There are three pigments that affect the color of an eggshell:
protoporphyrin,
biliverdin and
biliverdin-
zinc chelate.
Protoporphyrin is the main pigment in brown and light-brown eggshells, whereas very little
protoporphyrin is found in white eggshells. Eggshell
protoporphyrin is derived from the
heme formation in birds.
Coproporphyrinogen III oxidase (CPOX) and
ferrochelatase (FECH) represent rate-limiting
enzymes for the
heme-biosynthetic pathway.
Breast cancer resistance
protein (BCRP),
feline leukemia virus receptor (FLVCR), and
heme-responsive gene-1 (HRG1) serve as primary transporters for both
protoporphyrinogen and
heme. Finally, four organic
anion transporting
polypeptide family members (including solute carrier
organic anion transporter family, SLCO1C1, SLCO1A2, SLCO1B3 and LOC418189) may affect pigment transport within eggshells. Here we measured gene expression levels in key tissues of egg-producing hens. We analyzed three different types of hens that generated distinct eggshell colors: white, pink or brown. Our data revealed three ways in which eggshell color was genetically influenced. First, high-level expression of CPOX generated more
protoporphyrinogen and a brown eggshell color. In contrast, high expression of FECH likely converted more
protoporphyrinogen into
heme, reduced
protoporphyrinogen levels within the eggshell and generated a light color. Second,
heme transporters also affected eggshell color. High-level expression of BCRP, HRG1 and FLVCR were associated with brown, white and generally lighter eggshell colors, respectively. Finally,
protoporphyrin precipitation also affected eggshell color, as high expression of both SLCO1A2 and SLCO1C1 were associated with brown eggshell color. As such, we have identified seven genes in which expression levels in different tissues were associated with eggshell color.