2015-09-27 13:00:00
One of the most prominent characteristics of life on our planet is that it occupies practically all of the ecological niches where it can develop. The adaptation capacity of living beings, if not unlimited, is very broad and, in general, surprising. We can find a wide variety of creatures in the most unexpected places, from the tops of cold mountains to the darkness of deep caves.
Obviously, this ability to adapt depends on genes, which are what ultimately make possible the physiological mechanisms to adapt to various external conditions, such as intense heat or cold, water scarcity, or food scarcity. It is this last condition that is frequently encountered, among other animals, by fish that live in the waters accumulated at the bottom of grottoes and caverns.
Cave fish can rarely become big fish, because food is very scarce. In the darkness of the caves, the plants are not able to grow, lacking all light, and they cannot, therefore, be the ones to provide the food that the fish need. These are nourished by organic substances carried in the mud and rainwater that reach the caves from the surface, or by foods as appetizing as bat excrement. How have these fish evolved to adapt to these terrible conditions, which not even Angela Merkel would dare imagine for the Greeks?
A group of researchers from Harvard and New York universities decided to study this issue by comparing possible genes that could contribute to the adaptations of a species of Mexican cavefish (Astyanax mexicanus). The interest of this species of fish lies, first of all, in the fact that there is a form of it that lives outdoors and a form that lives in caves. This already allows a comparison between them to analyze the evolutionary changes that have occurred between one and the other. However, the story does not end here. There are also different populations of cave fish in different caves, which use different adaptation strategies. All cave populations of A. mexicanus have a slower metabolism than outdoor species, and some can only use this strict energy austerity as their only means of survival. However, in addition to asceticism and resistance to famine, A. mexicanus populations in some caves are capable of soaking themselves with food on the rare occasions when it is abundant, thus storing energy for when worse times come. These fish populations accumulate much more body fat than others, although they also spend it slowly, which makes them susceptible to obesity. What is the genetic cause of these adaptive differences?
The hormone responsible
The studies carried out by the researchers, in which they compare the sequence of several genes involved in the control of metabolism and appetite, have identified a gene, called MC4R, that could explain these differences. This in itself is a surprising result, because in general variations in a single gene are not enough to cause important adaptive differences.
The MC4R gene produces a receptor protein for the hormone melanocortin. Some variants of this gene were already known to be related to the control of several important functions, depending on whether they are better or worse activated by the hormone. Studies with laboratory mice, carried out at the end of the last century and the beginning of this one, revealed that MC4R participates in appetite control and sexual behavior, including erectile function. In 2009, two genomic association studies carried out with humans found that some changes in the DNA sequence, located near this gene, although not in it, were associated with the development of obesity and type 2 diabetes. Finally , more recent studies have detected a modification of the protein due to a change of one amino acid for another that is also associated with obesity. Today there is little doubt that this gene participates in the susceptibility to becoming obese.
The studies carried out now reveal that the MC4R gene of cave fish that eat with great appetite when food is abundant has three changes, three mutations, which, in this case, are found in the heart of the gene and not in a region close to it. The changes affect the nature of the amino acids that make up the receptor protein, which modifies its sensitivity properties to the hormone. Curiously, one of these changes is identical to that detected in humans susceptible to being obese.
These studies, although carried out with beings that dwell in darkness, shed new light on our own evolutionary history and our susceptibility or not to becoming obese. It is possible that, as has happened with these fish, during our evolution, adaptation to periods of food scarcity went through the development of different strategies (depending on the mutations that occurred in different individuals), one of which has been and is eat a lot when food is available. Unfortunately, this adaptive strategy today, in a world of caloric hyperabundance, is tragic for many people.
Referencia: Ariel C. Aspiras et al. (2015). Melanocortin 4 receptor mutations contribute to the adaptation of cavefish to nutrient-poor conditions.
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