Transcriptional responses in feeder time-trained foragers suggest diverse interactions between the circadian clock and mushroom bodies in honey bees

A hundred years ago Karl von Frisch and his students demonstrated that honey bees use time-memory to schedule their daily foraging flights. However, till today little is known about molecular processes and functional interactions between memory centers and the circadian clock underlying the capability to form time-memories in animals. Combining feeder time-training of foragers with time-series RNA sequencing and RNAscope labeling revealed molecular features associated with the expectation of foraging activity: (i) anticipatory activation of the transcription factor Egr1 and the receptor for pigment dispersing factor (pdfr) in the small-type Kenyon cells (KCs), (ii) synchronized peak-expression of more than 850 genes including Egr1 downstream genes and well-known memory-related genes during training time, and (iii) groups of KCs and cells associated with the central complex co-expressing per and cry2. With respect to earlier studies characterizing behavioral correlates of time-memory, we speculate that anticipatory initiation of physiological and transcriptional activity in the small-type KCs might function in preparing the worker bee for its foraging activity including reactivation and reconsolidation of foraging related memories. The expression of clock genes in addition to pdfr in KCs suggests an unexpected complexity of functional interactions between memory centers and the clock in honey bees.