Eggshell Composition of Squamate Reptiles: Relationship between Eggshell Permeability and Amino Acid Distribution

Abstract

Abstract-Most snakes and lizards produce eggs with flexible shells that interact with the environment to maintain water balance. Geckos produce rigid eggshells that are independent of an external source of water and can be oviposited in more open, dryer locations. In this study, we analyzed and compared the amino acid composition of 24 lizard species, six snake species, and four outgroups (including avian and reptilian elastin and chicken eggshell). Rigid Gecko eggshells had significantly lower levels of seven of the 17 amino acids evaluated. Multivariate analysis showed that proline was the most important amino acid in distinguishing between these two groups of eggshells, occurring at significantly higher levels in flexible eggshells. High levels of proline have also been observed in the eggshells of other species. Proline and other amino acids are associated with the alleviation of water and salt stress in plants.

A comparative study of eggshells of Gekkota with morphological, chemical compositional and crystallographic approaches and its evolutionary implications

“…Thus, it is a biologically crucial component [ 46 ] and commonly found in keratin associated proteins, especially in cysteine [ 47 , 48 ]. Moreover, cysteine and methionine are present in soft and rigid gekkotan eggshells [ 16 ].…”

“…Diverse scientific approaches have been applied to gekkotan eggshells during the past two decades: measuring the relative amount of amino acids in eggshells [ 16 ], water vapor permeability [ 17 – 19 ], and evolutionary implications for egg shape and size [ 3 , 20 ]. However, fundamental studies for microstructural features of gekkotan eggshells have not been done exhaustively.…”

Contrasting models of parity‐mode evolution in squamate reptiles

“…If lineages representing putative regains of the eggshell present with drastically different eggshell compositions, this would potentially support hypotheses of reversal. For instance, gecko eggs are very different from other squamates in terms of histology and amino‐acid profiles (Sexton et al, ). If such differences were common across different lineages, it would support hypotheses of reversal, indicating that the eggshell has re‐evolved, and was re‐constructed differently each time.…”

Dinosaur Integument

“…The external shell morphology revealed closely spaced knobby projections and (Fig. 7.13) pores for gas exchange similar to modern avian eggshells but differing from the flexible shell of lepidosaurians (Sexton et al 2005). Electron microscopic examination revealed organic cores immediately internal to the mineralized shell.…”