Development of the Astyanax mexicanus circadian clock and non-visual light responses

Abstract

Most animals and plants live on the planet exposed to periods of rhythmic light and dark. As such, they have evolved endogenous circadian clocks to regulate their physiology rhythmically, and non-visual light detection mechanisms to set the clock to the environmental light-dark cycle. In the case of fish, circadian pacemakers are not only present in the majority of tissues and cells, but these tissues are themselves directly light-sensitive, expressing a wide range of opsin photopigments. This broad non-visual light sensitivity exists to set the clock, but also impacts a wide range of fundamental cell biological processes, such as DNA repair regulation. In this context, Astyanax mexicanus is a very intriguing model system with which to explore non-visual light detection and circadian clock function. Previous work has shown that surface fish possess the same directly light entrainable circadian clocks, described above. The same is true for cave strains of Astyanax in the laboratory, though no daily rhythms have been observed under natural dark conditions in Mexico. There are, however, clear alterations in the cave strain light response and changes to the circadian clock, with a difference in phase of peak gene expression and a reduction in amplitude. In this study, we expand these early observations by exploring the development of non-visual light sensitivity and clock function between surface and cave populations. When does the circadian pacemaker begin to oscillate during development, and are there differences between the various strains? Is the difference in acute light sensitivity, seen in adults, apparent from the earliest stages of development? Our results show that both cave and surface populations must experience daily light exposure to establish a larval gene expression rhythm. These oscillations begin early, around the third day of development in all strains, but gene expression rhythms show a significantly higher amplitude in surface fish larvae. In addition, the light induction of clock genes is developmentally delayed in cave populations. Zebrafish embryonic light sensitivity has been shown to be critical not only for clock entrainment, but also for transcriptional activation of DNA repair processes. Similar downstream transcriptional responses to light also occur in Astyanax. Interestingly, the establishment of the adult timing profile of clock gene expression takes several days to become apparent. This fact may provide mechanistic insight into the key differences between the cave and surface fish clock mechanisms.

Evolution of gastrointestinal tract morphology and plasticity in cave-adapted Mexican tetra, Astyanax mexicanus

“…Proliferation and gene expression in the intestine is under circadian control and disruption of clock genes alters renewal of the epithelium (46)(47)(48). Having evolved in complete darkness, cavefish have altered circadian rhythm as evidenced by developmentally delayed and reduced amplitude of clock gene expression, lack of circadian cycles of metabolism, and reduced sleep (25,(49)(50)(51). It is plausible that alterations to the gene regulatory network controlling circadian rhythm influenced homeostasis of the intestinal epithelium during cavefish evolution.…”

Circadian Clocks in Fish—What Have We Learned so far?

“…Zebrafish has one per2 gene, in contrast to many other teleost species, such as Astyanax mexicanus , where there has been a clear duplication of the per2 gene. The distinct roles of per2a and per2b are unclear, but in A. mexicanus , the expression patterns in light-dark (LD) and constant dark (DD) differ, indicating that they may have distinct roles [1]. The situation for the multiple cryptochromes in zebrafish is equally unclear, where there are at least six distinct cry genes.…”

“…Entrained zebrafish and S. senegalesis embryos restrict hatching to a particular time window in the day. Constant light conditions disrupt this timed regulation, with resulting ultradian bouts for zebrafish and 24-h delays or advances in DD and LL respectively for flatfish hatching [1] . Such results have significant implications for commercial fisheries, which often employ constant light conditions to influence early larval growth rates.…”

“…During the first 6 h of development, however, zebrafish rely on maternally deposited 6-4 photolyase transcript. The large quantities of maternally deposited 6-4 photolyase is not only found in zebrafish, but also in cavefish embryos, and undoubtedly in many other species of fish [1,32].…”

“…In this respect, various fish models offer unique tools with which to study the wide significance of light responsiveness, and this is no more true than in the use of naturally occurring cave populations of fish, in particular the blind Mexican cavefish ( Astyanax mexicanus ) and the Somalian cavefish ( Phreatichthys andruzzii ). These animals, which have evolved in complete darkness, can effectively be viewed as circadian/light responsive “mutants”, and as such offer considerable potential for exploring the global importance of light [1,2,3,4].…”