Using RNASeq to investigate the involvement of the Ophiocordyceps clock in ant host infection and behavioral manipulation

Introduction Parasites can modify host behavior to ensure their own growth and transmission. Multiple species of the fungi Ophiocordyceps infect ants, but in a species-specific manner; one fungal species co-evolved to successfully modify the behavior of one ant species. However, several characteristics of the behavioral modification seem to be similar across different Ophiocordyceps-ant systems, including a preference for the time of the day for manipulating host behavior. In this study, we explored the various mechanisms via which the circadian clock of Ophiocordyceps might be playing a role in modifying host behavior. We studied O. camponoti-floridani that modifies the behavior of its ant host Camponotus floridanus. To separate the role of the clock in behavior manipulation, from its role in growth and survival, we compared the daily gene expression profile of O. camponoti-floridani to a generalist, non-manipulating fungal parasite, Beauveria bassiana, which also successfully infects the same ant host.

Results Majority of the 24h rhythmic O. camponoti-floridani genes show peak expression before or at the transitions between light and dark. Rhythmic genes in O. camponoti-floridani, for which B. bassiana lacks an ortholog, were overrepresented for enterotoxin genes. Around half of all genes that show 24h rhythms in either O. camponoti-floridani or B. bassiana showed a consistent difference in their temporal pattern of daily expression. At the halfway mark in O. camponoti-floridani infections, when diseased ants show a loss of 24h rhythms in daily foraging, several fungal clock genes, including Frequency, showed differential expression. Network analyses revealed a single gene cluster, containing White Collar 1 and 2, that showed overrepresentation for genes oscillating every 24h in liquid culture as well as genes differentially expressed while growing inside the ant head.

Conclusion This study identifies several sets of putatively clock-controlled genes and biological processes in O. camponoti-floridani that likely plays a role in modifying the behavior of its ant host. Differential expression of O. camponoti-floridani clock genes or 24h-rhythmic genes during infection is suggestive of either a loss of daily rhythm or a change in the amplitude of rhythmic gene expression. Both possibilities would suggest that a disease-associated change occurs to the functioning of the O. camponoti-floridani clock, and its output, while the fungi grows inside the ant head.