Eumelanin- and pheomelanin-based colour advertise resistance to oxidative stress in opposite ways

Abstract

The control mechanisms and information content of melanin‐based colourations are still debated among evolutionary biologists. Recent hypotheses contend that molecules involved in melanogenesis alter other physiological processes, thereby generating covariation between melanin‐based colouration and other phenotypic attributes. Interestingly, several molecules such as agouti and glutathione that trigger the production of reddish‐brown pheomelanin have an inhibitory effect on the production of black/grey eumelanin, whereas other hormones, such as melanocortins, have the opposite effect. We therefore propose the hypothesis that phenotypic traits positively correlated with the degree of eumelanin‐based colouration may be negatively correlated with the degree of pheomelanin‐based colouration, or vice versa. Given the role played by the melanocortin system and glutathione on melanogenesis and resistance to oxidative stress, we examined the prediction that resistance to oxidative stress is positively correlated with the degree of black colouration but negatively with the degree of reddish colouration. Using the barn owl (Tyto alba) as a model organism, we swapped eggs between randomly chosen nests to allocate genotypes randomly among environments and then we measured resistance to oxidative stress using the KRL assay in nestlings raised by foster parents. As predicted, the degree of black and reddish pigmentations was positively and negatively correlated, respectively, with resistance to oxidative stress. Our results reveal that eumelanin‐ and pheomelanin‐based colourations can be redundant signals of resistance to oxidative stress.

Alternative reproductive strategies and the maintenance of female color polymorphism in damselflies

“…A key aspect in understanding the maintenance of the female‐limited color polymorphism in ischnuran damselflies that has thus far, however, been neglected is the possibility that color morphs could be signaling alternative reproductive strategies (see Roulin, 2004; Roulin & Bize, 2006 and references therein). This idea has recently received considerable support, with increasing evidence suggesting that the pigments necessary to produce alternative colors may have pleiotropic effects on physiological attributes (Armbruster, 2002; Eliason, Shawkey, & Clarke, 2016; Forsman, Ringblom, Civantos, & Ahnesjö, 2002; Merrill, Van Schooten, Scott, & Jiggins, 2011; Roulin, Almasi, Meichtry‐Stier, & Jenni, 2011). Such alternative strategies have only been studied in a handful of male‐polymorphic organisms (Ahnesjo & Forsman, 2003; Hutchings & Myers, 1994; Lank, Smith, Hanotte, Burke, & Cooke, 1995; Tuttle, 2003) and in three female‐polymorphic organisms: butterflies (Ellers & Boggs, 2002), fishes (Craig & Foote, 2001), and recently, reptiles (Galeotti et al., 2013).…”

Contrasted effects of an oxidative challenge and α-melanocyte-stimulating hormone on cellular immune responsiveness: an experiment with red-legged partridges Alectoris rufa

“…Accordingly, recent studies have found a positive correlation between the level of expression of eumelanic traits and some measures of resistance to oxidative stress (e.g., Roulin et al 2011). A key issue remains to better evaluate the possible costs associated with the production and maintenance of such traits, since these costs would ensure honest signaling of individual quality.…”

Melanin Chemistry and the Ecology of Stress

“…Thus, pheomelanogenesis represents a consumption of glutathione, the main intracellular antioxidant and physiological reservoir of cysteine, while cysteine is not consumed during eumelanogenesis (Ito and Wakamatsu 2008). This has led to the realization that producing eumelanin entails different benefits and costs than producing pheomelanin and that pheomelanogenesis may limit the ability to cope with physiological processes or environmental conditions that generate high levels of oxidative stress (Karell et al 2011;Roulin et al 2011;Galván and Møller 2013;Galván et al 2014). However, the diversity of melanins is more than just eumelanin and pheomelanin.…”