Data from: Physiological conditions and genetic controls of phaeomelanin pigmentation in nestling barn swallows

Phaeomelanin is a common pigment that confers a reddish color to animals. Since phaeomelanogenesis requires the sulfhydryl group from cysteine or glutathione (GSH), which is an important antioxidant, this pigmentation and the associated coloration may be an honest signal, whereby only high-quality individuals (e.g., with lower oxidative stress) are able to develop showy plumage. The present study tested the mechanisms underlying the honest signal hypothesis using nestling barn swallows, Hirundo rustica gutturalis, which exhibit phaeomelanic throat plumage patches. We examined the relationship between phaeomelanin pigmentation levels and physiological condition during trait development, and the expression of the phaeomelanin-related gene agouti-signaling protein (ASIP) and the GSH-related gene glutathione S-transferase (GST) in throat feather follicles. We found that during phaeomelanogenesis, heavier nestlings produced more pigmented feathers, indicating that nestlings with high phaeomelanin concentrations are in better condition. We also found that phaeomelanin concentration was negatively correlated with total GSH level, but not significantly related with measures of oxidative stress. Among the GST genes, GSTM3 exhibited the highest expression in the developing feathers during phaeomelanogenesis. The expression levels of ASIP were positively associated with the amount of phaeomelanin deposition and negatively associated with the expression of GSTM3, reducing the amount of GSH that was available as an antioxidant. These findings suggest that high-quality individuals produce high concentrations of phaeomelanin in their plumage without experiencing increased oxidative stress, despite phaeomelanin production, which is triggered by ASIP, potentially actively consuming the sulfhydryl group from GSH.褐黑素（Phaeomelanin）是一类广泛分布于动物体内的色素，可赋予生物体红色调体色。由于褐黑素生成（phaeomelanogenesis）过程依赖半胱氨酸或谷胱甘肽（glutathione, GSH）提供的巯基，而谷胱甘肽是一类重要的抗氧化剂，因此该色素沉着及其相关体色表现可作为一种诚实信号（honest signal）：即仅生理质量较高的个体（如氧化应激水平更低的个体），方可发育出艳丽的羽毛。本研究以具有褐黑素喉部羽斑的家燕（Hirundo rustica gutturalis）雏鸟为实验对象，对该诚实信号假说的潜在机制进行验证。我们分析了性状发育阶段褐黑素沉着水平与生理状态的关联，并检测了喉部羽毛囊中褐黑素相关基因刺豚鼠信号蛋白（agouti-signaling protein, ASIP）以及谷胱甘肽相关基因谷胱甘肽S-转移酶（glutathione S-transferase, GST）的表达水平。研究结果显示，在褐黑素生成阶段，体重更大的雏鸟所长出的羽毛色素沉着程度更高，这表明羽毛褐黑素浓度较高的雏鸟生理状态更佳。此外，我们发现褐黑素浓度与总谷胱甘肽水平呈负相关，但与各项氧化应激（oxidative stress）指标无显著关联。在谷胱甘肽S-转移酶家族基因中，GSTM3在褐黑素生成期的发育羽毛中表达量最高。刺豚鼠信号蛋白（ASIP）的表达水平与褐黑素沉积量呈正相关，而与GSTM3的表达呈负相关，该过程会减少可作为抗氧化剂的谷胱甘肽可用量。上述结果表明，尽管由刺豚鼠信号蛋白触发的褐黑素生成过程可能会主动消耗谷胱甘肽中的巯基，但高质量个体仍可在未出现氧化应激升高的情况下，在其羽毛中合成高浓度的褐黑素。