The accumulation of the amino acid cysteine in lysosomes produces toxic substances, which are avoided by a gene (CTNS) coding for a transporter that pumps cystine out of lysosomes. Melanosomes are lysosome-related organelles that synthesize melanins, the most widespread pigments in animals. The synthesis of the orange melanin, termed pheomelanin, depends on cysteine levels because the sulfhydryl group is used to form the pigment. Pheomelanin synthesis may, therefore, be affected by cysteine homeostasis, although this has never been explored in a natural system. As diet is an important source of cysteine, here we indirectly tested for such an effect by searching for an association between food abundance and pheomelanin content of feathers in a wild population of Northern goshawk Accipiter gentilis. As predicted on the basis that CTNS expression may inhibit pheomelanin synthesis and increase with food abundance as previously found in other strictly carnivorous birds, we found that the feather pheomelanin content in nestling goshawks, but not in adults, decreased as the abundance of prey available to them increased. In contrast, variation in the feather content of the non-sulphurated melanin form (eumelanin) was only explained by sex in both nestlings and adults. We also found that the feather pheomelanin content of nestlings was negatively related to that of their mothers, suggesting a relevant environmental influence on pheomelanin synthesis. Overall, our findings suggest that variation in pheomelanin synthesis may be a side effect of the maintenance of cysteine homeostasis. This may help explaining variability in the expression of pigmented phenotypes.
Keywords: Animal pigmentation; Cysteine homeostasis; Melanogenesis; Phenotypic plasticity; Raptors.
Gyrfalcons Falco rusticolus adjust CTNS expression to food abundance: a possible contribution to cysteine homeostasis.Oecologia. 2017 Aug;184(4):779-785. doi: 10.1007/s00442-017-3920-6. Epub 2017 Jul 20. Oecologia. 2017. PMID: 28730346
Predation risk determines pigmentation phenotype in nuthatches by melanin-related gene expression effects.J Evol Biol. 2018 Dec;31(12):1760-1771. doi: 10.1111/jeb.13379. Epub 2018 Oct 13. J Evol Biol. 2018. PMID: 30256489
Impairment of mixed melanin-based pigmentation in parrots.J Exp Biol. 2020 May 11:jeb.225912. doi: 10.1242/jeb.225912. Online ahead of print. J Exp Biol. 2020. PMID: 32393547
Quantitative analysis of eumelanin and pheomelanin in humans, mice, and other animals: a comparative review.Pigment Cell Res. 2003 Oct;16(5):523-31. doi: 10.1034/j.1600-0749.2003.00072.x. Pigment Cell Res. 2003. PMID: 12950732 Review.
Current update and trends in melanin pigmentation and melanin biology.Keio J Med. 1995 Mar;44(1):9-18. doi: 10.2302/kjm.44.9. Keio J Med. 1995. PMID: 7760535 Review.