Abstract
All living organisms synchronize biological functions with environmental changes; melatonin plays a vital role in regulating daily and seasonal variations. Due to rhythmic activity of the timezyme aralkylamine N-acetyltransferase (AANAT), the blood level of melatonin increases at night and decreases during daytime. Whereas other vertebrates have a single form of AANAT, bony fishes possess various isoforms of aanat genes, though the reasons are still unclear. Here, we have taken advantage of multiple unpublished teleost aanat sequences to explore and expand our understanding of the molecular evolution of aanat in fish. Our results confirm that two rounds of whole-genome duplication (WGD) led to the existence of three fish isoforms of aanat, i.e., aanat1a, aanat1b, and aanat2; in addition, gene loss led to the absence of some forms from certain special fish species. Furthermore, we suggest the different roles of two aanat1s in amphibious mudskippers, and speculate that the loss of aanat1a, may be related to terrestrial vision change. Several important sites of AANAT proteins and regulatory elements of aanat genes were analyzed for structural comparison and functional forecasting, respectively, which provides insights into the molecular evolution of the differences between AANAT1 and AANAT2.
Highlights
Light stimulus is a key factor for maintaining the physiological balance of fish
A total of 84 aanat sequences were derived from 37 vertebrate species; these sequences and their encoded proteins were used for our data analysis
Amphibians, reptiles, birds, sarcopterygii, and chondrichthyes, only one aanat gene was identified in their genomes
Summary
Light stimulus is a key factor for maintaining the physiological balance of fish. The pineal gland of lower vertebrates, such as fish, can detect changes in environmental light levels. The primary function of the pineal gland is to produce and release melatonin in response to circadian and lighting stimulation. Circulating melatonin levels are much higher at night than during daytime in essentially all vertebrates. The difference in blood melatonin levels between daytime and night provides information to the rest of the organism regarding the time of day and duration of the night period. Melatonin can regulate the endocrine activities through binding to its receptors throughout the fish body [2], it is considered a key component of the circadian system. Melatonin is thought to influence wide-ranging physiologies including reproduction, sleep, the immune system, and so on [7,8,9,10]
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