Pleiotropy in the melanocortin system: expression levels of this system are associated with melanogenesis and pigmentation in the tawny owl ( Strix aluco )

Abstract

The adaptive function of melanin‐based coloration is a long‐standing debate. A recent genetic model suggested that pleiotropy could account for covariations between pigmentation, behaviour, morphology, physiology and life history traits. We explored whether the expression levels of genes belonging to the melanocortin system ( MC 1R , POMC , PC 1/3 , PC 2 and the antagonist ASIP ), which have many pleiotropic effects, are associated with melanogenesis (through variation in the expression of the genes MITF , SLC 7A11, TYR , TYRP 1 ) and in turn melanin‐based coloration. We considered the tawny owl ( Strix aluco ) because individuals vary continuously from light to dark reddish, and thus, colour variation is likely to stem from differences in the levels of gene expression. We measured gene expression in feather bases collected in nestlings at the time of melanin production. As expected, the melanocortin system was associated with the expression of melanogenic genes and pigmentation. Offspring of darker reddish fathers expressed PC 1/3 to lower levels but tended to express PC 2 to higher levels. The convertase enzyme PC 1/3 cleaves the POMC prohormone to obtain ACTH , while the convertase enzyme PC 2 cleaves ACTH to produce α‐melanin‐stimulating hormone (α‐ MSH ). ACTH regulates glucocorticoids, hormones that modulate stress responses, while α‐ MSH induces eumelanogenesis. We therefore conclude that the melanocortin system, through the convertase enzymes PC 1/3 and PC 2, may account for part of the interindividual variation in melanin‐based coloration in nestling tawny owls. Pleiotropy may thus account for the covariation between phenotypic traits involved in social interactions (here pigmentation) and life history, morphology, behaviour and physiology.

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