Nature exhibits many unsuspecting and unusual symbiotic
relationships, to which all are not beneficial to both parties. The different
symbiotic relationships include commensalism, mutualism, and parasitism.
Parasitism is the negative end of the spectrum whereby one symbiont benefits at
the cost of another. An explicit example of parasitism is the Cordyceps fungus
with various species of forest ants.
The Cordyceps fungus varies widely over the large array of
arthropods, most of which are species / host specific. Ants in particular are
especially susceptible to the fungus which is able to wipe out
entire colonies in a matter of weeks. It is estimated that approximately 8
million ants can occupy a single hectare in a forested environment; this may
partially explain why Cordyceps is more prevalent in forested locations. Currently
is it hypothesised that Cordyceps may be responsible for the population
regularity and stability in arthropod species in tropical forest environments,
for the reason that no one species gains the upper hand so to speak. Cordyceps, like most other fungi releases spores to reproduce with an array of different methods (Evans, 1982).
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| Figrure 1. Forest ant infected with Cordyceps fungus. Photographer: L. Austin (2003) |
Once an individual ant has become infected they can start to
show symptoms, typically in the change of behavioural patterns. Their focus turn to attacking themselves, almost in
an attempt to rid their bodies of the crippling fungus. It almost seems to take control
of the ants as it forces them to head to higher ground. This is advantageous to the fungus in terms of reproductive success, dispersal and infection. The individual soon dies in
a with its mandibles tightly locked around any structure that will prevent it from falling,
after several weeks the reproductive structure of the Cordyceps erupts from the
back of the ants head growing into extraordinary arrangements. Some ant species have, by some means,
come to recognise infected individuals and will carry and dump the infected away
from the colony. Obviously this is to prevent the remainder of the colony from becoming infected (Holder and Keyhani, 2005).
There are thousands of varieties of Cordyceps to which most
specialise on a single species of arthropod. Virtually nothing can save an
individual once infected though some ants have come to recognise the symptoms
of Cordyceps and will dispose of infected individuals. This is generally advantageous and poses minimal risk to other arthropods it is species specific. However, if the same species of ant is within close proximity to the colony that is disposing of ants they may become infected.
References:
EVANS, H. 1982.
Entomogenous fungi in tropical forest ecosystems: an appraisal. Ecological Entomology, 7, 47-60.
HOLDER, D. J. & KEYHANI, N. O. 2005. Adhesion of the
entomopathogenic fungus Beauveria (Cordyceps) bassiana to substrata. Applied and environmental microbiology,
71, 5260-5266.
Figure 1. L. Austin (2003) Forest ant infected with Cordyceps fungus. https://www.utexas.edu/courses/zoo384l/sirena/species/fungi/
Video with thanks to BBC: https://www.youtube.com/watch?v=XuKjBIBBAL8
This idea of “zombie ants” is quite interesting. Do you what type of fungus Cordyceps is? Is there any suggestion in the literature of how this symbiotic relationship has evolved? Is there also any evidence that some individuals are more resilient and less likely to be infected than others (within the same species)?
ReplyDeleteCordyceps is 1 of 400 known species of ascomycete fungi or sac fungi. The fungus is commonly referred to as entomopathogenic fungi as it primarily hosts on insects and other small arthropods. There is some evidence to suggest that the relationship has existed for more than 40 million years by fossilised leaves. The evolution of the relationship is poorly known. I could not find a clear answer to your question of resilience though species of ants have evolved defense techniques that are behavioural based, primarily avoiding contact with the fungus. The ants will also groom themselves if spores are detected. It would seem logical to think that the ants that primarily attend to the queen or spend almost all their time in the nest / colony will be less likely to come into contact with the fungus; the explores / soldiers of the colony would be more susceptible to contact.
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