Survival of the Sightless: 瞎者生存： By Roshni Printer Adaptation by Losing an Ability One of the most fascinating aspects of evolution is how population growth is linked to the inheritance of favorable traits. One famous example involves the domestication of animals, increased availability of milk, and the selective advantage to adults who can digest lactose in milk [1]. However, in some cases, evolution also manifests as the loss of traits or functions. An example is the Mexican tetra, Astyanax mexicanus, a freshwater fish species found in caves and streams in Central America. Mexican tetra exists as surface form and cavedwelling form. Unlike the former which lives near the water surface and have well-developed eyes, the latter lives in caves and have lost their eyes during evolution [2]. Early eye development begins as usual but is disrupted prematurely in the cavefish embryo, and then their eyes degenerate within a few days [2, 3]. The striking difference between the two morphs of the species raises an important question: Why did they lose their sight, which is a fundamental survival function for almost all animals? Although it seems counterintuitive to lose a function, the answer may lie in energy conservation. In the dark environment of underwater caves where the food supply is scarce, maintaining eyesight poses an unnecessary energy demand to the fish [4]. The metabolic cost of this complex function on the neural tissue to process visual information simply inflicts a heavy burden. Studies found that the cost of vision in young surface-dwelling Mexican tetra can reach up to 15% of their resting energy expenditure, a cost that can be avoided by its cave-dwelling counterpart [4]. How Do Cavefish Lose Their Eyes? Therefore, one can expect the loss of eyes to be a trait favored by natural selection in dark environments. Then, how can embryonic eye degeneration be achieved at the genetic and molecular levels? Sonic hedgehog (Shh; footnote 1) protein is a morphogen that plays a role in the differentiation of embryonic cells into the brain and spinal cord, eyes, and many other parts of the body [5]. Scientists observed that the Shh protein is expressed in an expanded region along the cavefish embryonic midline [2, 6]. The overexpression of the shh gene was found to inhibit the development of two eye structures, namely lens and optic cup, eventually leading to the degeneration of the eye. One may speculate that the overexpression is caused by some mutation in the two shh genes of the cavefish, but unfortunately no mutation was found in shh [6]. There is a possibility that the mutations are in other genes
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