Cordyceps: curiosities about the fungus from the HBO series The Last of Us



by LORENZO CIOTTI

Cordyceps: curiosities about the fungus from the HBO series The Last of Us

The Last of Us is definitely the series of the moment. Airing weekly on HBO, the post-apocalyptic series has the true villain of the story as the Cordyceps fungus, which infects people until they turn into zombies. And it's a fungus that really exists in nature, but that has different effects than those shown in The Last of Us!

Cordyceps are divided into two groups. The first, which includes parasites of underground fungi, includes only about fifteen species in the world. The second, which includes parasites mainly of insects but also of spiders, is much larger and includes more than 250 species distributed mainly in tropical regions and in Asia.

Only about twenty species of those mentioned in the mycological literature have been found in Western Europe. The spores, when mature, are dispersed in the air. When a spore fragment contacts the body of the suitable host, the spore is stimulated, germinates, and enters the insect, usually as a larva.

The fungus then spreads throughout the insect's body and ends up killing it in a few days. When the insect is dead, the mycelium completely fills the body, which from the outside looks normal, like a mummified one. The fungus thus waits for favorable conditions to bear fruit.

Cordyceps: curiosities about the fungus from the HBO series The Last of Us

Other species can parasitize spiders or butterflies, producing, following the infection, in an initial stage, psychotropic substances which induce the animal to reach high places.

In a second stage the exoskeleton of the host is perforated by the numerous hyphae of the fungus which thus produces the fruiting bodies, and therefore the spores can be released in favorable positions and fall back on other arthropods.

The fruiting bodies, also called stromata, are made up of a stem which is much longer than the host the deeper it is buried, and of a fertile head called capitula which encloses the perithecia. These contain long, thin-walled asci except at the apex where it is much thickened.

Each ascus contains eight long, thread-like, septate spores. The spores of most species fragment when mature before being expelled from the ascus and leaving the perithecia through a small hole called the ostiole. Cordyceps, like many other ascomycetes, can also fructify in an imperfect stage that does not resemble the perfect stage.

Anamorphs, which have often been described under different names, are not easy to relate to teleomorphs because they only very rarely appear simultaneously.