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3.
The Felt Fungi
Species of Septobasidium (Fig. 14-84) form
a felty crust of growth over scale insects that infest plants, thus the
name “felt fungi.”
Fig. 14-84.
Scale insects overgrown with Septobasidium pseudopedicellatum.
They are Basidiomycota closely related to rust fungi.
Scale insects (Fig. 14-85) are “sap suckers” that extend
their long snouts (probosces) into plant tissues to derive their
nutrients.
Fig. 14-85. A close up of the felt fungus showing
numerous scale insects (arrows) attached to the twig.
Spores of the felt fungi germinate on the surface of
young scale insects, send haustoria into their coelomic cavities, and
thereby obtain their nutrient (Fig. 14-86).
Fig. 14-86. A cross section of a scale insect
showing the felt fungus covering it. Note, basidial and spore
production on the surface. (See Couch, 1938)
Septobasidium will then form an extensive,
felty, mycelial growth that will enshroud the scale insect. The fungus is biologically
clocked to form basidia and spores on the felt surface at the time
young scale larvae emerge from the felt and move on to new feeding sites.
Once the scale insect sets up housekeeping at a new location, the spores
will germinate and build a fungal house over the scale. Why did such an
association develop? How does the scale insect benefit? The answer is, the
fungus provides a home that protects the scale insect from birds and other
prey and from stressful environmental changes. Likewise, the fungus is
benefited by nutrients provided by the scale and dessimination of its
spores to new feeding sites.
4.
Fungus Culturing Ants
(Some
of the images in this section came from: www.zoo.org/virtualtour/TRF/ants.htm)
Leaf-cutting ants amaze us by their ability to cut and carry
back to their nest leaf pieces 10 times their own weight (Fig.
14-87; Fig. 14-88; Fig.
14-89).
Fig.
14-87. A diagramatic sketch of the nest of leaf-cutting ants.
Fig.
14-88.
The entrance to a leaf-cutting ant nest.
Fig.
14-89.
A brush painting of a trail of leaf cutting ants.
Dr. Neal Weber of the Department of Biological Science, Florida State
University has done extensive studies on fungus culturing by ants. The
leaf-cutting Attine ants have developed a mutualistic relationship
in which the vegetative mycelium of a particular fungus is passed from one
generation of ants to another solely through excretions. They grow the
fungus on piles of leaf disks (Fig.
14-90).
Fig.
14-90. Ants tending the fungal garden within their nest.
The ants are unable to digest cellulose, but the fungus can and is
thereby able to convert it into carbohydrates, fats and proteins that the
ants can utilize. The larvae pick up the fungus as the mycelium
proliferates in the ant galleries, forming large fungus gardens. These
nests extend 3 meters or more underground (Fig.
14-91).
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Fig.
14-91. An excavated nest showing the details of a fungal garden.
Most of these fungi are wood-rotting basidiomycetes such as species of Rozites,
Leucoagaricus (Fig.
14-92), and Lepiota
(Fig.
14-93).
Fig.
14-92. Leucocoprinus luteus, one
of the mushrooms cultivated in the fungal garden.
Fig.
14-93. Lepiotia
cristatiformis, another mushroom cultivated by leaf-cutting ants.
The leaf pieces are carefully handled by the ants and put into special
compartments, mixing them with saliva, and the ants actually inoculate the
fungus into the newly acquired bits of leaves. The fungus is not allowed
to grow to the surface of the ant mound by the continued licking and
feeding of the ants. The basidiocarps sporulate annually, usually during
the rainy season. Old, decomposed leaves are carried several meters away
from their nest. To prevent invasion by other fungi, the ant colony
produces more than 20 different antibiotics.
5.
Fungus Culturing Termites:
A
similar mutualistic relationship occurs between termites and fungi. Two
fungal groups are commonly found growing symbiotically with termites (Fig.
14-94), Termitomyces (a mushroom) and Xylaria
(a stromatic Ascomycete, the “dead man’s fingers”).
Fig.
14-94. A queen termite with workers, reproductives, and soldiers. (Provided
by Barbara Thorne)
Species
of Termitomyces are some of our
largest mushrooms, reaching 100 CM in diameter on African termite mounds (Fig.
14-95, Fig. 14-96).
Fig.
14-95. Giant termite mounds typical in Nigeria and other African
countries. (Photo
by Bob and Joann Parham)
Fig.
14-96. Termitomyces
robustus, one
of the mushrooms cultivated by mound-building termites.
This
mushroom is the favorite species used for food by the Yoruba tribe in
Nigeria. A very popular mushroom in China is the jizong mushroom, Collybia
albuminosa, a shaggy capped fungus that grows in association with 7
different species of termites. It appears in the mountainous areas after
torrential rains and for several weeks serves as the dominant food for
monkeys in the areas.
The size and complexity of termite nests are
remarkable (Fig.
14-97).
Fig.
14-97. Some fungi associated with termites: A and B) Termitomyces letestui, C) T.
microcarpus, D) Podaxon pistillaris. (Sands,
1969. Academic Press)
An interesting aspect of this complexity is the thermoregulation
within the nests. Mound shape is determined by the climate and locality
of the mound because it is important that the fungus garden be kept at a
comfortable temperature. This is done with a unique array of ventilation
chambers in the nests. If it
gets too cold, the termites place obstructions in the vents to maintain
warmth. When temperatures rise during the day, the vent closures are
removed and cooling can commence. Isn’t that neat! Is this where GE
gets their ideas? Guess what? In Zimbabwe, they have built office
buildings using this same thermoregulation principle and have found that
these buildings require less than 10% of the energy requirements that
are demanded in traditional buildings; and all because the
termites had to keep their fungal gardens cool! (McNeil, 1997, NY
Times, Green Building News). The fungus gardens in termite mounds are
very similar to those of leaf-cutting ants in that the termites have
designed special compartments for the fungi and give special nutritional
treatment for colony growth. The fungus in turn provides food and
special chemical metabolites that influence the reproductive dynamics of
the termite colonies. The cellulosic and pectinolytic enzymes of these
wood-rotting fungi help to break down the substrates for termite use.
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