Figure 1.
Hypothesized mechanism at the basis of the cost of melanin-based signals.
High glutathione levels simultaneously neutralize reactive oxygen species (ROS) and inhibit melanin production. Individuals fully expressing melanin-based signals pay the cost of a decrease in their antioxidant power. The information content of the signal reflects the capacity to mobilize alternative antioxidant systems. Different mechanisms have been proposed to explain the inhibition of melanisation by glutathione: 1. Reaction of thiol groups of glutathione with L-dopaquinone to form dopa-thiol conjugates, which are precursors of phaeomelanin, leading to an apparent inhibition of pigmentation due to the lighter color of phaeomelanin [67], [68]. 2: Interaction of thiol groups with the tyrosinase active site, inhibiting tyrosinase hydroxylation [67]. Tyrosinase is the main catalytic agent for melanin production in vertebrates [69]. 3: Depletion of free radicals and peroxides by glutathione, as some of these agents stimulate tyrosinase activity [70].
Figure 2.
Effect of a glutathione inhibitor on the antioxidant machinery of great tits.
Nestlings treated with different doses of a glutathione inhibitor (DL-buthionine-S,R-sulfoximine; BSO) during development presented lower total glutathione levels in erythrocytes (A) and higher total antioxidant status (TAS) in blood plasma (B). TAS values were Ln-transformed to meet the normality assumption. Least squared means±SE from mixed models including the nest identity as a random factor.
Figure 3.
The reduction in glutathione levels was translated into signal expression.
Great tit nestlings treated with different doses of a glutathione inhibitor (DL-buthionine-S,R-sulfoximine, BSO) throughout their development showed larger breast stripes. Least squared means±SE from mixed models including the nest identity as a random factor. Pictures on left and right sides of the figure illustrate a control and a highest BSO-dose nestling.
Figure 4.
The level of total glutathione (tGSH) in erythrocytes was negatively related to the circulating levels of antioxidants (i.e. total antioxidant status; TAS) in control birds.
Dots represent real data, whereas the linear adjust was obtained from the model including the nest identity as random factor.