Subdigital integumentary microstructure in Cyrtodactylus (Squamata: Gekkota): do those lineages with incipiently expressed toepads exclusively exhibit adhesive setae?

In taxa such as insects, spiders, bats, frogs, and lizards, adhesive structures at the distal ends of their limbs have independently evolved, enabling the animals to adhere to inclined or even inverted surfaces. The adhesive apparatus of geckos functions via a complex interaction among muscles, bones, vascular tissue, and microscopic epidermal microstructures. The microstructures of geckos are classifiable as spinules, spines, prongs and setae, but only setae, which possess spatulate tips, promote adhesive competency sufficient to support body mass employing van der Waals forces. Several studies indicate that the form of toepad microstructures might be specific to the exploitation of the attributes of the substrata employed during habitat use. The species-rich genus Cyrtodactylus exhibits extensive variation in the shape of the subdigital scales associated with different habitats, making it a promising candidate for studying toepad evolution. We investigated the subdigital microstructures of 27 Cyrtodactylus species occupying a wide range of habitats, and exhibiting a spectrum of subdigital morphology, from the presence of the ancestral condition of small, rounded scales to the early-stage development of macroscopically visible incipient toepads. Using SEM and phylogenetic comparative analyses, our objectives were to (a) clarify how integumentary microstructural traits relate to the presence of incipient toepads and (b) identify potential adaptations linked to specific habitat types. We hypothesized that (1) species showing incipient toepad development will possess setae, while those lacking obvious macrostructural modifications should exhibit only spines, prongs, or spinules. Additionally, we hypothesized that either (2) the presence of setae is associated with arboreal lifestyles and, to a lesser extent, with rock-dwelling ecotypes; or (3) alternatively, microstructural traits are more strongly influenced by phylogeny, with closely related species exhibiting more similar toepad features than those more distantly related. We found setae, spines, and prongs on the incipient toepads. Spines were found to be the ancestral subdigital microstructures of Cyrtodactylus, with multiple independent transitions to prongs (three times) and setae (twice). One shift towards setae defines a largely seta-bearing clade, exhibiting a strong phylogenetic signal and supports our third hypothesis. Most transitions to incipient toepads occurred within this clade, consistent with hypothesis 1, and we reveal that the evolution of setae likely preceded that of broadened scales. Although microstructure types did not significantly correlate with ecotype, specific morphometric traits varied significantly among both microstructure types and ecotypes.