Retention of the full visual opsin repertoire in Australia's cryptozoic lizards

Australian scincid lizards in the sister-genera Lerista and Ctenotus are a prominent system for understanding adaptation in the transition from surface to fossorial life. The approximately 205 species in this group exhibit extreme diversity in morphology and ecology. Lerista and Ctenotus both include diurnal and surface-active species that are fully pentadactyl, and Lerista also contains many specialised limb-reduced and limbless sand-swimmers. To understand how the visual systems of these lizards have responded to their varied photic environments, we examined the five opsin genes encoding the pigments that mediate colour and dim-light vision. These genes were sequenced for 59 species of Lerista and Ctenotus and analysed for variation in selection pressures among amino acid sites and across branches in the species tree. All five opsins are present and intact in all species of Lerista and Ctenotus examined, and we identified signals of positively selected substitutions in all five opsin genes -RH1, which mediates scotopic vision, and four cone opsins associated with photopic vision (SWS1, SWS2, RH2, LWS). Most comparisons of selection pressures did not show significant differences according to broad ecological divisions. Only LWS showed a signal of relaxed selection in sand-swimming (limb reduced) versus less fossorial (fully limbed) Lerista. These results suggest that photopic abilities are retained across both clades, even in the most fossorial species, highlighting a need for studies of visual ecology of Australian skinks, and prompts caution with regards to generalisations about degenerate vision in fossorial squamates.