The expression of the gene Slc7a11 promotes the antioxidant capacity of cells by providing them with cysteine that can be used for the synthesis of glutathione (GSH), the most important intracellular antioxidant. In melanocytes, intracellular cysteine can also enter melanosomes and get incorporated in the pheomelanin pigment synthesis pathway, thus decreasing cysteine availability for GSH synthesis and potentially creating chronic oxidative stress. We thus hypothesized that a mechanism limiting the use of intramelanocytic cysteine for pheomelanin synthesis in environmental conditions generating oxidative stress may be physiologically advantageous and favored by natural selection. Here we searched for evidence of such a mechanism by comparing the influence of melanocytic Slc7a11 expression on pheomelanin‐based pigmentation in developing Eurasian nuthatch Sitta europaea nestlings from two populations differing in predation risk, a natural source of oxidative stress. Pheomelanin synthesis and pigmentation tended to increase with Slc7a11 expression in the low‐risk population as expected from the activity of this gene, but decreased with Slc7a11 expression in the high‐risk population. The same was not observed in the expression of five other genes influencing pheomelanin synthesis without affecting cysteine availability in melanocytes. The influence of body condition on the intensity of pheomelanin‐based pigmentation also differed between populations, being positive in the low‐risk population and negative in the high‐risk population. The resulting pigmentation of birds was more intense in the high‐risk population. These findings suggest that birds perceiving high predation risk may limit the use of cysteine for pheomelanin synthesis, which becomes independent of Slc7a11 expression. Some birds may have thus evolved the ability to adjust their pigmentation phenotype to environmental stress.

中文翻译：

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Slc7a11 表达和体况对两个不同捕食风险的欧亚五子鱼 Sitta europaea 种群基于褐黑素的色素沉着的差异影响

基因 Slc7a11 的表达通过为细胞提供半胱氨酸来促进细胞的抗氧化能力，半胱氨酸可用于合成谷胱甘肽 (GSH)，谷胱甘肽是最重要的细胞内抗氧化剂。在黑素细胞中，细胞内的半胱氨酸也可以进入黑素体并被纳入褐黑素色素合成途径，从而降低用于 GSH 合成的半胱氨酸，并可能产生慢性氧化应激。因此，我们假设在产生氧化应激的环境条件下限制使用黑素细胞内半胱氨酸合成褐黑色素的机制可能在生理上有利并受到自然选择的青睐。在这里，我们通过比较黑素细胞 Slc7a11 表达对来自两个捕食风险不同的种群（氧化应激的天然来源）发育中的欧亚五子雀 Sitta europaea 雏鸟中基于褐黑色素的色素沉着的影响来寻找这种机制的证据。正如该基因的活性所预期的那样，低危人群中的褐黑素合成和色素沉着随着 Slc7a11 的表达而增加，但在高危人群中随着 Slc7a11 的表达而下降。在影响黑色素合成而不影响黑素细胞中半胱氨酸可用性的五个其他基因的表达中没有观察到相同的情况。身体状况对基于褐黑色素的色素沉着强度的影响也因人群而异，低危人群为阳性，高危人群为阴性。由此产生的鸟类色素沉着在高危人群中更为强烈。这些发现表明，感知到高捕食风险的鸟类可能会限制半胱氨酸在褐黑素合成中的使用，这与 Slc7a11 的表达无关。因此，一些鸟类可能已经进化出根据环境压力调整色素沉着表型的能力。