Temporal transcriptomic and chromatin dynamics underlying class-level recognition in cleaner wrasse

Class-level recognition, whereby receivers learn signaller cues and associate them with class-specific information such as familiarity, guides partner choice in mutualisms and supports community stability. Yet its molecular basis is poorly understood. In Labroides dimidiatus, class-level recognition shapes its behavioural ecology: a single individual can engage in 2,300 daily interspecific interactions, favouring rapid partner assessment and categorisation based on learned cues. To probe the molecular basis of this capacity, we coupled a familiar-unfamiliar two-choice social preference test with forebrain RNA-sequencing and H3K27ac profiling at 0, 30, and 120 min, linking behaviour, transcription, and chromatin state. Behaviourally, familiarisation reduced time spent near the familiar client. At the molecular level, transcriptomic profiles varied across time points, with early differences in genes related to synaptic release and chemosensory processes, followed by changes at 30 min in genes associated with GABAergic/homeostatic functions and, at 120 min, in genes linked to neuronal remodelling, consistent with a temporally structured molecular response. Chromatin profiling revealed broad baseline accessibility with modest between-condition differences and stronger within-condition temporal shifts, suggesting that chromatin supports, rather than drives, transcriptional change. Overall, this work provides a molecular framework for class-level recognition and suggests temporally structured molecular dynamics associated with social information processing.