Evidence for oscillating circadian clock genes in the copepod Calanus finmarchicus during the summer solstice in the high Arctic.

Lukas Hüppe,Kim Last,Bettina Meyer,David Wilcockson,Laura Payton,Elizaveta Ershova

doi:10.1098/rsbl.2020.0257

Abstract

The circadian clock provides a mechanism for anticipating environmental cycles and is synchronized by temporal cues such as daily light/dark cycle or photoperiod. However, the Arctic environment is characterized by several months of Midnight Sun when the sun is continuously above the horizon and where sea ice further attenuates photoperiod. To test if the oscillations of circadian clock genes remain in synchrony with subtle environmental changes, we sampled the copepod Calanus finmarchicus, a key zooplankter in the north Atlantic, to determine in situ daily circadian clock gene expression near the summer solstice at a southern (74.5° N) sea ice-free and a northern (82.5° N) sea ice-covered station. Results revealed significant oscillation of genes at both stations, indicating the persistence of the clock at this time. While copepods from the southern station showed oscillations in the daily range, those from the northern station exhibited an increase in ultradian oscillations. We suggest that in C. finmarchicus, even small daily changes of solar altitude seem to be sufficient to entrain the circadian clock and propose that at very high latitudes, in under-ice ecosystems, tidal cues may be used as an additional entrainment cue.

Highlights

Biological clocks are ubiquitous, ancient and adaptive mechanisms enabling organisms to track and anticipate environmental cycles and regulate biological processes
The persistence of zooplankton diel vertical migration (DVM) during the high Arctic Midnight Sun period is still debatable [9,10,11,12,13] and raises the question whether associated clock gene oscillations are maintained at this time or whether the clock stops ‘ticking’ and only reinitiates once clear light/dark cycles resume? Here we address this by determining circadian clock gene expression in C. finmarchicus during the Midnight Sun period
During the time of sampling at station JR85 (18– 19/06/2018, 82.5° N, sea ice-covered), daily cycles in solar altitude were lower when compared to the time of sampling several days later at station B13 (30/06/2018 – 01/07/2018, 74.5° N, sea ice-free, figure 1c)

Summary

Introduction

Biological clocks are ubiquitous, ancient and adaptive mechanisms enabling organisms to track and anticipate environmental cycles and regulate biological processes . Since circadian clocks of most organisms use the daily light/dark cycles as a Zeitgeber (literally, time giver) to maintain synchrony with the environment (entrainment), the capacity of the mechanism to persist under Midnight Sun conditions remains uncertain [4,5]. The persistence of zooplankton DVM during the high Arctic Midnight Sun period is still debatable [9,10,11,12,13] and raises the question whether associated clock gene oscillations are maintained at this time or whether the clock stops ‘ticking’ and only reinitiates once clear light/dark cycles resume? We address this by determining circadian clock gene expression in C. finmarchicus during the Midnight Sun period. For each replicate (n = 3–5 per time point), 15 C. finmarchicus CV stage copepods were sorted from the samples using morphological characteristics. Lett. 16: 20200257 solar altitude (°) solar altitude (°) tidal height (m) tidal height (m) clock cycle period timeless relative gene expression

B13 JR85 relative gene expression relative gene expression

Results

Discussion