Abstract
Cave dwelling invertebrates display many adaptations such as a lack of pigmentation, elongated appendages, and reduced visual systems. Such species can be found in the rich karst terrains of Kentucky, Tennessee, Alabama, and Georgia. The cave-adapted beetles of the Ptomaphagus genus inhabit caves in these states and display significant reductions in their visual systems. Within this genus are also the surface-dwelling P. cavernicola and the soil-dwelling P. shapardi. Thus, Ptomaphagus provides an exciting comparison among closely related species with the fully developed visual system of P. cavernicola, the intermediate system of P. shapardi and the substantially reduced systems of the cave species. The phenotypic differences have already been examined among the species of this genus, but the genetic changes associated with adaptation to cave life have not received such attention. Beetles typically have two light-sensitive opsin genes – a long-wavelength (LW) opsin and an ultraviolet (UV) opsin. We examined LW and UV opsins in 18 cave dwelling Ptomaphagus species, as well as P. shapardi and P. cavernicola, to understand how the visual systems of the beetles evolved during their transition to cave environments. We found that although the UV opsin gene is present in P. cavernicola and P. shapardi, it has been lost in all of the cave dwelling Ptomaphagus. In contrast, the LW opsin gene was present in all of the cave species, with no indication of mutations that would render the gene nonfunctional. These beetles can detect light, as indicated by light-dark tests we performed on P. hatchi, found in caves in the Sewanee area. The tests showed a general avoidance of light by these beetles. These results suggest that although they have lived underground for millions of years, these beetles still have functional visual systems, possibly to prevent them from wandering out of their cave habitats into environments they are less suited to survive in.
