Abstract
Pigment-based coloration is a common trait found in a variety of organisms across the tree of life. For example, calcareous avian eggs are natural structures that vary greatly in color, yet just a handful of tetrapyrrole pigment compounds are responsible for generating this myriad of colors. To fully understand the diversity and constraints shaping nature’s palette, it is imperative to characterize the similarities and differences in the types of compounds involved in color production across diverse lineages. Pigment composition was investigated in eggshells of eleven paleognath bird taxa, covering several extinct and extant lineages, and shells of four extant species of mollusks. Birds and mollusks are two distantly related, calcareous shell-building groups, thus characterization of pigments in their calcareous structures would provide insights to whether similar compounds are found in different phyla (Chordata and Mollusca). An ethylenediaminetetraacetic acid (EDTA) extraction protocol was used to analyze the presence and concentration of biliverdin and protoporphyrin, two known and ubiquitous tetrapyrrole avian eggshell pigments, in all avian and molluscan samples. Biliverdin was solely detected in birds, including the colorful eggshells of four tinamou species. In contrast, protoporphyrin was detected in both the eggshells of several avian species and in the shells of all mollusks. These findings support previous hypotheses about the ubiquitous deposition of tetrapyrroles in the eggshells of various bird lineages and provide evidence for its presence also across distantly related animal taxa.
Highlights
Evolutionary processes acting on color production mechanisms have generated an astounding diversity of colors and patterns in animals that serve many communicative functions, including parasitic mimicry, antipredator defenses, prey-attraction, sexual displays, parent-offspringPLOS ONE | DOI:10.1371/journal.pone.0143545 December 9, 2015Avian and Molluscan Shell Pigments co-financed DH
Biliverdin was detected from the eggshells of T. migratorius, M. ater and C. japonica, and protoporphyrin was detected from eggshells of G. gallus, M. ater and C. japonica
Mass spectrometry analysis of the negative control A. mississippiensis and bird eggs with only biliverdin detected by UV-Vis absorption spectra did not yield protoporphyrin by UHPLC/Mass Spectrophotometry (MS) analyses, confirming that protoporphyrin detected in molluscan shells was not a result of contamination
Summary
Evolutionary processes acting on color production mechanisms have generated an astounding diversity of colors and patterns in animals that serve many communicative functions, including parasitic mimicry, antipredator defenses, prey-attraction, sexual displays, parent-offspring. Regardless of vast evolutionary and ecological differences, bird eggs and mollusk shells share a similar diversity in coloration (at least to the human eye), and in both taxa pigments are embedded within a calcium carbonate shell matrix [13,14,15] For these reasons we expect that pigment convergence may be found in these groups. The information gap concerning pigment composition in diverse taxa is addressed here by investigating the potential convergence and biochemical basis of color expression in avian eggshells and molluscan shells To this end, eggshell pigment composition of eleven extinct and extant paleognath avian species, and four molluscan species’ shells (Fig 1) was examined to identify the presence or absence of protoporphyrin and biliverdin and the potential overlap in pigment composition across representatives of diverse phyla (Chordata and Mollusca). Avian and molluscan shells are composed of a comparable calcium carbonate matrix [13,14,15], and have similar colors that suggest similar pigments might be found in both avian eggshells and molluscan shells
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