Thursday, May 24, 2007

This Week's Citation Classic

This week's citation classic is Avise, J.C. & Selander, R.K. 1972. Evolutionary genetics of cave-dwelling fishes of the genus Astyanax. Evolution 26: 1-19.

The results of this paper do not revolutionize science or subvert the dominant paradigm or even overturn long cherished dogma. In fact, the results are fairly inconclusive. The main result is that troglobitic (i.e. cave-dwelling) fish exhibit much less genetic diversity than do epigean (i.e. surface-dwelling) fish, but the authors cannot do more than offer speculative hypotheses for why this should be. Nonetheless, I still think this paper is pretty damn cool. John Avise is currently a Distinguished Professor at UC: Irvine, and this was his very first publication consequent his thesis work under Robert Selander at the University of Texas. Not bad for a Master's student.

The main reason why I dig this paper so much was that it was my first exposure to the cave-dwelling fishes. The cave-dwelling fishes are very much like their epigean counterparts, but they lack eyes! I repeat, they are eyeless... they have no eyes. Well obviously, you say, eyes are not required in the stygian blackness of the subterranean world so mutations in eye genes were no longer deleterious and they accumulated more and more mutations until the eyes finally faded away.

Not so fast... it turns out that there is more to the story than first appears, and, in fact, it is a testament to the strength of natural selection. See there's a gene called Hedgehog that is crucial to the development of the eye. If you favored the neutral mutation theory, you'd expect that in Astyanax, Hedgehog is inactive due to accumulating mutations. Not quite. Hedgehog is upregulated in cave-dwelling fish! Much more of the developmental protein is produced in cave fish and in surface fish, and, what's more, this overproduction is responsible for the eye deformities.

Why should this protein be overproduced? The likely answer is that the gene is pleiotropic; Hedgehog also controls the development of the teeth, cranium, and tastebuds among other things. Perhaps upregulation of Hedgehog aided Astyanax foraging, and in surface fish, this benefit was traded-off against the need to have functioning eyes. The loss of eyes does not matter for Astyanax so they are able to take full advantage of Hedgehog upregulation. Natural selection favored eyeless fishes because they were better able to survive and reproduce than eyed fishes. How cool is that?

William Jeffery published a paper on this material in Journal of Heredity. Carl Zimmer and PZ Meyers also wrote about cave fish for their respective blogs: The Loom and Pharyngula.

Photo from the Jeffery Laboratory at UMD, the leading laboratory studying the cave-dwelling fishes.

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