15 June 2011

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

ResearchBlogging.orgCaves 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.


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


Cornelia said...

Hello Dr Zen, I enjoyed your article very much as I am very curious about the cave crayfish.
I am just back from a speleological vacation in Kentucky where I had the chance to see crayfish in three different stages, in the same small "pond" not far off the entrance of the cave (at Horse Cave). The largest was colored and with eyes, there was one smaller and slightly discolored and with eyes but less popping and there was one completely eyeless and colorless.
I thought I heard our guide (Peggy Nims, which she is the director of education of the cave preservation Association) say that once they enter the cave it takes a couple of generations to become eyeless... now I am not sure anymore, it seems to be too soon just a couple of generations.
So I am searching but not finding a straight answer to this question and though you maybe able to answer it: how many generations apart would you say is a crayfish who wondered in a cave and stayed from its eyeless descendents?

Thank you!

Zen Faulkes said...


Hm. I do not know off the top of my head. Will do a little digging.