Eggshell Biliverdin and Protoporphyrin Pigments in a Songbird: Are They Derived from Erythrocytes, Blood Plasma, or the Shell Gland?

Abstract

Biliverdin and protoporphyrin pigments are deposited into the eggshell when the developing egg is in the shell gland. However, the site of synthesis of eggshell pigments is still uncertain, although it may influence the possible costs and potential functions of eggshell coloration in avian species. Eggshell pigments may be derived from red blood cells or be produced in other organs and then transferred to the shell gland, or they may be synthesized de novo in the shell gland. We studied in the canary (Serinus canaria) whether eggshell blue-green and brown pigmentations are associated with experimentally elevated anemia, female hematocrit level, immature erythrocyte percentage, and feces and plasma pigment levels during egg laying to find out the possible origin of eggshell pigments. We found no significant effects of hematocrit level or experimentally elevated anemia on intensity of eggshell blue-green and brown pigmentations; therefore, we consider it less likely that eggshell pigments are derived from erythrocytes. In addition, we found no significant associations between female feces biliverdin concentration during egg laying and intensity of eggshell blue-green pigmentation, suggesting that eggshell biliverdin may not originate from the spleen or liver. We found a negative association between plasma and feces protoporphyrin concentrations during egg laying and eggshell brown chroma. This result suggests that an increased production of protoporphyrin in the liver, which could have elevated plasma and feces protoporphyrin concentrations, could inhibit eggshell protoporphyrin pigmentation, probably through affecting enzymatic activities. We suggest that both pigments are produced de novo in the shell gland in the canary, but circulating pigment levels may influence shell gland pigment synthesis, thus connecting the physiological status of the female to eggshell coloration.

Eggshell spotting pattern is related to hatching asynchrony, hematocrit value and growth of nestling great titsParus major

“…In our population, female great tits that lay eggs in the beginning of the breeding season, when caterpillar food availability is scarce, and those that have smaller clutches lay darker‐spotted eggs (Hargitai et al , c), indicating that food limitation may lead to more pigmented eggshells. Moreover, some experimental and observational studies of other bird species also indicated that food limitation resulted in darker spotted eggshells (Duval et al , Hargitai et al , 2018a, Badás et al ). Therefore, it is possible that females in less favourable conditions laid darker spotted eggs, and due to energetic constraints they delayed the start of incubation, resulting in more synchronous broods.…”

“…Red‐brown eggshell maculation is generated by protoporphyrin pigment (Kennedy and Vevers , Mikšík et al , Gorchein et al ) that is most likely produced in the shell gland (Zhao et al , Wang et al , Hargitai et al ). Apart from its widespread role to camouflage the eggs (Stoddard et al , Spencer et al ), )a great number of hypotheses have been proposed in recent years for the evolution of eggshell pigmentation (Kilner , Reynolds et al , Maurer et al ).…”

“…For example, it is possible that eggshell pigments modify the amount and wavelength of light transmitted through the eggshell and thus affect embryonic development (Maurer et al 2015, Lahti and Ardia ). Eggshell pigments may play a thermoregulatory role (Bakken et al , Westmoreland et al ), and pigments may also decrease trans‐shell contamination by microbes (Ishikawa et al 2010, but see Fargallo et al , Dearborn et al ). It is also possible that eggshell pigments strengthen thin eggshells (Gosler et al , García‐Navas et al 2010, Hargitai et al ).…”

“…Thicker-shelled eggs could result in better hatchability due to preventing eggshell breakage or excessive water loss (Graveland et al 1994, Eeva and Lehikoinen 1995, Bureš and Weidinger 2003. Finally, it was shown that eggshell pigmentation could also be associated with hatching success and offspring growth (Hanley and Doucet 2009, Sanz and García-Navas 2009, Poláček et al 2017a Red-brown eggshell maculation is generated by protoporphyrin pigment (Kennedy and Vevers 1976, Mikšík et al 1996, Gorchein et al 2009 that is most likely produced in the shell gland (Zhao et al 2006, Wang et al 2009, Hargitai et al 2017. Apart from its widespread role to camouflage the eggs (Stoddard et al 2011, Spencer et al 2014), a great number of hypotheses have been proposed in recent years for the evolution of eggshell pigmentation (Kilner 2006, Reynolds et al 2009, Maurer et al 2011.…”

Effects of food limitation on the intensity of blue‐green and brown eggshell coloration: an experimental study with the canary

“…Plasma biliverdin is mainly produced in the spleen from degrading erythrocytes, and then released into the circulation, and transferred to the liver (Murray 2003). However, several studies of poultry (Yamada 1972, Stevens et al 1974, Zhao et al 2006, Wang et al 2009, 2011, and a recent study of the canary (Hargitai et al 2017) suggest that eggshell biliverdin is most likely produced in the shell gland by the oxidation of heme, which could be produced in the cells of the shell gland. If circulating biliverdin inhibits by some negative feed-back mechanism the activity of enzymes in the synthesis pathway of eggshell biliverdin in the shell gland, it could provide a possible explanation for the effect of food/antioxidant availability and body condition on eggshell blue-green coloration.…”