Genetics: The evolution of gecko toepads

Geckos are lizards that belong to the infraorder Gekkota. Many 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, called "padded" geckos, the setae branch into multiple flat, highly deformable tips called spatulae, which 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 shows the morphological differences between the toepads of padded and padless species.

Figure 1 Morphological differences between non-adhesive toepads (A and B) and an adhesive toepad (C)

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 regions in nuclear DNA, as well as one mitochrondrial gene. Figure 2, taken from this study, illustrates the reconstruction of a single clade of gecko into a "maximum likelihood tree," where P(toepads) represents the probability that the most recent common ancestor possessed adhesive toepads. Shaded gray boxes indicate padless species without adhesive toepads, and bolded species were physically examined to confirm the presence or absence of adhesive toepads.

Figure 2 Maximum likelihood tree

Padless species C. angulifer and P. rangei typically occur in sand dunes, whereas padded members of this clade are typically rupicolous (occur in rocky areas).