Expression of rhodopsin and opsin in late‐stage epigean and hypogean salamander embryos

Abstract

Evolutionarily, sensory systems have undergone extreme diversifications that have allowed vertebrates to survive diverse environments. The salamander clade Eurycea of Central Texas has branched out of its epigean ancestral roots and has successfully invaded subterranean environments. The San Marcos salamander (E. nana) and the Barton Springs salamander (E. sosorum) live above ground in springs and streams whereas the Texas blind salamander (E. rathbuni) occupies caves and recesses in the Edwards Aquifer. From an evolutionary perspective, it is thought that the hypogeal state is derived, but what remains a mystery is the developmental adaption of its sensory system to life under ground. The habitational broadening of the Eurycea salamander has been associated with the phenotype of reduced eyes exhibited in the Texas blind salamander. It is not clear whether this phenotype represents incomplete development, regression, or a combination of both. The clade of Eurycea is a prime model for comparing the molecular mechanisms that are involved in diverging neural systems, since species within the same clade present two extremes of phenotypes that are associated with their environment. Retinal development in embryonic E. rathbuni has been observed; however, the extent to which the retina develops is unknown. We identify retinal rod and cone photoreceptor cells along with their associated visual proteins (rhodopsin and opsin, respectively) in blind species and in epigean salamanders with fully developed eyes. Immunohistochemistry followed by confocal microscopy was applied for the investigation of sections of late embryonic stages of these salamander species. The results suggest opsin is expressed in the retina of E. nana and E. sosorum, and rhodopsin is expressed in all three species. Based on previous work indicating absence of visual pigments in adult Texas blind salamanders, we conclude expression of rhodopsin is transient or variable in this species. In future studies we will examine the expression of visual pigments at other stages to get a clearer picture of the time course and species‐level variability in expression.