Leaf- cutter ants are part of a larger ant group
called the fungus garden ants (Myrmicinae), each
species of which, remarkably, cultivates a particular
species of symbiotic fungus, which makes up its
principal food source (discussed below). There are
approximately 200 fungus garden species, of which 37
are leaf- cutters (Attini). The remaining species, most
of which are inconspicuous, cultivate their fungus
on some combination of decaying plant or animal
organic matter. Though most abundant in the tropics,
fungus garden ants also occur in warm temperate and
subtropical grasslands. One species even occurs as far
north as the New Jersey pinewoods.
Leaf- cutter ants do not consume leaves but rather clip
and carry leaf fragments back to their colonies. There
they convert the leaves to media that serve to culture
a specific fungus. This fungus is the ants’ food, though
they also taste and ingest the sap of the leaves they cut,
sometimes using it as an additional food source. Leaves
brought to the colony are clipped into small pieces and
chewed into a soft pulp. Before placing the pulpy mass
on one of many fungus beds within the colony, a worker
ant holds it to its abdomen and defecates a droplet of
enzyme- and protein- rich fecal liquid on it. The chewed
leaf is then added to the fungus- growing bed, and small
fungal tufts are placed atop it. Other ants sometimes add
their fecal droplets to the newly established culture. The
fungus bed is termed a cultivar.
Worker ants collecting leaves avoid those that contain
chemicals potentially toxic to the fungus. For example,
the tree Hymenaea courbaril, a legume, has been shown
to contain an antifungal terpenoid, and Atta ants, not
surprisingly, avoid clipping its leaves. The tree has
evolved a protection from Atta not by defending against
the ant but by defending against its fungus.
The ants culture only a few fungal species, all of which
are members of the family Lepiotaceae, in the class
Basidiomycetes, a group whose free- living members
include the familiar mushrooms. The fungi are never
found free- living outside of ant colonies. The fungus
garden is protected from contamination from other
fungal species by constant “weeding” by ants (but see
more on these fungi below). Without the attention of
the ants, the fungus will be overtaken by other fungal
species. Both ants and fungi are totally interdependent,
an example of an obligate mutualism. Ant and fungus
are coevolved, like the fig wasps and fig plants, to
produce essentially two united genomes. Fungus and
ants disperse together: only the queen reproduces, and
when a queen ant founds a new colony she takes some
of the precious fungus with her inside her mouth.
Studies of the fungus- ant relationship at the
biochemical level have revealed that ants play multiple
roles in culturing the fungus. The ants clean the leaves
as they chew them to make the culture bed pure.
Ant fecal fluid contains ammonia, allantoic acid, the
enzyme allantoin, and all 21 common amino acids.
These compounds are all low- molecular- weight
nitrogen sources, and they are the key ingredients in
making the culture optimal for the fungus. The fungus
lacks certain enzymes that break down large proteins
(all of which are made up of chains of amino acids).
Thus it depends totally on the ant fecal fluid to supply
its amino acids. Experiments attempting to grow the
fungus in a rich protein medium have failed. It can
grow only in a medium of small polypeptides and
amino acids. Ants also supply enzymes necessary to
aid in breaking down protein chains. Michael Martin
summarized the functions of the ants:- fungal dispersal
- planting of the fungus
- tending the fungus to protect it from competing
species, - supplying nitrogen in the form of amino acids
- supplying enzymes to help generate additional
nitrogen from the plant medium.
While this may seem complex, it actually gets even
more complex, because the system is not confined to just
ants and fungus. There is a second fungus, Escovopsis,
which is antagonistic, invading and consuming the ants’
cultivar. But ants are able to hold this fungus in check
because actinomycete bacteria (order Actinomycetales)
that live on the ants manufacture antifungal chemicals
that inhibit the Escovopsis. However, yet another
organism, a black yeast, feeds on the actinomycete
bacteria, making it more difficult for the ants to protect
their cultivars. Thus what appears to be a tightly linked
two- species system (a very traditional way in which to
view a mutualistic relationship) is, in fact, a far more
complex five- species interaction of antagonistic and
mutualistic interactions that one researcher has called
a “balancing act.”
Each fungus garden is unique. The cultivar in any
given colony is isolated from cultivars in other attine
gardens because queens that found new colonies
transport only the fungi from their initial colony.
The genetic composition of the fungus from each of
the colonies is distinct, and there appears to be no
176 chapter 10 tropical intimacy: mutualism and coevolution