Blind cave fish, Astyanax mexicanus.
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Do you keep tropical freshwater fishes? I have kept tropical fishes for most of my life and was always intrigued by the so-called “blind cave fish”, Astyanax mexicanus, that were sometimes offered for sale to the public. These fish evolved from a sighted species that live in surface streams, but since the blind cave fishes lived in caves since the mid-Pleistocene or earlier, they were not exposed to light for at least one million years. Eventually these fishes lost their ability to grow eyes at all because the lack of sight presented no detriment to survival and in fact, because eyes are energetically expensive structures to make, there was selective pressure against their development and maintenance.
But what most hobbyists don’t know about these pinkish-white fishes is they actually are comprised of 29 distinct populations found in several caves in north-eastern Mexico (see figure 1).
Why is this so interesting, since all the populations are blind? This is important because some hybrids between different populations have had their sight restored in just one generation, according to a study that was just published.
Earlier research revealed that blindness in blind cave fish was the result of mutations at distinct locations, or loci, on the suite of genes that are responsible for making a fully functional eye. Further, this previous work showed that each population of fish had mutations at different loci, so they were blind for different genetic reasons. Based on this information, Richard Borowsky, an evolutionary biologist at New York University, hypothesized that genetic mutations found in separate populations might complement each other so that partially or fully functional eyes would be restored in the hybrid offspring.
“Given the large number of mutations at different loci that have accumulated in these populations, we reasoned that hybridization among independently evolved populations might restore visual function,” Borowsky said.
When blind cave fish development was observed, all embryos began developing eyes but these structures rapidly diminished into vestigial organs as the animal aged in purebred individuals (see Pachon cave fish and Tinaja cave fish, figure 2). The fish’s body then grows skin and scales over them. Further, not only were the fishes’ eyes affected, but the muscles required to move the eyes along with the brain structures necessary to interpret incoming visual information were affected.
However, the hybrid embryos (Tinaja/Pachon F1 and Tinaja/Molino F1, figure 2) developed fully functional, although smaller than normal, eyes (see surface fish, figure 2, for comparison).
Interestingly, the resulting hybrids demonstrated that this complementation depends upon the genetic distinctiveness of each population: when two nearby populations of fish were hybridized, they produced fewer sighted offspring than did those that were more distantly located from each other. This implies that there is a connection between nearby caves that allows some gene flow so these nearby fish populations share some of the same mutations.
“Restoration of the ability to see comes in a single generation because the populations residing in different caves are blind for different reasons — [for example] different sets of genes are non-functional in the different populations,” said Borowsky. “[In the hybrids], the deficiencies in one lineage are compensated for by the good gene copies in the other lineage, and vice versa.”
Borowsky plans to study these fish further to identify the actual genes responsible for blindness. Such research will likely contribute to identifying the genetic causes of human blindness and vision problems, such as myopia and macular degeneration.
Equally as important, this research adds insights into the evolution of cavefish in particular, and also serves to illustrate a general evolutionary principle: there is more than one molecular mechanism to accomplish the same result.
“Evolution’s palette is varied,” Borowsky asserted. “There are numerous genetic ways to accomplish the same change.”
This study was published in the peer-reviewed journal, Current Biology.
R BOROWSKY (2008). Restoring sight in blind cavefish Current Biology, 18 (1) DOI: 10.1016/j.cub.2007.11.023.
National Geographic (quotes, photograph)