Abstract
Antarctic krill (Euphausia superba) is a high latitude pelagic organism which plays a central role in the Southern Ocean ecosystem. E. superba shows daily and seasonal rhythms in physiology and behaviour, which are synchronized with the environmental cycles of its habitat. Recently, the main components of the krill circadian machinery have been identified and characterized. However, the exact mechanisms through which the endogenous timing system operates the control and regulation of the overt rhythms remains only partially understood. Here we investigate the involvement of the circadian clock in the temporal orchestration of gene expression by using a newly developed version of a krill microarray platform. The analysis of transcriptome data from krill exposed to both light-dark cycles (LD 18:6) and constant darkness (DD), has led to the identification of 1,564 putative clock-controlled genes. A remarkably large proportion of such genes, including several clock components (clock, period, cry2, vrille, and slimb), show oscillatory expression patterns in DD, with a periodicity shorter than 24 hours. Energy-storage pathways appear to be regulated by the endogenous clock in accordance with their ecological relevance in daily energy managing and overwintering. Our results provide the first representation of the krill circadian transcriptome under laboratory, free-running conditions.
Highlights
Due to its central position in the food web[10], the ongoing environmental changes in its habitat[11], and increasing commercial interest[12], the Antarctic krill Euphausia superba has become a model organism for the study of high latitude endogenous clock machinery and its effect on daily and seasonal life-cycle functions[13,14,15,16]
Total RNA was extracted from the head, including brain, eyestalk, and compound eyes, which play a crucial role in the modulation of neuroendocrine and behavioural circadian rhythms
Krill heads were selected in order to reduce background noise deriving from peripheral clocks, and to compare gene expression data with similar data previously obtained from krill heads, as described in De Pittà et al.[14] and Piccolin et al.[24]
Summary
Due to its central position in the food web[10], the ongoing environmental changes in its habitat[11], and increasing commercial interest[12], the Antarctic krill Euphausia superba (hereafter referred as krill) has become a model organism for the study of high latitude endogenous clock machinery and its effect on daily and seasonal life-cycle functions[13,14,15,16]. The same authors observed a bimodal oscillatory pattern in oxygen consumption and in the catalytic activity of four metabolic enzymes, which appeared to mirror cyclic locomotor activity patterns[2] (i.e. diel vertical migration). These observations suggest a link between the endogenous clock and metabolism. The temporal orchestration of specific biological processes (translation, proteolysis, energy and metabolic process, visual transduction and stress response), under prolonged almost continuous light conditions, suggest that in krill an endogenous oscillator controls and synchronizes the progression of biochemical and physiological events through the 24-hour cycle. The functional characterization of these genes revealed that krill possesses an “ancient” circadian clock exhibiting both mammalian and insect features[22]
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