"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

November 12, 2013

Ophiocordyceps sessilis

There are many species of fungi that infect insects and some of the most well-known species are the ones that infect ants, better known to most as the "zombie ant fungus". We have previously featured one such fungus and its ant-jacking antics on this blog. But while most people might think that there's just a single zombie ant fungus out there which is responsible for creating this intriguing wonder (or nightmare) of nature, there are actually many different species of such fungi and they are found all over the world infecting various different insects. In the Ophiocordyceps genus alone there are over a hundred species and there might be some undescribed fungi that are hiding in plain sight because they have been misidentified and misclassified as a previously known species.
Photo of Ophiocordyceps sessilis from
Fig. 1 of the paper

Today, we are going to be featuring one such fungus and it hails from Japan where they are called Kobugata-aritake which means the "bump-neck ant fungus". The fungi specimen described in the paper we are discussing today were originally collected in 2006 from a forest near the village of Iitate, Fukushima. They were initially thought to be specimen of a fairly commonly found species call Ophiocordyceps pulvinata, but upon reexamination, researchers noticed a number of key differences which separated O. sessilis from O. pulvinata.

Both fungi were found sprouting from dead ants which had their mandibles clamped tightly around a branch in the typical "zombie ant" pose, but whereas O. pulvinata produce a bulbous fruiting body that sprouts from the back of the ant's head (see photo on lower left), ants infected with O sessilis are covered in spiny fruiting bodies jutting out all over the ant's body (see photo on upper right).

Further difference between the two fungi can be seen under the microscope; O. pulvinata produce discrete spores that are long and slim, but the spores of O. sessilis look like beads on a necklace which readily breaks apart into small "part-spores". These part-spores of O. sessilis can also germinate on malt-extract agar plates within two days, growing into soft, velvety colonies of fungal mass, whereas O. pulvinata spores failed to grow on such artificial medium. Finally, comparisons of sequences from selected genetic markers revealed that O. sessilis is clearly a very different species to O. pulvinata.

Photo of Ophiocordyceps pulvinata from
Fig. 1 of the paper
A peculiar thing the researchers noticed is that O. sessilis is only ever found in ants that are also infected with O. pulvinata. They suggested that O. sessilis is actually a parasite of O. pulvinata itself and noted other Ophiocordyceps species are often found in pairs, so what had previously be considered as coinfections may in fact be a case of hyperparasitism (whereby a parasite is itself infected by a parasite).

However, there is another possibility that the researchers did not mention in their paper, which was that O. sessilis needs O. pulvinata to pave the way in order for them to colonise the ant's body. An example of this is can be found among fluke-snail host-parasite systems. Like most digenean trematodes, the blood fluke Austrobilharzia terrigalensis they needs to infect a snail for the asexual part of its life cycle, but unlike those other species, A. terrigalensis cannot infect a snail on its own and is always found in snails that are already infected with another species of fluke. The coinfecting species always appear shriveled and emaciated in the presence of A. terrigalensis and it has been suggested that while A. terrigalensis lacks the ability to subvert or suppress the immune defences of snails, they are capable of colonising a snail once its defences have been knocked out by another species, at which point they barge in, overpower the resident parasite and take over the host.

So either O. sessillis is a hyperparasite (or a "mycoparasite" - a parasite of a fungus) of O. pulvinata, or it cannot colonise a host on its own and instead piggybacks on O. pulvinata, eventually usurping it and taking over the ant for its own. Either way, it appears that O. sessilis is a fungus that can hijack a fungus which is used to hijacking ants.

Reference:
Kaitsu, Y., Shimizu, K., Tanaka, E., Shimano, S., Uchiyama, S., Tanaka, C., & Kinjo, N. (2013). Ophiocordyceps sessilis sp. nov., a new species of Ophiocordyceps on Camponotus ants in Japan. Mycological Progress 12: 755-761.

P.S. I recently wrote an article for The Conversation about parasites that can survive freezing - including the hairworm (otherwise known as the parasite that gives crickets nightmares). To read it, just follow this link here.

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