Evolutionary divergence in transcription levels of neuropeptide receptors linked to both social and stress-response behaviour between two threespine stickleback populations exhibiting distinct behavioural types

Organism and background: Threespine stickleback (Gasterosteus aculeatus) from freshwater populations in Lake Temiscouata and Rond Lake (Quebec, Canada) differ in predator defence morphology and behaviour. Individuals from Lake Temiscouata have more lateral plates, longer pelvic and dorsal spines, and a longer pelvic girdle than fish from Rond Lake. When raised in a common environment, Lake Temiscouata fish are also significantly less aggressive and more limited in their locomotor activity than those from Rond Lake. Neurological background: Several neuropeptides and their receptors are known to be key players in both the molecular networks that underlie variation in social behaviour, and those that govern the physiological response to stress. These molecules include arginine vasotocin (AVT), isotocin (IT), corticotropin-releasing factor (CRF), and their receptors. Thus individuals that differ in social (aggression, sociality) and stress-response behaviours (locomotor activity, exploration, response to predators) might also differ in the activity of these neuropeptides and their receptors. Question: Do the juveniles of Lake Temiscouata and Rond Lake diverge in the expression of these neuropeptides and their receptors, particularly in the context of a response to an acute stressor. Methods: We quantified the genomic reaction norm of common-environment-reared juveniles from each population by measuring expression in the brain of genes coding for AVT, IT, CRF, and receptor subtypes (AVTR1a, ITR, CRFR1, respectively) before and after an acute stress using quantitative PCR. Results: We found no significant effect of population of origin, stress treatment, or their interaction on the expression of the three neuropeptides studied (AVT, IT, CRF) or of the AVTR1a receptor. We found a significant difference in expression of the ITR receptor between the two populations, with Temiscouata fish exhibiting higher expression of that gene, both before and after a stress. We observed a tendency for Temiscouata fish to show a larger transcriptional stress response for the CRFR1 receptor. Thus receptors in these neuropeptide networks have evolved divergently in these two populations and might be functionally implicated in behavioural divergence.