The evolution of gecko toepads

Geckos are lizards belonging to the order Gekkota. There are over $1500$‍ species of geckos currently known, with the vast majority of species occurring in warm climates. Many, but not all, species of gecko exhibit adhesive toepads that enable them to cling to both hydrophobic and hydrophilic surfaces. Geckos with adhesive toepads can easily scale vertical and even inverted surfaces. These toepads, in spite of their apparent stickiness, remain clean of dust particles and effectively repel water.

The phalanges of all geckos are covered with microscopic, keratinaceous hairs called setae, arranged in plate-like layers known as lamellae. In species with adhesive toepads, each setae branches into multiple flat, highly deformable tips called spatulae that allow each setae to interact with nearly all surfaces via van der Waals attractive forces. While individual spatulae interact rather weakly with surfaces, the sheer number of spatulae provides enough surface area in contact with the surface to create a strong adhesive force.

Figure 1: Morphological differences between adhesive and non-adhesive setae. (A) and (B) show non-adhesive toepads, while (C) shows an adhesive toepad.

A group of researchers carried out a phylogenetic analysis of multiple genes in over $100$‍ species of gecko. The analysis was based on approximately $4100$‍ nucleotide bases found in five distinct protein-coding nuclear DNA, as well as one mitochrondrial gene. Figure 2, taken from this study, illustrates the reconstruction of a single clade of geckos.

Figure 2: Maximum likelihood tree illustrating ancestral reconstruction of a single clade of geckos. P(toepads) represents the probability that the most recent common ancestor possessed adhesive toepads. Shaded gray boxes indicate padless species without adhesive toepads. Padded members of this clade typically occur in rupicolous (rocky) areas. Padless C. angulifer and P. rangei typically occur in sand dunes. Bolded species were physically examined to confirm the presence or absence of adhesive toepads.

Figures adapted from: Gamble, Tony, Eli Greenbaum, Todd R. Jackman, Anthony P. Russell, and Aaron M. Bauer. "Repeated origin and loss of adhesive toepads in geckos." PLoS One 7, no. 6 (2012): e39429.

The specific form of keratin found in a gecko’s setae contains a large number of beta-pleated sheets, lending the structure a significant degree of rigidity. This feature is associated with which level of protein structure?