Evolution and Hedgehog

I read an article in Seed, a science publication positioned between the serious research journal and the wide circulation science magazines, the other day that sparked my interest.

The article focused on the role of hedgehog, a gene I mentioned in a post the other week, in how the Mexican blind cavefish--pictured over there to the left--lost its eyes over time.

The article, written by Pz Myers, explores various biological explanations for why a species would lose functionality, such as flightless birds or eyeless fish. The species he focuses on is the Mexican blind cavefish which dwells in underground rivers where they are never exposed to light.

The first explanation, economical adaptation, is the one I learned in college, long before DNA analysis was available. This type of adaptation says that it requires a finite amount of time, energy, and effort to build an eye so that over time, if the eye doesn't contribute to the welfare of the species, the eye will cease to be made. This explanation has the advantage of common sense, which is probably why it was taught when I was in college, however it doesn't fit the facts. You see, the eye is built in Mexican blind cavefish embryos. It's only when it comes time for the lens to be created that the process stops. So the energy and effort are expended for the creation of the eye--it's just never finished in the adult fish.

The second explanation is the neutral theory of the loss of unused chracters which says that genes that express for development of unused features like the cavefish eye don't have any advantage or any cost. They're developmentally neutral. This means that since there's no conservation of genes involved, random mutations would eventually lead to broken genes. However that hasn't happened in the blind cavefish. Their ability to create functioning eyes are still intact except for their lack of lens creation.

So far it's a puzzle but the third evolutionary explanation sheds some light on this, if you'll forgive the pun. Pleiotropy and developmental interactions provide the necessary clues. You see, most genes have multiple functions. A gene might, for example, regulate mitotic spindle formation at one time and given different circumstances that same gene could aid in the movement of tubulin in the cell. In the case of the blind cavefish, there's two genes that interact with one another that both affect the formation of the face. Pax6 is the master gene, which controls the formation of the eye, and hedgehog, which is a gene--also referred to as hh--that helps with development in species from houseflies to humans. By the way, when a gene stays intact from species to species it's referred to as conservation. So hedgehog is a highly conserved gene.

As it turns out, these two genes, hedgehog and pax6, are in a negative relationship with one another. That is, when the expression of either gene is increased then the other gene is less expressed. This is important because hedgehog promotes development of the lower part of the face of the fish--and this section includes the jaw, mouth, and teeth--and increased sensitivity and development in this area strongly impacts the survival ability of a fish that hunts food by feeling with its jaw, as does the blind cavefish. For this overdevelopment of the lower face to occur, hedgehog function crowds out that of pax6 and that's what ultimately dooms the fate of the cavefish eyes. It's ultimately not that the cavefish doesn't need its eyes--though it doesn't--but rather that the cavefish needs greater sensitivity and development in its jaw more.