest that was four years old contained 1027 subterranean chambers, of which 390 contained fungus gardens. Another, approximately six years old, had 1920 chambers, or which 248 contained fungus gardens. Gardens are usually 20-30 cm (8-12 in) in diameter and weight approximately 300g (10.5 oz). It is estimated that these colonies had consumed 6000 kg (13,200 lbs) of vegetation.

In a young colony, the queen and the first workers to hatch from eggs establish the first fungus garden by excavating a chamber and filling it with vegetation brought in by the workers and then inoculating it with the fungus. Different species will utilize different substrate material for their fungus gardens. The Attine ants are commonly called leaf-cutting ants because they forage for leaves and cut them into pieces with their mandibles before carrying them back to their colony. Once they have returned with the leaf cutting, the workers cut the material into smaller pieces, lick it all over and often deposit anal excreta on it. The excreta, which serve as additional nutrients for the fungus garden, and plant material is then wedged into the garden and a tuft of mycelium placed on it. The gardens are sponge-like in appearance and is composed of numerous cavities which the workers walk through. In walking through, the workers probe the mycelium with its antennae, lick it, deposit anal droppings on it and also eat the hyphae.

Regardless of the species of ants, the colony only contains one species of fungus. This is difficult condition to maintain since, as you should recall, from our lecture on decomposition, fungi and bacteria are everywhere ready to take advantage of whatever organic material that becomes available. The worker ants in probing the mycelium with their antennae are able to distinguish their fungus from alien fungi. When foreign fungi are detected, the workers remove them. Also, some species have antibiotics that they secrete that will eliminate foreign insects. Some foreign fungi, undoubtedly, are present, but with the far more prevalent, cultured fungus, they are unable to compete and do not make up an appreciable part of the garden. When colonies are abandoned because of disturbance or migration, the fungus garden left behind deteriorates and becomes contaminated with other fungi and bacteria. Before abandoning their colony, the Attine ants always take some of the fungus garden with them as an inoculum to start their new fungus garden.

As the mycelium grows, swollen hyphal tips are formed, called the bromatia, which is the part of the hyphae that the ants consume. Although a great deal of plant material is brought into the colony, apparently the ants consume none of it. It is used entirely to feed the fungus and the ants only feed upon the fungus.

The discovery of the identity of the fungal species involved in these ants were determined by taking pure cultures into the lab and in some cases fruiting bodies have formed. In most instances, they have been determined to be species of the mushroom genera, Leucoagaricus and Lepiota. Other fungi that have been fruited include Auricularia, and Xylaria, a member of the Ascomycota. For reasons unknown, these species do not form fruiting bodies around or near the colony.

Termites and Termitomyces

Termites that consume wood are one of the worst pests that we can imagine in many cultures. However, in nature they play an important role in the decomposition of plant material. Most termites have symbiotic bacteria in their gut that allow them to digest cellulose and thereby recycling plant material. In addition because of their large biomass, they also serve as a food source for a number of other species of animals.

Where the ants that cultivate fungi are in the new-world tropics, the termites that cultivate fungi are native to the old-world tropic. These termites belong to the subfamily Macrotermitinae, which include approximately twelve genera that are distributed in Africa, Madagascar, India and much of south-east Asia. And unlike their new-world counterparts, these termites do not have bacterial in their gut to digest cellulose. Instead, as was the case in the Attine ants, the fungi that they cultivate will digest the plant material and the termite will eat the fungus.

The start of new colonies is similar to that of the Attine ants. The colony begins with a winged male and female rather than a winged female that has already been impregnated. The two termites will wall themselves in an underground "royal chamber" from which they will never leave. When mature, the queen will be quite large, relative to the other termites. The abdomen containing the eggs make up most of the large size of the queen, which may be up to 10 centimeters long in Macrotermes bellicosus. The queen then begins laying eggs, and the workers that result bring food to the couple, take new eggs away for incubation and add further to the nest. The workers continue to build the fungus garden around the royal chamber. Above the royal chamber, the workers build mounds that may be as much as six meters tall and three meters across at the base. The mound have air shafts that leads to the fungus garden, which by this time, may be a large central structure that is 50 cm (20 in) in diameter and weighing as much as 25 kg (55 lbs) or may be made up of a number of smaller chambers.

Colonies may contain as many as a million termites that forage for plant debris, mostly in the form of wood. Unlike the Attine ants, the termites will eat the plant material where they find it and upon returning to the colony will place their fecal droppings in the fungus garden. The fungus garden is sponge-like in appearance and on the surface arise spherical structures that are composed of clustered conidiophore and conidia. While tending the garden, the workers will nibble on the fungus. The cellulolytic enzymes, that are in the mycelium, remain active in the gut of the workers. The king, queen, soldiers and nymphs do not eat the fungus directly, and live on the salivary secretion of the workers. It can be seen that the mound building termites also have a complex caste system like that of the leaf cutter ants.

The fungi that are in these termite mounds, unlike those in the Attine ant colonies, are well known since they fruit readily in nature. The mushrooms formed have been observed to be connected to the fungus gardens of the termites. When there is rainfall of more than 2 cm/day, the fungus gardens will produce mushrooms with long stalks that will grow through the soil surface and produce the mushroom cap. These mushrooms have been identified as Termitomyces, a genus known only from the termite mounds. There are thirty species in this genus. In some species, a member of the Ascomycota, a species of Xylaria, may also be found growing with the mushrooms.

