Expression of glucocorticoid and mineralocorticoid receptor genes co-vary with a stress-related colour signal in barn owls

Glucocorticoid hormones are important intermediates between an organism and its environment. They enable an organism to adjust its behavioural and physiological processes in response to environmental changes by binding to mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) expressed in many tissues, including the integument. The regulation of glucocorticoids covaries with melanin-based colouration in numerous species, an association that might result from the melanocortin system and evolve within a signalling context. Most studies have focused on the circulating levels of glucocorticoids disregarding the receptors that mediate their action and could partly account for the covariation between the regulation of stress and melanin-based coloration. We investigated the association between melanin-based coloration and the gene expression levels of GR and MR but also of the melanocortin system and melanogenesis in the growing feathers of nestling barn owls (Tyto alba). We also considered the expression of genes related to the melanocortin system and melanogenesis to better understand the origin of the link between the expression of receptors to which corticosterone binds and melanin-based coloration. Nestling barn owls displaying larger black feather spots expressed GR and MR at lower levels than did smaller-spotted individuals. Because the expression of the GR and MR genes was positively, rather than negatively, correlated with the expression of melanogenic genes, we suggest that the link between melanin-based coloration, GR and MR is not directly associated to melanogenesis. The finding that the expression of GR and MR was positively associated with the expression of the PCSK2 gene (encoding the protein convertase responsible for post-translational modification of the proopiomelanocortin hormone) suggests that the melanocortin system may be implicated in the establishment of the covariation between melanism and the expression of receptors to which glucocorticoids bind. Together with previous studies, our results suggest that coloration is not only related to the production of glucocorticoids but also to the sensitivity of the integument to glucocorticoids.