Unlike wild and domestic canaries (Serinus canaria), or any of the three dozen species of finches in genus Serinus, the domestic urucum breed of canaries exhibits bright red bills and legs. This novel trait offers a unique opportunity to understand the mechanisms of bare-part coloration in birds. To identify the mutation producing the colorful phenotype, we resequenced the genome of urucum canaries and performed a range of analyses to search for genotype-to-phenotype associations across the genome. We identified a nonsynonymous mutation in the gene BCO2 (beta-carotene oxygenase 2, also known as BCDO2), an enzyme involved in the cleavage and breakdown of full-length carotenoids into short apocarotenoids. Protein structural models and in vitro functional assays indicate that the urucum mutation abrogates the carotenoid-cleavage activity of BCO2. Consistent with the predicted loss of carotenoid-cleavage activity, urucum canaries tended to have increased levels of full-length carotenoid pigments in bill tissue and reduced levels of carotenoid-cleavage products (apocarotenoids) in retinal tissue compared with other breeds of canaries. We hypothesize that carotenoid-based bare-part coloration might be readily gained, modified, or lost through simple switches in the enzymatic activity or regulation of BCO2 and this gene may be an important mediator in the evolution of bare-part coloration among bird species.

中文翻译：

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金丝雀裸露部位类胡萝卜素装饰从头出现的遗传基础。

与野生和家养的金丝雀（Serinus canaria）或Serinus属的三打雀中的任何一种不同，家养的金丝雀乌鲁姆犬繁殖出鲜亮的红嘴和腿。这一新特性为了解鸟类裸露部分着色的机理提供了独特的机会。为了鉴定产生彩色表型的突变，我们对大黄蜂的基因组进行了重新测序，并进行了一系列分析，以寻找整个基因组中基因型与表型的关联。我们在基因BCO2（β-胡萝卜素加氧酶2，也称为BCDO2）中鉴定出一个非同义突变，该酶参与将全长类胡萝卜素裂解和分解成短的类胡萝卜素。蛋白质结构模型和体外功能分析表明，urucum突变消除了BCO2的类胡萝卜素切割活性。与预测的类胡萝卜素切割活性丧失一致，与其他种类的金丝雀相比，乌鲁那金丝雀倾向于在票据组织中增加全长类胡萝卜素色素的水平，并在视网膜组织中降低类胡萝卜素裂解产物（类胡萝卜素）的水平。我们假设基于类胡萝卜素的裸露部分的着色可能很容易通过酶活性或BCO2调节中的简单转换而获得，修饰或丢失，并且该基因可能是禽类裸露部分着色演变中的重要介体。