Animals are living all over the world, at different environment types (moderate-harsh). In extreme environments, only few animals survive. However, various collection of animals have adapted to life in permanent darkness of caves. These animals are united by the convergence of some regressive phenotypes. The regressive evolution refers to the loss of unused traits over time. Cave animals appeared one of the best examples of regressive evolution. Astyanax Mexicanus has been chosen as the model species for the study of evolution and adaptation in dark environment. It has allowed very significant advances in identifying mutations involved in cave phenotypes. The two forms of Astyanax Mexicanus are surface fish which inhabits the rivers of Mexico and the south of USA, and cavefish which live in permanent darkness which considered as extreme environment. Cavefish have undergone a number of changes at the morphological, physiological, and behavioural levels, however, they can breed with their surface fish conspecifics.
Permanent absence of light and lack of food environment of caves, have forced strong selective pressures on colonizing species. Because of the darkness there is no photosynthetic products. Thus, cave animals completely depend on the food supplies from outside the caves. Bats and seasonal flood supplier the food in the caves, but its limited and infrequent. Multiple adaptations to these circumstances have been acquired by cavefish, such as reduces metabolic rate, enhance fat storage, improve starvation resistance and increased metabolic efficiency. Also, they have reduced circadian rhythms that allows to save 30% of energy. Cavefish have been studied to understand how genes function or effects metabolic evolution. Despite the various morphological and behavioural differences between surface fish and cavefish, they remain interfertile.
The features of metabolic rate of both cave and surface populations provide chance to reveal the genetic causal metabolic variation in vertebrates and how cave animals can adapt to the limited and rare food supply.
The study of starvation resistance of both cavefish and surface fish uncovered that each individuals of cavefish (Pachon, Tinaja, and Molino) lost half much weight as surface populations during 2-months fast. Beside this observation, as all adult fish were received regular food, Tinaja cave fish found to have a higher fat storage than surface fish. Similar results were found on other independently cavefish population Pachon. All cave population shows the same trend but, Tinaja cavefish have the highest fat levels. And they distinct in the location of fat storage within their bodies. Tinaja cavefish have a large liver that indicates the presences of lipids. Enlarge liver were not seen in other cave fishes. No significant change in life span or health of the Tinaja cavefish who developed fatty livers under high nutrient conditions. That means it have developed a mechanism to use liver as a fat storage organ without causing it any harm.
To reveal how cavefish, acquire high fat storage researchers have studied the food consumption between both cavefish and surface fish populations. So, they studied the differences in appetite by measuring the amount of black worms (Lumbriculus variegatus) consumed at specific period of time in cavefish and surface fish. The outcomes indicate significant differences in appetite modulation among populations. For instance, Fig (A-C) displays no difference in appetite between surface fish and Pachon cavefish even that it had higher fat levels than surface fish. In addition, there were no differences in appetite between fed Tinaja cavefish and the appetite of 3-weeks of fasting surface fish. The appetites of both Pachon and Tinaja cavefish were not affected by three weeks of fasting whereas surface population have increased their appetite. This implies that there is significant in appetite modulation between populations. Overall, Tinaja cavefish shows the highest appetite between populations (Fig 1. A-C). These information indicate that there are significant differences in appetite modulation between populations.
To identify the underlying genetic cause of these phenotypes, researchers analysed F1 hybrids between Pachon and Tinaja cavefish and between surface and Tinaja cavefish. This study have revealed that both F1 hybrids displays appetite similar to Pachon and surface fish during a fed state. This have