Turning light and going blind: A tale of caves and genes

Caves seem to be dead ends of the most literal sort. Not so. Caves are bubbling cauldrons of evolutionary experimentation, and pose wonderful opportunities to study convergence. Because cave systems tend to be unconnected, they can easily become little isolated pockets, and a larger non-cave-dwelling population might invade caves repeated times. Or different species might enter the same cave. In any case, the habitat is so unique and so demanding that the changes are large.

Asellus aquaticus is a widespread isopod crustacean. Some live in caves, some don’t. The ones in caves show “an increase in body length, an increase in the length of certain appendages, an increase in the length of aesthetascs (thought to be chemoreceptors), a decrease in the degree of body and eye pigmentation, a decrease in the size of the eye, and a changed setal pattern.”

But the cave and surface forms are nominally the same species. What has happened genetically to these? Are they still able to interbreed? Are all those cave-related features reversible?

Protas and colleagues tried breeding the two forms, and were able to get hybrids, all females. Why the first generation were all females isn’t clear.

When they started doing genetics, a perhaps unsurprising pattern emerged for the genes they examined: the cave dwellers were homozygous, while the hybrids and surface dwellers were heterozygous. This to me suggests the cave dwellers are showing classic recessive Mendelian genes.

The eyes of these cave crusties have a wide range of appearances. Some are truly eyeless, while others have eyes that are... messed up. Those also appeared in the second generation. The eye loss seems to be a small number of genes with large effects.

The authors found multiple genetic mutations responsible for pigment loss - there are two ways, involving three genes, to become albino. There are also different genes for small eyes and the complete lack of eyes. This is very similar to the situation in Mexican blind cave fish, where “small eyes” and “no eyes” have are the result of two different genes, not variation within a single gene.

The paper goes on to detail much more about the genes, most of which is “above my pay grade,” as they say. But it’s cool to find these genes, with such clear and large effects, that are so clearly correlated with the environment, and with such unusual parallels to an unrelated species.

Anyone know of third blind cave species that a geneticist might tackle? Or maybe someone is willing to do the experiment of releasing a population of fruit flies into a cave, and coming back in a decade or two to see what has happened.

Reference

Protas M, Trontelj P, Patel N. 2011. Genetic basis of eye and pigment loss in the cave crustacean, Asellus aquaticus. Proceedings of the National Academy of Sciences 108(14): 5702-5707. DOI: 10.1073/pnas.1013850108