Why Did Cavefish Go Blind? ： 洞穴魚為何失去視力？ which in turn influence the expression of shh [6]. Interestingly, it was reported that the overexpression of shh simultaneously increases the number of taste buds and jaw size in cavefish, enhancing their ability to taste and smell. This is terminologically known as “pleiotropy,” in which a single gene influences multiple seemingly unrelated traits [7]. Considering the loss of eyes as a tradeoff, some scientists even hypothesized that it is the trait of enhanced oral and taste bud development that is favored by natural selection [6]. Nevertheless, regardless of the different guesses regarding the “true meaning” of the trait, a more recent study on the cavefish originated from Pachón cave in Mexico suggested that the loss-of-eyes phenotype could be a result of epigenetic gene silencing, meaning that the phenotype may not be caused by DNA mutations (alternations in DNA sequence) but other mechanisms that turn off the eye development genes [8]. The research team attributed the phenotype to the DNA methylation of those genes, in which methyl groups (–CH3) are added to DNA to inhibit transcription. The expression of the modified genes is therefore stopped, so as their functions to support normal eye development. It is also worth mentioning that many of those inactivated genes are also found in humans and associated with human eye diseases. Thus, further investigation could potentially deepen our understanding of those diseases in human. Adaptive Strategy Beyond the Loss of Eyes Cavefish also compensate for the lack of vision through other heightened senses, such as vibration attraction behavior (VAB) [9]. VAB is the ability of some cavefish populations to locate prey by sensing water disturbance. This shift from visual to non-visual methods of survival shows the reallocation of resources that happens because of the environment. Cavefish have also shown evolution in traits such as losses of melanin pigmentation and circadian rhythms, increases in fat stores and body weight, and as mentioned before, the enhanced gustatory and olfactory systems [2, 9]. Different Roads Lead to Rome While different populations of cavefish develop similar adaptive traits, termed “convergent evolution,” the underlying genetic events or mechanisms are not necessarily the same [10]. The surface fish ancestors which presumably got trapped in different caves and formed different populations have probably developed those traits independently [2, 3, 10]. An example is the albinism in the Molino population and the Pachón population [10]. oca2 is a pigmentation gene required to maintain normal pigmentation of Mexican tetra. In those populations, a deletion can be found in oca2 gene, which causes the production of nonfunctional Oca2 protein. Notably, the deleted sequence within the gene is different in the two populations, meaning that the mutations occurred independently after the ancestors had settled in their caves. Therefore, evolution does not always follow a single path; parallel and convergent evolution of the same species can take place. This makes the Astyanax mexicanus an interesting model to study 3
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