QTL for magnitude of the plasma cortisol response to confinement in rainbow trout

Better understanding of the mechanisms underlying inter-individual variation in stress responses and their links with production traits is a key issue for sustainable animal breeding. In this study, we searched for QTL controlling the magnitude of the plasma cortisol stress response and compared them to body size traits in five F2 full-sib families issued from two rainbow trout lines divergently selected for high or low postconfinement plasma cortisol level. Approximately 1000 F2 individuals were individually tagged and exposed to two successive acute confinement challenges (one month interval). Post-stress plasma cortisol concentrations were determined for each fish. A medium density genome scan was carried out (268 markers, overall marker spacing less than 10cM). QTL detection was performed using QTLMap software, based on an interval mapping method (http://www.inra.fr/qtlmap). Overall, QTL of medium individual effects on cortisol responsiveness (<10% of phenotypic variance) were detected on nineteen chromosomes, strongly supporting the hypothesis that control of the trait is polygenic. While a core array of QTL controlled cortisol concentrations at both challenges, several QTL seemed challenge specific, suggesting that responses to the first and to a subsequent exposure to the confinement stressor are distinct traits sharing only part of their genetic control. Chromosomal location of the steroidogenic acute regulatory protein (StAR) makes it a good potential candidate gene for one of the QTL. Finally, comparison of body size traits QTL (weight, length and body confirmation) with cortisolassociated QTL did not support evidence for negative genetic relationships between the two types of traits.