Cavefish have lost ability to generate eyes. New research examines the evolutionary mechanisms that caused this change.
One of the beautiful things about evolution is that species can adapt to survive in the most inhospitable environments, at least from our perspective. One of such situations is adaptation to the dark. There are several species living in dark places, such as caves, that rarely see the light of day. As diverse as these species may be, due to the need to adapt to a similar condition, in this case to perpetual darkness, they all evolved similar characteristics. This is an example of convergent evolution (different paths leading to a similar end result). Cavefish are an amazing example of such adaption.
One of the main adaptions of cavefish is that they are blind and lack pigment. At a first glance this may not look beneficial, but the loss of these features is compensated by other adaptations like increased jaws and better taste buds. We may be tempted to think that fish that kept all of these features would be favoured by natural selection, but that would require energy not only to develop but also to maintain. Therefore, fish that lost features that are otherwise useless in the dark are favoured as that energy can be used for something else.
Eye development is an extraordinary example of parallel evolution (different species develop a similar trait through similar mechanisms). There are many types of eyes in nature but they all seem to be formed by a similar mechanism. Of special importance is a gene (genetic information contained in the DNA) called pax6. This gene is what is called as a master gene. What this mean is that the developmental processes that lead to eye development seems to be initiated by pax6, and this is true for all animals that have eyes. This was initially found in fruit flies, where activation of this gene in different body parts would lead to formation of rudimentary eyes.
While the basic genetic mechanisms leading to the formation of the eye is well understood in several species, this is not the case for the mechanisms that lead to eye loss/degeneration. Therefore, cavefish presents an attractive model to study this process as different species have been living in darkness for different amounts of time, and while all species lack a proper eye, there are some small differences in their anatomy. This may indicate that while the end result is the same, the path to reach that result may be different . This is the question that researchers from Heidelberg University aimed to answer.
To tackle this question, the researchers studied Phreatichthys andruzzii, the Somalian cavefish. This fish has been isolated in a cave for 2 million years and developed extreme adaptations to the dark; they even lack scales. In order to understand what leads to the degeneration of the Somalian cavefish eye, the researchers investigated the activity of several genes involved in different steps of eye formation during embryonic development. What the researchers found was that genes involved in the early development of the eye do not seem to be affected in this species, and in fact they still start to develop a retina. However, it was observed that these immature retinal cells are not maintained and in fact start to die shortly after and eye tissue degenerates and sinks under the surrounding tissue becoming barely visible. Roughly at the same time neuronal connections to the retina seem to be lost and there is an increase in olfactory neurons.
These results suggest that in the Somalian cavefish the mechanism that leads to eye degeneration does not interfere with the activity of the master regulator of eye formation, pax6, which activity seems to be normal. These findings seem to contrast with what has been previously found in a more well studied specie of cavefish, Astyanax mexicanus, the Mexican cavefish, which separated from the surface counterparts only 10 thousand years ago. In the Mexican cavefish, eye loss has been attributed to an interference with the activity of pax6, leading to a much earlier loss of the eye.
The loss of any organ is not something which is easy to achieve, as genes used to originate the organ are often used by other organs, so mechanisms that would lead to a complete abolishment of that organ could possibly affect other organs as well. This is not restricted to the genetic program of organ formation, often the physical presence of an organ in the embryo is necessary for the development or maturation of another organ. It is remarkable that two different species with similar characteristics evolved two completely different mechanisms to eliminate the same organ, especially one which formation is as conserved as is the case of the eye. It is a true example of convergent evolution. It would be interesting to know if this is a special case in evolution or if it applies for the loss of other organs in other species.