Molecular characterisation of boldness in rainbow trout

Boldness is linked to behavioural variation with a continuum from bold to shy individuals in many species. Bold individuals are more aggressive, more active and take greater risks than shy, timid conspecifics. Individual variation in boldness may be influenced by physiological differences, since the reactivity of the HPI (hypothalamo-pituitary-interrenal) axis and genotype is thought to be crucial in generating these divergent phenotypes. Therefore, bold and shy rainbow trout, Oncorhynchus mykiss, were characterised physiologically by measuring the production of cortisol, and an examination of differential expression of candidate genes in the brain. Genetic differentiation was conducted by microsatellite analysis to identify markers for the boldness trait. The subjects originate from an outbred population and two lines selected for divergent physiological responses to stress. These high and low stress responders correspond to shy and bold fish, respectively. This novel information will contribute to a better understanding of the molecular basis of these important individual traits.