Evolutionary Developmental Biology

There are many studies that recognise how genetic mechanisms can affect the phenotype in animals, including butterflies (Beldade and Brakefield, 2002), honeybees (Toth and Robinson, 2007) and fish (Protas et al., 2006). For example, the unsighted cavefish (Astyanax mexicanus) represents a good form of the morphological features in evo-devo. Because of the adjustment to living in darkness, they have no place eyes; however, they have also gained a number of helpful characteristics. Later studies suggested that there was an increase in sonic hedgehog (Shh) (gene is essential for many developmental processes in vertebrates) (Bingham et al., 2001) midline, indicating that this was not directly the cause of the eye loss in cavefish. This led to novel ways to search for possible modifications in the cavefish forebrain, for example that the most anterior part of vertebrates' central nervous system develops under the control of the main Shh morphogen (Retaux et al., 2008). In addition, studies estimate the time of deviation from about 1 million years, and maybe this relates to independent proceedings which occurred at different caves and different period. Studies by (Yamamoto et al. (2004) have shown that sonic hedgehog (shh) and tiggy-winkle hedgehog (twhh) gene expression is complete along the anterior embryonic midline in some different populations of cavefish. The increase of hh signalling is thought to result in genes' hyper-activation and the avoidance eye development and growth. Each of these characteristics exists in surface fish with overexpression of shh and/or twhh, supporting the idea that hh signalling has a function in cavefish eye loss. These studies have suggested that the formation of the smaller primeval eye cup in the cavefish was caused by expanded expression of hh, which occurs during the first stage of eye development. In this phase, the smaller primeval eye cup in the cavefish also induces a smaller lens placode. There is significant dissimilarity in eye sizes, even in the hybrids among surface and cavefish (Wilkens, 2010). Evo-devo tries to describe the mechanisms of these staged phenotypes. Through phases of time, in a minimum four different populations, Astyanax cavefish may have independently exhibited various degrees of eye degeneration. Yamamoto et al. (2004) showed that cavefish's regressive positive morphological characters as well as to blindness include the loss of melanin pigment, adjustments in the orbital skeleton and increases in jaw size, maxillary teeth and taste buds.