By Erich G. Vallery, USDA Forest Service - SRS-4552, Bugwood.org, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=4476437.
I read an interesting National Geographic page that summarised the findings of a paper, which described the first observation of the parasitic fungus Entomophthora muscae infecting wild vinegar fruit flies, Drosophila melanogaster.1
This fungus causes very specific behaviour in flies it infects, especially the fly’s final day of life: The fungus forces the fly to climb the highest point in its environment, before the fly is stuck there via fungal hyphae growing out of the fly’s extended proboscis.2 After several hours, the fly is forced to raise its wings, after which fungal structures called conidiophores burst through the fly’s exoskeleton at its weakest points, forming a distinct, fuzzy banding pattern on its abdomen.2 The spores are ejected from the conidiophores to spread to new hosts.2 Spores that stick onto another host bore through the fly’s exoskeleton to enter its body.2 Once the fungus has consumed the fly’s hemolymph (the invertebrate equivalent of blood) and its fat stores, it induces the mind control behaviour of its host, allowing it to reproduce.2
The aim of this paper was to study how this fungus causes these behavioural changes in its host. Laboratory bred versions of Drosophila melanogaster were exposed to the fungus to observe changes in the fly’s gene expression or neural activity, in comparison to control fruit flies.2 They saw that the genes associated with the fly’s immune system had increased activity 24 hours after infection.2 After 48 hours the fungus had infiltrated the ventral nerve cord (the invertebrate equivalent of a spinal cord), and after 96 hours (4 days) the structure of the internal organs was disrupted, after which the fly died.2 The conidiophores emerge from the fly’s dead body 2-3 hours later.2
Other social insects such as ants (which are also infected by parasitic fungi) have defensive behaviour against infection by fungal parasites, such as leaving the colony to die in isolation, and in doing so preventing the fungus spreading to other members of its colony.3 Other social insect species such as termites have learned to detect the musty odour of their fungal parasite; one study found that termites exposed to fungus (diluted 1:4 in water) induced mutual grooming, to try and clear the spores before they infect the host.4
I was also interested to read about other parasitic organisms that alter the behaviour of their hosts. some examples are:
- Ants infected by the parasitic fungus Ophiocordyceps unilateralis which causes similar climbing behaviour as the fruit flies
- a parasitic worm which makes infected crickets drown themselves so the worm
- a parasitic flatworm (Leucochloridium paradoxum) driving it’s host snail into plain view of bird predators
- The protozoan Toxoplasmosis gondii causes rats and mice to be attracted to pheromones in cat urine to increase likelihood of being eaten.5
- Langley L (2018). Watch these flies turn into zombies; meet the bizarre organism that acts as a puppet master. National Geographic. published August 10th 2018, accessed 6th October 2019, available from: https://www.nationalgeographic.com/animals/2018/08/zombie-flies-mind-control-fungus-animals/
- Elya C, Lok TC, Spencer QE, McCausland H, Martinez CC and Eisen M (2018). Robust manipulation of the behaviour of Drosophila Melanogaster by a fungal pathogen in the laboratory. eLife, published July 8 2018. Accessed 27th October 2018. Available from: https://elifesciences.org/articles/34414
- Heinze, J, Walter B (2010) Moribund ants leave their nest to die in social isolation. Current Biology, vol. 20, No 3, published January 8th 2010, accessed 29th October 2018, available from: https://www.sciencedirect.com/science/article/pii/S0960982209021551?via%3Dihub Doi: https://doi.org/10.1016/j.cub.2009.12.031
- Yanagawa A, Fugiwara-Tsujii N, Akino T, Yoshimura T, Yanagawa T, Shimizu S. Musty odor of entomopathogens enhances disease prevention behaviours in the termite Coptotermes formosanus. Journal of Invertebrate physiology, vol. 108, No 1, Science Direct, published 12 June 2011, accessed 29th October 2018, available from: https://www.sciencedirect.com/science/article/pii/S0022201111001066?via%3Dihub Doi: https://doi.org/10.1016/j.jip.2011.06.001
- Jones L (2015). Ten sinister parasites that control their hosts minds. BBC Earth, published 16th March 2015, accessed 28th October 2018, available from: http://www.bbc.com/earth/story/20150316-ten-parasites-that-control-minds
- Shang Y, Feng P, and Wang C. Fungi That Infect Insects: Altering Host Behavior and Beyond. PLoS Pathog. 2015 Aug; 11(8): e1005037. Published online 2015 Aug 6. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4527712/ doi: 10.1371/journal.ppat.1005037 - for a review of entomopathogenic fungi and their interaction with their hosts.
- Roode JCe and Lefèvre T. Behavioural immunity in insects. Insects, vol 3, no 3, pg. 789-820. published September 2012, accessed 30th October 2018, available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4553590/ doi: 10.3390/insects3030789 – for a review on behaviours used by insects to prevent infection by parasites.
- Cordyceps: attack of the killer fungi - Planet Earth Attenborough BBC – a video example of how entomopathogenic fungal conidiophores burst through the exoskeleton of their host. This kills the host and endangers other individuals of the host’s species. This ant species tries to prevent this by moving fungal infected corpses as far away from the colony as they can.