Question

Imagine you are a doctoral student studying evolution of a species of teleost, Astyanax, which lives in two populations; in rivers above ground and in underground aquatic caves. These populations have evolved different morphologies, those above ground can see and are pigmented, those that are underground are blind and unpigmented. Despite the morphological differences, the surface and cave fish can mate, leading to progeny with ranges of intermediate phenotypes. Assume the above ground Astyanax is the ancestral population.

a) In previous work, it was found that the homeodomain transcription factor Pax6 in cave-dwellers harbors multiple mutations that may be linked to loss of eyesight by affecting the progression of eye development. Without knowing what the mutations ARE, make an argument predicting the effect of different types of mutations on Pax6 and the role of these mutations in generating variation.

b) As a highly observant doctoral student, you notice some fish living above ground are also unpigmented, like their cave-dwelling cousins. Please argue what the possible sources of this variation might be and discuss how this feature may have arisen in the cavefish after relocation underground, independently in the above ground fish, or by convergent evolution. Lastly, predict which scenario is most likely to be true using parsimony.

Answer 1

a) Scientists have already discovered that eyes begin to form in embryonic cave fish, but it is killed later in development when the brakes are put on a gene known as pax6.

pax6 is found to be downregulated in cave-dwellers' developing eyes, other genes were turned up, ones controlled by a management-level gene known as sonic hedgehog that's active in the midline. When extra sonic hedgehog was injected into surface-dwelling embryos, the fish ended up with smaller retinas and almost no lens. And when they injected a chemical that inhibits hedgehog's activity into cavefish embryos, the fish had 30% more lens than normal, the researchers report in the 13 October issue of Nature. The finding suggests that sonic hedgehog prevents eye formation by dialing down pax6 and perhaps revving up other genes.

Interestingly, the morphology of the small eye primordium in cavefish embryos resembles the Small eye phenotype in mouse, which is caused by a mutation in the Pax6 gene. PCR analysis and library screen have detected a single Pax6 gene in Astyanax. Astyanax Pax6 is expressed in the lens placode, presumptive retina, and in parts of the central nervous system during early development. Later, Pax6 expression becomes restricted to the lens epithelial cells, the ganglion and amacrine cells of the retina, and the corneal epithelium.