Saturday, January 03, 2009

Convergent loss of pigmentation in cavefish   posted by p-ter @ 1/03/2009 09:04:00 AM

One of the established cool examples of convergent evolution (which for my purposes here I'll define loosely here as the evolution of different populations to the same phenotype via different mutations) has been the repeated loss of pigmentation (and eyes) in fish that have adapted to life in light-poor, nutrient-poor caves. In 2006, a group reported that albinism (panel J in the picture) in several of these caves was due to mutations in OCA2 (a SNP in a regulatory region of this gene also causes blue eyes in humans).

Not all cavefish however, are fully albino--in some populations, there also exists a "brown" phenotype (panel "G" in the picture) with reduced pigmentation. In a new paper, the gene underlying this phenotype is shown to be MC1R (this gene, of course influences pigmentation in all sorts of species), and, similarly to OCA2, two different mutations have arisen in different populations.

One might imagine that light pigmentation in cavefish could just be due to simple drift--a random mutation that knocks out pigmentation is no longer selected against in a place where there's little light, and so could drift up to high frequency. But the fact that this phenotype has arisen so many times, and reached high frequency in the presumably short time period that these fish populations have been isolated (I say presumably short because I can't find any numbers on this, but the different populations can interbreed freely) suggests a role for strong positive selection for this phenotype in adaptation to the cave environment.

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