Ontogeny of the circadian system during embryogenesis in rainbow trout (Oncorhynchus mykyss) and the effect of prolonged exposure to continuous illumination on daily rhythms of per1, clock, and aanat2 expression.

It is widely held that the development of the circadian system during embryogenesis is important for future survival of an organism. Work in teleosts has been, to date, limited to zebrafish, which provides little insight into the diversity of this system within such a large vertebrate class. In this study, the authors analyzed the diel expression of per1, clock, and aanat2 in unfertilized rainbow trout oocytes and embryos maintained under either a 12:12-h light:dark (LD) cycle or continuous illumination (LL) from fertilization. 24-h profiles in expression were measured at fertilization as well as 8, 21 42, and 57 days postfertilization (dpf). Both per1 and clock were expressed in unfertilized oocytes and all embryonic stages, whereas aanat2 expression was only measureable from 8 dpf. A reduction in both per1 and clock mean expression levels between unfertilized oocytes/0-1 dpf embryos and 8-9 dpf embryos was suggestive of a transition from maternal RNA to endogenous mRNA expression. Although aanat2 expression was not clearly associated with photic conditions, photoperiod treatment did alter the expression of per1 and clock expression/rhythmicity from as early as 8 dpf (per1), which could suggest the presence and functionality of an as yet unidentified "photoreceptor." As a whole, this work demonstrates that clock systems are present and functional during embryonic development in rainbow trout. Further studies of their expression and regulation will help understand how the environment interacts with embryonic development in the species.