Task allocation among social insect workers is an ideal framework for studying the molecular mechanisms underlying behavioural plasticity because workers of similar genotype adopt different behavioural phenotypes. Elegant laboratory studies have pioneered this effort, but field studies involving the genetic regulation of task allocation are rare. Here, we investigate the expression of the
foraging
gene in harvester ant workers from five age- and task-related groups in a natural population, and we experimentally test how exposure to light affects
foraging
expression in brood workers and foragers. Results from our field study show that the regulation of the
foraging
gene in harvester ants occurs at two time scales: levels of
foraging
mRNA are associated with ontogenetic changes over weeks in worker age, location and task, and there are significant daily oscillations in
foraging
expression in foragers. The temporal dissection of
foraging
expression reveals that gene expression changes in foragers occur across a scale of hours and the level of expression is predicted by activity rhythms: foragers have high levels of
foraging
mRNA during daylight hours when they are most active outside the nests. In the experimental study, we find complex interactions in
foraging
expression between task behaviour and light exposure. Oscillations occur in foragers following experimental exposure to 13 L : 11 D (LD) conditions, but not in brood workers under similar conditions. No significant differences were seen in
foraging
expression over time in either task in 24 h dark (DD) conditions. Interestingly, the expression of
foraging
in both undisturbed field and experimentally treated foragers is also significantly correlated with the expression of the circadian clock gene,
cycle
. Our results provide evidence that the regulation of this gene is context-dependent and associated with both ontogenetic and daily behavioural plasticity in field colonies of harvester ants. Our results underscore the importance of assaying temporal patterns in behavioural gene expression and suggest that gene regulation is an integral mechanism associated with behavioural plasticity in harvester ants.
Variation in nest relocation of harvester ants is affected by population density and food abundance