The Macrotermitinae are major pests of tropical agriculture and cause damage to wooden structures. They take nutrients underground to their mounds where they remain locked up and unavailable for years. However, they are part of the food chain, serve as food for many animals, and the Termitomyces species have become highly prized, edible mushrooms, in the tropics, and attempts are underway to cultivate these species.

Ambrosia Fungi/Beetles

Some scolytid beetles are wood inhabiting insects, and form tunnels in trees that are diseased or have been cut. The tunnels have narrow openings to the outside which widen into a number of cave like chambers where the eggs and larvae will develop. The tunnels and chambers are lined with the ambrosia fungus and is usually a source of food for the adult and is the sole source of food for the larvae. This is a very highly evolved relationship with only certain beetles and fungi occurring together. Neither the fungus nor the beetle species are found free-living in nature. This relationship has also proven to be harmful to a number of economic plants. The European Bark Beetle is responsible for carrying the spores of the Dutch Elm Disease that has decimated the American Elm and in Hawai‘i, an introduced species of ambrosia beetle has been having an impact on the Acacia koa, a native koa. You can read here about the economic damage that is caused, by the symbiosis between these two organism, on the plants that they live. The Asian Ambrosia Beetle is a common species on the mainland that has causes disease in trees.

The adult female beetle carries the fungus with them during their migration and when they are hibernating for the winter. There is a specialized sac, the mycangia, that occur in various locations of the body in different species of insects. When the female finds a suitable plant on which to live, she bores a small hole by which she will have access, which will eventually form a series or gallery of tunnels. It is along the gallery walls that the fungus garden is started. The mycangia produce secretions that prevent the spores from drying and also provides nutrients needed for the germination of the spores. When new tunnels are bored, the beetles prepare a mixture of feces and wood fragments which they smear on the tunnel wall as a substrate for the fungus to grow. As in the case of the colonial insect, there is only one fungus growing on the tunnel walls. This fungus is used as food for both the new larvae that may be borne along the gallery or in special chambers called cradles, and it is responsible for the damage that is done to the tree. However, it is not known how the beetles maintain pure cultures of the ambrosia fungi. When the tunnels are abandoned, contaminating fungi develop abundantly, and grow over the ambrosia fungi.

Terms for Fungus-Animal Symbiosis

Ambrosia Fungi: Fungi that are cultivated by scolytid beetles, in tunnels or galleries, dug in trees. Many of the fungi cultivated are plant pathogens harmful to the tree on which they are growing.

Bromatia: Swollen tips of the mycelium on which the leaf-cutting ants feed.

Leaf-cutting ants: They are so called because they cut leaves into smaller pieces that can more readily be carried by a single ant. These ants are native mostly to Central and South America that gather plant material to cultivate a fungus garden that they use for food. The gathering of plant material is often devastating to the vegetation in the area.

Mound building termites: Termites that build large termites mounds that shelter the termite colony. Unlike termites with which we are familiar, termites in this group do not contain the protozoan symbiont in their gut that allows them to digest wood. Thus, they do not eat wood, but rather use the wood that they gathered to feed the fungus garden that they cultivate. It is the fungus that is utilized by the termite for food.

Mycangia: Specialized body part that female scolytid beetles carry the fungus that they will cultivate when they find a suitable tree host.

Scolytid beetles: Wood boring beetles that cultivate ambrosia fungi, in the galleries that they dig in the trees.

Septobasidium: A genus of Basidiomycota that forms colonies on top of plants that have scale insects. Because of its presence on plants, it is often thought to be a parasite, on the plant, but is actually growing in association with the scale insect, which is parasitizing the plant.

Suctorial Tube: Feeding tube of the scale insect that is inserted into the host plant in order to obtain its food.

Termitomyces: Genus of mushroom that is only found in association with mound building termites. Termites only eat the mycelium and not the mushrooms. However, the mushrooms are collected, by indigenous people, for food, in Africa, India, Madagascar, and South East Asia, where these termites occur.

Some Questions of Interest

• 1. Even though the Laboulbeniomycetes don't seem to harm their host and the host seem to live just as long as those that are not infected with the fungus, we call it a parasitic relationship. Why is that?

  2. How does ascospore germination and development of the ascocarp in the Laboulbeniomycetes differ from other mycelial Ascomycota?

  3. What is the specific type of symbiosis involved between fungi in the Laboulbeniomycetes and insects?

  4. The leaf cutter ants and mound building termites collect plant material to feed to their fungus garden and then eat the fungus that they cultivate. Why don't they just eat the plant material instead of going to all that trouble to cultivate a fungus garden?

  5. Haustoria of Septobasidium can definitely be found entering the scale insects with which they are associated? Thus, it would appear that this is a host-parasite relationship. What reasons can you give that would indicate that this is a mutualistic symbiotic relationship?

  6. Termites can be very destructive to our personal property, e.g. our homes. However, they may also be very beneficial. Name some important roles that they play in nature.

  7. How are the worker ants able to rid their fungus garden of weed fungi that may arise?

  8. Although the ambrosia fungi bore holes in the trees that they inhabit, the tunnels, themselves, really do not hurt the trees. Nevertheless, they may be responsible for a great deal of damage to the tree where they live. How is it that they may cause this damage?