Ocular Histology of Three South Central Texas Paedomorphic Salamander Species (Eurycea sosorum, Eurycea nana, and Eurycea rathbuni) and Comparative Ocular Development of Two Morphotypes
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The recent focus on conserved genes expressed through development has allowed for great headway in understanding the molecular mechanisms responsible for the variation seen among organisms. The expression of these integral developmental genes has implications with respect to evolutionary processes. The south central Texas Eurycea clade presents a unique continuum of karst phenotypes, having species representative of both subterranean and surface phenotypes. By describing the adult ocular morphology and the developmental processes leading to it, I hope to lay the foundation for better understanding the molecular mechanisms responsible for generating subterranean phenotypes in a karst salamander system.
Three species of salamanders (Eurycea rathbuni, Eurycea nana, and Eurycea sosorum) from the south central Texas Eurycea clade were obtained for examining adult ocular histology. All specimens were mortalities obtained from the U.S. Fish and Wildlife Service San Marcos Aquatic Resource Center. The adult histology revealed an underdeveloped eye in the subterranean species E. rathbuni and well-developed eyes in the surface species E. nana and E. sosorum. Interestingly, a prominent optic nerve was found in both surface and subterranean species. The optic nerve of the subterranean species E. rathbuni was further examined using transmission electron microscopy. A number of myelinated axons were observed, suggesting functional capability of the optic nerve.
I have described the adult ocular histology in three species of south central Texas Eurycea, and I am interested in describing the developmental processes leading to the divergent anatomy between the two morphotypes. Furthermore, I aim to understand how differences in gene expression influence the divergent outcomes of eye development between the two morphotypes; therefore, expression of genes involved in ocular development (pax6 and shh) was examined in E. rathbuni embryos and E. sosorum embryos. The proteins Pax6 and Shh are conserved among all animals and share similar expression patterns through development in species in which their expression has been examined. I found that both E. rathbuni and E. sosorum express Pax6 and Shh, but the time course and location of Pax6 and Shh expression in the developing eye of E. rathbuni differed from that in E. sosorum. Furthermore, I observed unexpectedly that the lens, which functions in inducing development of the retina in other organisms, persists in the latest stage of E. rathbuni, suggesting that they maintain a lens after hatching and potentially well into the juvenile state.
The two morphotypes examined share similar ontogeny, yet different spatial and temporal expression of Pax6 and Shh. Interestingly, a similar pattern can be seen by a cave adapted fish (Astyanax mexicanus), suggesting a degree of convergent evolution both through ontogeny and the expression of Pax6 and Shh. I conclude that these salamanders present an ideal system in which to study the evolutionary and developmental mechanisms that lead to the variation in subterranean morphotypes seen in the Eurycea clade. Moreover, this system represents an innovation from the fish system for understanding the evolutionary processes responsible for subterranean adaptation.