Many species of snakes exhibit epidermal surface nanostructures that form complex motifs conferring self-cleaning properties, and sometimes structural iridescence, to their skin. Using confocal microscopy, we show that different types of nanostructures are generated by cell borders and cell surface. Using scanning electron microscopy, we characterize the complexity and diversity of these surface gratings in 353 species spanning 19 of the 26 families of snakes. We then perform continuous-time Markov phylogenetic mapping on the snake phylogeny, providing an evolutionary dynamical estimate for the different types of nanostructures.
Using confocal microscopy, we show that the specialised cells bearing surface nanostructures on the epidermis of snakes can be greatly elongated along their left-right axis and that different types of nanostructures are generated by cell borders and cell surface. We then produced and analysed scanning electron microscopy images of skin sheds from 353 species and characterised the observed nanostructures with four characters. The full character matrix, as well as one representative SEM image of each of the corresponding species, is available as a MySQL relational database at https://snake-nanogratings.lanevol.org. Markov phylogenetic mapping of these characters on the snake phylogeny suggests that the presence of cell border digitations is the ancestral state for snake skin nanostructures which was subsequently and independently lost in multiple lineages. Our analyses also indicate that cell shape and cell border shape are co-dependent characters whereas we did not find correlation between a simple life habit classification and any specific nanomorphological character.
These results, compatible with the fact that multiple types of nanostructures can generate hydrophobicity, suggest that the diversity and complexity of snake skin surface nano-morphology are dominated by phylogenetic rather than habitat-specific functional constraints. The present descriptive study opens the perspective of investigating the cellular self-organisational cytoskeletal processes controlling the patterning of different skin surface nanostructures in snakes and lizards.
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Phylogenetic mapping of scale nanostructure diversity in snakes
Arrigo M.I., De Oliveira Vilaca M.L., Fofonjka A., Srikanthan A.N., Debry A. & M.C. Milinkovitch
BMC Evolutionary Biology 2019, 19: 91