Vertebrates smell using their olfactory system, a complex organ composed of tissues in both the nose and the brain. In mammals where the sense of smell has been well studied, the front line of the olfactory system is a tissue called the olfactory epithelium, the layer of cells lining the interior portion of the nose. These cells are neurons, nerve cells, and they connect to the long olfactory nerve running to the brain, specifically to a portion of the brain called the olfactory lobe, where “smelling” really happens. The cells of the olfactory epithelium contain olfactory “receptors”, proteins that recognize specific odor molecules, and amazingly these receptors are so varied that each recognizes only a few of the millions of possible odors. Researchers Linda Buck and Richard Axel won the Nobel Prize in 2004 for determining how this system functions to allow us to smell so many different things.
Early during embryonic development a gene called Pax6 is expressed in the portion of the embryo that will develop into the olfactory epithelium and other parts of the olfactory system. This gene is essential for olfactory system development, as mice carrying a Pax6 mutation do not form most parts of the system. Many aspects of embryonic development are highly conserved across vertebrate species, and studying developmental processes in basal (i.e. evolutionarily “earlier”) vertebrates can often tell us how these systems first came to be.
Sharks are famous for having a highly developed sense of smell, and are known to have large olfactory bulbs, but how their olfactory system develops has not been well-studied. Identification of the shark Pax6 protein showed that it is an astounding 95% similar to that of the mouse, which suggests that Pax6 function is likely to be well conserved between these two very different species.
In a recent paper, Ferreiro-Galve et al studied the expression of Pax6 in a favorite shark model for embryonic development, the lesser spotted dogfish, Scyliorhinus canicula. (Interestingly, the genus Scyliorhinus comes from the Greek, meaning – for reasons that are unclear to me – ‘shark nose’.) Colorimetric stains specific for Pax6 showed that the gene is active in the developing olfactory epithelium, olfactory nerve, and the olfactory bulb of the brain. See the figure, where the purple color indicates Pax6 gene expression in the highly folded olfactory epithelium of the dogfish. (From Ferreiro-Galve et al.)
This pattern is remarkably similar to that seen in mice, suggesting that Pax6 already played a role in building the olfactory system in the common ancestor of sharks and mammals, more than 450 million years ago. Sometimes the outward differences between species can detract from underlying similarities conserved across eons – when something works well evolution tends not to mess with it.
That paper is Ferreiro-Galve, S, Candal, E, and Rodríguez-Moldes, I. 2011. Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway. Journal of Experimental Zoology, 314B. It is available here (http://onlinelibrary.wiley.com/doi/10.1002/jez.b.21448/abstract), but unfortunately is not open access.