Evolving the electric swim

This is really cool:

The same genes that give sharks their sixth sense and allow them to detect electrical signals are also responsible for the development of head and facial features in humans, a new study suggests.
The finding supports the idea that the early sea creatures which eventually evolved into humans could also sense electricity before they emerged onto land.

The paper by Renata Freitas and colleagues is in Evolution and Development. Here is the suggestive part of the discussion:

Our finding that the rostral spread of the EphA4 expression domain prefigures the routes taken by mechanosensory and electrosensory axons is reminiscent of mouse ear innervation, in which EphA4 is expressed in the cells lining the auditory nerve pathway, where it directs axons to the cochlea (Pickles 2003). If this function is conserved in the shark laterosensory system, then EphA4 may be involved in guidance of sensory axons to electroreceptors and mechanoreceptors. Indeed, absence of EphA4 expression from ampullary placodes may also relate to the termination of growth cones at these positions. This may be important for both function and development of electroreceptors, as it has been suggested that the arrival of nerve fibers may induce formation of electroreceptive organs (although it is also possible that the placodes attract axons; Fritzsch et al. 1998).
Expression of EphA4 in the shark laterosensory system may represent a deeply conserved mechanism for establishing topographic maps of peripheral sensory inputs in vertebrates. In the mouse, EphA4 and EphrinA5 regulate development of the somatotopic map of projections from sensory whiskers to the barrel fields on the cortex (Vanderhaeghen et al. 2000). EphA4 has been shown to regulate thalamocortical projections, as well as the topographic projections of motor neurons from the spinal cord to the limb (Eberhart et al. 2000, 2002). Similar spatial patterning occurs in the auditory system, where topographic projections originating from the cochlea project to the nucleus magnocellularis, which in turn, innervates the nucleus laminaris in the brain to form a tonotopic map of high- to low-frequency sounds. Interestingly, EphA4 is expressed in a tonotopic gradient at the time when nucleus magnocellularis axons are forming synapses on the nucleus laminaris (Person et al. 2004). The association between expression of EphA4 and development of the shark electrosensory system suggests that EphA4 could play a role in establishing the topographic relationships between peripheral electroreceptors and their primary central targets. Regulation of EphA4 expression during development of the cephalic electrosensory system would therefore underlie how sharks localize the position of electrical stimuli relative to their spatial map of the body. This hypothesis is consistent with EphA4 playing a general role in the establishment of topographic maps during vertebrate embryogenesis (Vanderhaeghen et al. 2000; Yue et al. 2002; Dufour et al. 2003; Person et al. 2004).

This kind of deep genetic homology is not new -- one of the early insights of evo-devo was the widely shared genetic homologies underpinning sight in different lineages who "independently" evolved it. But this is interesting to me because electrosensory organs are easily seen as "exotic" characteristics of some lineages very distant to us. This study suggests that they may have served an important role in the early evolution of tetrapod ancestors. The genes that help generate electroreceptivity in sharks are still expressed in mammals, bent to other purposes.

The LiveScience article has an embryological note of caution from Glenn Northcutt, by the way:

Glenn Northcutt, a neuroscientist at the University of California at San Diego, who was not involved in the study, said the finding was interesting, but that more studies are needed before a direct link between neural crest cells and electroreceptors can be established.
"It still requires a definitive experiment, where the developing neural crest cells are marked with dye, the embryo develops and the dye clearly shows up in the electroreceptors," Northcutt said.
Evolution and Development is a really cool journal. Two of the other articles in the same issue relate to the sensory systems of cave fish -- a classic problem in evolution for which evo-devo approaches are giving interesting answers.

References:

Freitas R, Zhang GJ, Albert JS, Evans DH, Cohn MJ. 2006. Developmental origin of shark electrosensory organs. Evol Devel 8:74. DOI link