Chapter 13
Fungal symbiosis
This chapter is divided into the following major
sections:- the major types of symbiosis involving fungi
- mycorrhizal associations
- lichens
- Geosiphon pyriforme– a remarkable “new” symbiosis
- fungus–insect mutualisms
Fungi are involved in a wide range of intimate sym-
biotic associations with other organisms. Some of the
more important examples are discussed in this chap-
ter, and it would be no exaggeration to say that they
have shaped the history of life on land. In several
cases the fungi and their partners have become so
intimately dependent on one another that they have
lost the ability to live alone. In other cases the fungi
can be cultured in laboratory media but they are,
in effect, ecologically obligate symbionts because they
seldom if ever grow as free-living organisms in nature.
The many thousands of species of lichen are classic
examples of this. They grow in some of the most inhos-
pitable environments on earth, where no other organ-
isms can grow, including cooled lava flows and arid
desert sands, where they literally hold the place in place!
Examples of new types of symbiotic association
continue to be discovered. In 1996 a unique associ-
ation between a mycorrhizal fungus and a cyanobac-
terium was reported for the first time. In this case thefungus engulfs cyanobacteria, which then provide the
fungus with its source of sugars (Gehrig et al. 1996).
This “dual organism,” Geosiphon pyriforme, is known
from only a few natural sites in Germany. Even more
recently, a nonphotosynthetic liverwort Cryptothallus
mirabilis (related to the mosses) was shown to
form a partnership with a species of Tulsanella
(Basidiomycota), a mycorrhizal fungus of birch trees.
When birch seedlings were supplied with radiolabeled
CO 2 the label was translocated from the birch
seedlings to the liverwort, via the mycorrhizal hyphal
network, supplying the liverwort with organic carbon
nutrients (Bidartondo et al. 2003).
As we shall see in this chapter, the below-ground
networks of fungal hyphae provide potential links
between several different types of organism. There is
every reason to believe that further examples remain
to be discovered.
Symbiotic associations are also significant in economic
terms. The fungal endophytes of pasture grasses pro-
duce toxic alkaloids such as lolitrem B and ergovaline,
which are now known to be responsible for several
diseases of horses, sheep, and other grazing animals
(Chapter 14). These fungi have been found in over
60% of pastures in the USA. The many types of mycorr-
hizal fungi are exploited commercially in forestry and
cropping systems. And in a quite different context, fungi
form several mutualistic associations with insects,
some of which cause serious economic losses.