Expanding the eggshell colour gamut: uroerythrin and bilirubin from tinamou (Tinamidae) eggshells

Abstract

To date, only two pigments have been identified in avian eggshells: rusty-brown protoporphyrin IX and blue-green biliverdin IXα. Most avian eggshell colours can be produced by a mixture of these two tetrapyrrolic pigments. However, tinamou (Tinamidae) eggshells display colours not easily rationalised by combination of these two pigments alone, suggesting the presence of other pigments. Here, through extraction, derivatization, spectroscopy, chromatography, and mass spectrometry, we identify two novel eggshell pigments: yellow–brown tetrapyrrolic bilirubin from the guacamole-green eggshells of Eudromia elegans, and red–orange tripyrrolic uroerythrin from the purplish-brown eggshells of Nothura maculosa. Both pigments are known porphyrin catabolites and are found in the eggshells in conjunction with biliverdin IXα. A colour mixing model using the new pigments and biliverdin reproduces the respective eggshell colours. These discoveries expand our understanding of how eggshell colour diversity is achieved. We suggest that the ability of these pigments to photo-degrade may have an adaptive value for the tinamous.

A new molecular mechanism supports that blue-greenish egg color evolved independently across chicken breeds

“…Blue-greenish coloration ( BGC ) is an ancient and common eggshell color. It appears not only in nonavian dinosaurs ( Wiemann et al, 2017 ), but in some modern avian species ( Siefferman et al, 2006 ; Wang et al, 2013 ; Chen et al, 2020 ; Hamchand et al, 2020 ).…”

“…BGC is mainly caused by biliverdin IX, a product from oxidative degradation of heme ( Kennedy and Vevers, 1976 ; Ryter and Tyrrel, 2000 ). With the change of biliverdin contents and the involvement of protoporphyrin IX, another eggshell pigment responsible for red-brown color ( Kennedy and Vevers, 1976 ), BGC eggs show a wide spectrum of chroma from blue-greenish eggs laid by chicken and duck ( Wang et al, 2013 ; Chen et al, 2020 ), deep blue eggs laid by eastern bluebird ( Siefferman et al, 2006 ), olive-green eggs laid by elegant crested tinamou to blackish-green eggs laid by emu ( Hamchand et al, 2020 ). BGC was hypothesized to signal female quality to male which allocates more parental care for their offspring depending on antioxidant properties of biliverdin ( Stocker et al, 1987 ; Mereno and Osorno, 2003 ).…”

Biopyrrin Pigments: From Heme Metabolites to Redox-Active Ligands and Luminescent Radicals

“…18 These compounds, however, are likely present in multiple biological settings, and a biotripyrrin was recently isolated in tinamou eggshells. 19 Dipyrrolic propentdyopent compounds (Chart 2) have been observed consistently in studies of hyperbilirubinemia and other conditions related to abnormal heme metabolism. 20 In general, heme fragments in urine and cerebrospinal fluid are under investigation as potential biomarkers for a range of conditions ranging from heart disease 21 to neurodegeneration, 22 and their potential role in cerebral vasoconstriction is also under scrutiny.…”

“…In the 1990s, the tripyrrolic motif of uroerythrin (Chart ) was recognized in two urinary pigments (designated as biotripyrrin-a and -b) and associated with oxidative stress and inflammation in patients . These compounds, however, are likely present in multiple biological settings, and a biotripyrrin was recently isolated in tinamou eggshells . Dipyrrolic propentdyopent compounds (Chart ) have been observed consistently in studies of hyperbilirubinemia and other conditions related to abnormal heme metabolism .…”

Avian eggshell coloration predicts shell-matrix protoporphyrin content

“…;Lack 1958;Weidinger 2001;Westmoreland and Kilti 2007), strengthening eggshells (Underwood and Sealy 2002;Gosler et al 2005), and, for the embryo, mediating light exposure and modulating the thermal environment (Bakken et al 1978;Mauer et al 2015;Lahti and Ardia 2016). Two pigments predominate in avian eggshells, protoporphyrin, responsible for the pink, rusty-brown, and black hues, and biliverdin, for the blue-green hues (Mikšík et al 1994(Mikšík et al , 1996Gorchein et al 2009; but see Hamchand et al 2020). Protoporphyrin plays an important role in heme synthesis (Chiabrando et al 2014) and, because it is a strong pro-oxidant (Afonso et al 1999), its presence in eggshells has inspired a number of studies aimed at determining if variation in eggshell pigmentation reflects differences in females' condition, in particular their state of oxidative balance at the time of egg production (Moreno and Osorno 2003;de Coster et al 2013;Holveck et al 2019).…”