The mutually beneficial interaction
between fungus-growing ants (Hymenoptera: Formicidae: Attini) and their fungi (Agaricales: mostly
Lepiotaceae: Luecocoprineae) is said to have started 45-60 million years ago.
In this symbiotic relationship, the fungus-growing, or attine ants use leaves, flowers,
and detritus as well as dead insects and their feces to mature their fungal
gardens. In addition, they disinfect these added organic materials to protect
the fungi from bacterial infection by licking and chewing them. This also begins
the decomposition process. The
disinfection is attributed to the secretion of antibiotic substance such as phenylacetic
acid, 3-hydroxydecanoic and indoleacetic acid from the ant’s mandibular glands
and metapleaural glands. The thick whitesh granular deposits on the surface of
some of the attine ants are actually actinomycetes-filamentous bacteria, which
also secrete anti-biotic substances. Collectively, these anti-biotic substances
play a big role in suppressing anti-fungal pathogens such as Escovopsis. In return for the protection and gardening,
the fungi produce specialized structures called gongylidia -rich in lipids and
carbohydrates- for the attine ants to consume.
The mutualism between the attine
ants and their cultivars has been conserved for over 45 million years mainly
due to ‘vertical transmission’ of the cultivars. It is referred to as vertical
transmission because attine queen ants carry fungal inocula from their parents
to their new colonies. Lateral transmission of cultivars between colonies has
also been reported and is used to explain incidences when distantly related ant
species cultivate the same fungi or incidences when a single ant species cultivates
distantly related cultivars.
Comparison of free living fungi and
ant-associated fungi has shown similarities between the two. This suggests that
any observed differences between them arose due to domestication of formerly
free living fungi by ants. In relative terms,
recently domesticated fungi show closer links to currently free living fungi,
while fungi domesticated earlier on show closer links to ancient free living
fungi. This data can be explained by co-evolution: fungi cultivated by attine
ants evolved differently than free living fungi. For example, over time, the
cultivated fungi have lost their independent resistance to infection due to the
anti-biotic substances and constant grooming provided by the attine ants. On
the other hand, free living fungi have retained the independent resistance,
likely passed down from ancient species.
Word count: 373
Reference
Currie, C R. “A Community of Ants, Fungi, and Bacteria: a
Multilateral Approach to Studying Symbiosis.” Annual Review of Microbiology 55
(2001): 357–380
