Biology Reference
In-Depth Information
colonies by carrying a small amount of the fungus within a pouch in her oral
cavity to a new nest site. She uses the fungus as the inoculum for a new garden
(
Caldera et al. 2009
).
The ant-fungus symbiosis is complex: the ants have associations with at least
four microorganisms: 1) the fungi they grow as food in gardens, 2) a parasitic
microfungus (
Escovopsis
) that attacks the food fungus, 3) an actinobacterium
(
Pseudonocardia
) that produces antibiotics that inhibit the parasitic
Escovopsis
,
and 4) a black yeast that parasitizes the ant-actinobacteria mutualism (
Caldera
et al. 2009
).
The symbiosis between the ants, their fungal food and the
Escovopsis
para-
site originated
≈
50-65 million years ago (
Currie 2001, Currie et al. 2006, Caldera
et al. 2009, Poulsen and Currie 2010
). Ant workers culture the fungus garden
by grooming alien fungal spores from the garden or by removing infected leaf
material (grooming and weeding). The ants have paired metapleural glands that
produce antimicrobial compounds that help protect ants from insect-pathogenic
fungi. The ants pass their forelegs across the surface of the gland and then pass
their legs through their mouthparts during garden maintenance. It is thought
that metapleural gland grooming behavior allows application of chemicals
directly to fungus gardens that are infested with parasitic fungi. In addition, the
actinobacterium produces antibiotics that are effective against
Escovopsis
. For
a long time, it was thought that the ants had a waxy exudate on its body, but a
closer examination revealed these “waxes” are aggregations of the actinobac-
teria (
Pseudonocardia
) growing in elaborate cuticular crypts in the exoskeleton
and underneath these are specialized glands that produce secretions used by
the actinobacteria (
Figure 4.4
,
Currie et al. 2006
).
The fourth symbiont, black yeast (
Phialophora
), grows on the same locations
on the ant cuticle as the actinobacteria and parasitizes the system by acquiring
nutrients from the bacteria, which could reduce the ability of the bacteria to
suppress
Escovopsis
growth.
Are there any more microorganisms associated with attine leaf-cutter ants?
Future studies may identify other symbioses, according to
Caldera et al. (2009)
.
Pinto-Tomas et al. (2009)
demonstrated that fungus gardens of leaf-cutter ants
have a higher level of nitrogen than is present in the fresh leaves used to fer-
tilize them. Because ant workers had a higher level of N than was present in
the leaf material, it seems that N enrichment occurs as the ants process the
plant material. No nitrogen-fixing genes are present in the fungi or ants, but
they were found in
Klebsiella
bacteria isolated from the fungus gardens of leaf-
cutter colonies.
Klebsiella
are known to fix N
2
, as are bacteria in the genus
