The evolution of eu- and pheomelanic traits may respond to an economy of pigments related to environmental oxidative stress.

Dear Sir, Vertebrate animals produce two chemically distinct melanin pigments, eumelanin and pheomelanin, often simultaneously in the same cells but one usually prevailing on the other (e.g., Ozeki et al., 1997). The production of these pigments takes place in melanosomes, the specialized organelles of melanocytes. The first step in the melanogenesis pathway consists in the hydroxylation of L-tyrosine by tyrosinase to produce dopaquinone. Dopaquinone then undergoes a cyclization to form leukodopachrome leading to eumelanins. Alternatively, dopaquinone can react with thiol groups to form thioldopa conjugates, the precursors of pheomelanins (e.g., Ozeki et al., 1997). The switch from euto pheomelanogenesis is controlled by hormones such as the melanocyte-stimulating hormone (alpha-MSH) and the agouti signalling protein, which stimulate and inhibit, respectively, the action of tyrosinase. High tyrosinase activity favours eumelanogenesis as opposed to pheomelanogenesis (Ozeki et al., 1997). Because of the dependence of pheomelanins on thiol groups that react with dopaquinone, cysteine and cysteine-containing peptides also act as regulatory agents of the production of these melanins. Pheomelanogenesis preferentially occurs under conditions of high thiol concentrations and low tyrosinase activity, while opposite conditions are required for eumelanogenesis (Benathan et al., 1999). The fact that the thiol groups interact with the tyrosinase active site, thus inhibiting its activity (Jara et al., 1988), increases the opposed nature of conditions under which euand pheomelanogenesis occur. Whether thiol groups that are used during pheomelanogenesis are obtained from free cysteine or from reduced glutathione (GSH) is still under debate, but the most likely explanation is that free cysteine is transported from cytosol into melanosomes through a membrane transport mechanism (Potterf et al., 1999). In any case, GSH is the main physiological reservoir of cysteine, and thus it influences cysteine levels and potentially the process of pheomelanogenesis (Benedetto et al., 1981). GSH is also the most important intracellular antioxidant (Anderson, 1998; Wu et al., 2004). This means that, if eumelanogenesis takes place under conditions of low levels of thiol groups, this process requires proceeding with a diminished antioxidant capacity as compared to pheomelanogenesis. Therefore, oxidative stress (i.e. the imbalance between production of oxygen reactive species and availability of antioxidant compounds) might be higher in melanocytes where eumelanogenesis prevails on pheomelanogenesis because, although the latter requires cysteine consumption to proceed, there are generally lower levels of free thiol groups during eumelanogenesis. From kinetic considerations, tyrosine exclusively produces cysteinyldopas as long as cysteine is present since addition of thiol groups on dopaquinone is much faster than cyclization to leukodopachrome (Ozeki et al., 1997). Evidence that eumelanogenesis increases oxidative stress as a consequence of the low GSH levels it requires has recently come from birds, as great tit (Parus major) nestlings with experimentally decreased GSH levels developed more intensively an eumelaninbased plumage trait than controls, while they were forced to increase the levels of alternative, circulating antioxidants (Galván and Alonso-Alvarez, 2008). In mice, it is known that a mutation in the Slc7a11 gene decreasing extracellular cystine transport into melanocytes and thus GSH levels is responsible for a notably high susceptibility to oxidative stress in the individuals that present it (Chintala et al., 2005). Melanin-based phenotypic traits often evolve as signals of individual quality in vertebrate animals, an issue that has been mainly studied in birds (McGraw, 2008). Here we propose that, as euand pheomelanogenesis occur under conditions of low and high levels, respectively, of the key intracellular thiol-containing antioxidant (GSH), eumelanic and pheomelanic traits have the potential to signal different, even opposite, physiological information on the individuals that exhibit them, irrespective of