Evidences in duck (Anas platyrhynchos) by transcriptome data for supporting the biliverdin was mainly synthesized by shell gland.

ABSTRACT The blue-shelled egg not only plays a key role in helping birds to avoid predation as a result of crypsis and mimetism, but it also provides eggshell strength and filters solar radiation; moreover, it has an important economic trait for poultry. However, the source of biliverdin for blue-shelled egg remains unsolved in ducks. The current study detected the biliverdin content and localization of heme oxygenase 1 (HMOX1) in duck shell gland; moreover, RNA-seq analysis was performed in the shell gland of blue-shelled and white-shelled ducks. Results indicated that biliverdin is a primary pigment for blue-shelled egg in ducks, and the HMOX1 protein showed high expression in ciliated epithelial cells of shell gland between blue-shelled and white-shelled ducks. In the pathway of biliverdin synthesis, only 5-aminolevulinate synthase 1 expression level was significantly upregulated in blue-shelled ducks, and nuclear factor, erythroid 2 like 1 and period circadian clock 2 may be the essential elements in biliverdin synthesis of duck shell gland. Furthermore, some of the transporter genes, such as activator-Like and solute carrier family 13 member 5, may be involved in the formation of blue egg in duck. Results of the current study suggested that the biliverdin is most likely synthesized and secreted from epithelial cells of shell gland. In addition, ALAS1 may play a key role in the formation of blue egg in ducks.