Subterranean Traits of the Visual System in Mandarin Voles ( <i>Lasiopodomys mandarinus</i> ): Structural Remodeling and Functional Degeneration

The visual systems of subterranean mammals often exhibit significant physiological and structural modifications due to light-restricted habitats, as widely reported. However, as subterranean species with a relatively short divergence time from their above-ground relatives, Mandarin voles (Lasiopodomys mandarinus) have been the subject of limited research regarding their visual characteristics, and potential parallels in their visual system's environment-induced changes remain unclear. We systematically analyzed the visual system of Mandarin voles, using their above-ground sister species Brandt's voles (Lasiopodomys brandtii) and standard laboratory mice (C57BL/6J, Mus musculus) as reference controls. Behavioral results showed that Mandarin voles exhibit reduced visual function, including visual acuity (<0.02 cycles/degree) and depth perception. Anatomical examinations revealed structural remodeling in the retina and dorsal lateral geniculate nucleus (dLGN), characterized by a relative decrease in rods and an increase in cones within the retina compared to C57BL/6J mice, along with dLGN showing reduced volume but increased mature neuron density and connectivity compared to Brandt's voles and C57BL/6J mice. Notably, the primary visual cortex (V1) retained a conserved structure. An evolutionary dynamic analysis of vision-related genes identified pseudogenization of genes and contraction of gene families associated with retinol metabolism, crystallin proteins, signal transduction, and retinal structure. These findings suggest that Mandarin voles exhibited visual functional degeneration, structural remodeling of visual pathways, and pseudogenization of vision-related genes compared to surface-dwelling species. This study systematically characterized the visual features of Mandarin voles, providing novel experimental evidence for understanding environmental adaptation in mammalian visual systems.