28 minutes later: investigating the role of aflatrem-like compounds in Ophiocordyceps parasite manipulation of zombie ants

Abstract

Coevolutionary relationships between parasites and their hosts can lead to the emergence of diverse phenotypes over time, as seen in Ophiocordyceps fungi that manipulate insect and arachnid behaviour to aid fungal spore transmission. The most conspicuous examples are found in ants of the Camponotini tribe, colloquially known as ‘zombie ants’. While the behaviours induced during infection are well described, their molecular underpinnings remain unknown. Recent genomics and transcriptomics analyses of Ophiocordyceps camponoti-floridani have identified several highly upregulated biomolecules produced by the fungus during infection of Camponotus floridanus. Among them is an ergot alkaloid related to the mycotoxin aflatrem, known to cause ‘staggers syndrome’ in cows. Staggering, defined as unsteady movements side to side, is also observed in C. floridanus ants during late-stage infection. To test whether aflatrem-like compounds could be responsible, we injected healthy ants with aflatrem and recorded their behaviour for 30 min. Using both the automated object-tracking software MARGO and manual behavioural quantification, we found that aflatrem reduced ant activity and speed and increased staggering behaviours. To examine underlying transcriptomic changes, we performed RNA-seq on the heads of aflatrem-injected ants, keeping in step with previous transcriptomic work on Ophiocordyceps-manipulated ants. We identified 261 genes that were significantly dysregulated in the aflatrem-injected ants compared to sham-injected controls. When compared with RNA-seq data from Ophiocordyceps-manipulated ants, we found that both groups shared 113 differentially regulated genes. These included sensory neuron membrane protein genes, several odorant-binding protein genes and musculoskeletal genes such as titin and obscurin. Together, these results indicate that aflatrem-like compounds significantly affect neuromuscular and sensory function in C. floridanus. The conservation of staggers phenotype between C. floridanus and Bos taurus suggests that behaviour-manipulating strategies exhibited across the Tree of Life may be more similar in approach, if not widely different in application, than we realize.