Time-course RNASeq of Camponotus floridanus forager and nurse ant brains indicate links between plasticity in the biological clock and behavioral division of labor

Abstract

Background Circadian clocks allow organisms to anticipate daily fluctuations in their environment by driving rhythms in physiology and behavior. Inter-organismal differences in daily rhythms, called chronotypes, exist and can shift with age. In ants, age, caste-related behavior and chronotype appear to be linked. Brood-tending nurse ants are usually younger individuals and show “around-the-clock” activity. With age or in the absence of brood, nurses transition into foraging ants that show daily rhythms in activity. Ants can adaptively shift between these behavioral castes and caste-associated chronotypes depending on social context. We investigated how changes in daily gene expression could be contributing to such behavioral plasticity in Camponotus floridanus carpenter ants by combining time-course behavioral assays and RNA-Sequencing of forager and nurse brains. Results We found that nurse brains have three times fewer 24 h oscillating genes than foragers. However, several hundred genes that oscillated every 24 h in forager brains showed robust 8 h oscillations in nurses, including the core clock genes Period and Shaggy. These differentially rhythmic genes consisted of several components of the circadian entrainment and output pathway, including genes said to be involved in regulating insect locomotory behavior. We also found that Vitellogenin, known to regulate division of labor in social insects, showed robust 24 h oscillations in nurse brains but not in foragers. Finally, we found significant overlap between genes differentially expressed between the two ant castes and genes that show ultradian rhythms in daily expression. Conclusion This study provides a first look at the chronobiological differences in gene expression between forager and nurse ant brains. This endeavor allowed us to identify a putative molecular mechanism underlying plastic timekeeping: several components of the ant circadian clock and its output can seemingly oscillate at different harmonics of the circadian rhythm. We propose that such chronobiological plasticity has evolved to allow for distinct regulatory networks that underlie behavioral castes, while supporting swift caste transitions in response to colony demands. Behavioral division of labor is common among social insects. The links between chronobiological and behavioral plasticity that we found in C. floridanus, thus, likely represent a more general phenomenon that warrants further investigation.

Hijacking time: How Ophiocordyceps fungi could be using ant host clocks to manipulate behavior

“…This resulted in Modules 4, 11 and 12 being indicated as day-peaking clusters and the other two (Modules 1 and 7) as night-peaking clusters (Figure 1A). Additionally, we found that the 'plasticity genes' that were significantly higher expressed in forager brains as compared to nurses 77 (for-UP) were significantly overrepresented in Module 9. In contrast, the genes that were significantly higher expressed in nurse brains (or, lower expressed in foragers, for-DOWN) were significantly enriched in Module 6.…”

“…Indeed, a 24-h time-course study on C. floridanus foragers and nurses of the same colony demonstrated that foragers, entrained to provide 12-h:12-h LD cues, showed robust nocturnal foraging peaks, while nurses remained within the nest. 77 Conducting RNA-Seq on the brains of ants collected from these behavioural castes throughout a 24-h day showed that evident chronobiological differences in gene expression are plausibly underlying these phenotypes. Forager brains exhibited 24-h rhythmicity in three times more genes than nurse brains.…”

“…Among these genes were homologs of animal core clock genes such as Period and shaggy. 77 Furthermore, several genes that are known to encode for proteins involved in the regulation of the division of labour in insects were found to be either differentially rhythmic (e.g., vitellogenin) or differentially expressed (e.g., trophallactic fluid proteins) in forager versus nurse brains. As such, this study found a link between chronobiological and behavioural plasticity in C. floridanus, which would allow a colony to have its worker ants swiftly transition between forager and nurse castes when conditions require it.…”

“…20,68 Many carpenter ant species, including C. floridanus, are nocturnal; their foragers leave the nest to search for food during the night-time. 68,77 Assuring that infected individuals wander away from the nest and summit during hours that are outside of the activity peak of healthy nest mates could, therefore, be another way for the fungus to increase its transmission chances.…”

“…The above-mentioned transcriptomics data on ant daily rhythms and behavioural plasticity in C. floridanus have been used to identify rhythmic genes and plasticity genes potentially involved in the behavioural division of labour into foragers and nurses. 77 Additionally, research on the behavioural manipulation of C. floridanus by O. camponoti-floridani has brought about transcriptomics data that, in addition to fungal genes, also identified ant genes that are potentially involved in manipulated summiting and biting behaviour. 20 Even though the latter study was not designed with the detection of clock gene expression in mind, enrichment analyses indicated that ant genes that have been functionally annotated to be involved in circadian rhythms and light sensing are affected during the manipulation phase of the Ophiocordyceps infection.…”