Functional characterization of the circadian clock in the Antarctic krill, Euphausia superba

Alberto Biscontin,Cristiano Bertolucci,Cristiano De Pittà,Gabriella Mazzotta,Gabriele Sales,Leonard Janke,Achim Kramer,Rodolfo Costa,Thomas Wallach,Bettina Meyer,Astrid Grudziecki,Elena Sartori

doi:10.1038/s41598-017-18009-2

Abstract

Antarctic krill (Euphausia superba) is a key species in Southern Ocean ecosystem where it plays a central role in the Antarctic food web. Available information supports the existence of an endogenous timing system in krill enabling it to synchronize metabolism and behavior with an environment characterized by extreme seasonal changes in terms of day length, food availability, and surface ice extent. A screening of our transcriptome database “KrillDB” allowed us to identify the putative orthologues of 20 circadian clock components. Mapping of conserved domains and phylogenetic analyses strongly supported annotations of the identified sequences. Luciferase assays and co-immunoprecipitation experiments allowed us to define the role of the main clock components. Our findings provide an overall picture of the molecular mechanisms underlying the functioning of the endogenous circadian clock in the Antarctic krill and shed light on their evolution throughout crustaceans speciation. Interestingly, the core clock machinery shows both mammalian and insect features that presumably contribute to an evolutionary strategy to cope with polar environment’s challenges. Moreover, despite the extreme variability characterizing the Antarctic seasonal day length, the conserved light mediated degradation of the photoreceptor EsCRY1 suggests a persisting pivotal role of light as a Zeitgeber.

Highlights

Antarctic krill (Euphausia superba), further named krill, is a key species in Southern Ocean food webs[1], and plays a central role in ecosystem processes and community dynamics of apex predators[2]
In order to understand whether the functioning of the positive feedback loop is more similar to the Drosophila or mammalian model, we investigated whether EsCLK and EsCYC/BMAL were able to replace the primary component of the CLK:CYC/BMAL dimer in Drosophila and in mammals, respectively
In order to test whether the identified clock components show circadian oscillations at the transcriptional level, we examined the temporal expression profiles in krill eyestalks and brain sampled from nature during the Antarctic summer[32]

Summary

Introduction

Antarctic krill (Euphausia superba), further named krill, is a key species in Southern Ocean food webs[1], and plays a central role in ecosystem processes and community dynamics of apex predators[2]. Krill exhibit rhythmic behavioral and physiological features on a daily and seasonal scale in the Southern Ocean. A characterization of the diurnal gene expression pattern of krill during the Antarctic summer revealed that important processes most likely related to daily environmental changes (such as energy and metabolic process, visual transduction, and stress response) exhibited certain rhythmicity. These observations underline that a circadian regulation of krill metabolism and physiology occurs under natural conditions[10]. The high level of conservation with known circadian clock genes suggests their involvement in the biological clock systems of these species. A first step toward the definition of a molecular clock model in krill was taken by Mazzotta et al.[32] who identified a cryptochrome gene (here after termed EsCRY2), one of the cardinal components of the clock-work machinery in several terrestrial organisms

Methods

Results

Conclusion