Fig. 7

A balance hypothesis explaining that differential expression of ion transporters can cause blue-green eggshell color. Given the uncertainties in the cellular localizations, transfer directions, and ion types for some ion transporters, a large part but not all of differentially expressed ion transporters were incorporated in the balance model. Cellular localizations of ion transporters were determined according to the ion transfer model proposed by Jonchère et al. [40] and GO annotations. Transporter names shown in red represent that they were upregulated in the blue-shelled groups; the green indicate downregulation; the gray mean that the transporters were incorporated in the Jonchère’s model [40] but were not detected by this RNA-seq; ones shown in bold black letters are several potential biliverdin transporters which were expressed in shell glands of both blue-shelled and white-shelled ducks but did not show consistent expression trends among all cross-phenotype comparisons. The sign of “(+)” represents that upregulation of two H⁺ extruders can stimulate the transport activity of OATP2B1; the sign of “(−)” indicates that a competition for ATP can exist between multidrug resistance protein 1/3 (MRP1/3) and ion pumps (ATP2A2, ATP2B2, ATP2C2, ATP1B1, and ATP12A). The letters of “Q”, “I”, “II”, “III”, “IV”, and “Cyc” respectively represent coenzyme Q, complex I-IV, and cytochrome c which are involved in the mitochondria electron transport. CD320 is the chicken orthologue of duck ENSAPLG00000000921. Because the gene was annotated with the mitochondrial electron transport, NADH to ubiquinone (GO:0006120), it can exert a role in the first step of the electron transport chain