Animal visual systems have evolved at multiple levels of the visual perception pathway according to a given species' visual needs in the light environments it inhabits. Among the most impactful adaptations are changes to the visual opsin proteins, which are the protein component of the light-sensitive visual pigments housed in the photoreceptor cells of the retina that detect and respond to light signals. Schott et al. (2024) examine the diversity and evolution of visual opsins specifically the gene duplication/ loss and sequence evolution across 122 frog species representing 34 families with a diversity of life histories and ecologies. They find most species express four visual opsins with evidence for gene loss in two lineages (Arthroleptidae and Dendrobatidae), which represents a much lower level of gene loss than that inferred in other ancestrally nocturnal vertebrate groups like mammals, snakes, geckos, and crocodilians. This relatively limited gene loss in anurans suggests they have heavily invested in their visual systems despite being predominantly nocturnal. They also find evidence for shifts in selective pressure on each visual opsin associated with differences in ecology (scansorial and aquatic lifestyles) and life history (direct development), highlighting the need for functional studies to better understand how particular substitutions may impact visual pigment function. Finally, Schott et al. measure the spectral sensitivities of visual pigments in a subset of species, greatly expanding the previously known ranges for all frog visual pigments. Surprisingly, much of this variation is not explained by mutations at known "spectral tuning sites" identified in other vertebrates, suggesting that frogs have used unique molecular pathways to achieve this high level of spectral diversity. In sum, their study furthers our understanding of adaptive evolution in frog visual systems and represents an important advance in our growing understanding of vertebrate visual evolution.

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