Beach-dwelling crustaceans have both circadian and circatidal clocks which operate in parallel brain regions using overlapping genetic components
Animals which live on beaches rely on internal body clocks to keep track of the circadian day-night cycle and the changing tides. While the molecular mechanisms which drive circadian timekeeping in terrestrial animals has been extensively studied, little is known about the nature of circatidal clocks. To remedy this, Michael Hastings’s group in the LMB’s Neurobiology Division has worked with David Wilcockson at Aberystwyth University to study the crustaceans Eurydice pulchra and Parhyale hawaiensis to unravel how these animals adapt to multiple environmental cycles. This revealed distinct sets of circadian and circatidal timers localised to separate brain regions.
Circadian clocks are powered by a group of proteins acting in a repeating loop named the transcriptional-translational feedback loop (TTFL). Within this auto-regulatory system, positive factors Clock (CLK) and BMAL1 activate the expression of negative regulators Period (PER), Cryptochrome2 (CRY2) and/or Timeless (TIM), whose protein products accumulate and inhibit CLK-BMAL1 activity in a roughly 24-hour cycle. Led by Andrew Oliphant and Chee Ying Sia, members of Michael’s group, the team set out to identify and map the activity of these clock genes in field-collected E. pulchra and lab-reared P. hawaiensis to better understand their internal clocks.
The crustaceans were kept in different conditions to simulate different environmental cycles; light-dark cycles, constant darkness, and mechanical tides simulated by shaking. The activity of both species was closely monitored to determine if they were exhibiting circadian (~24 hour) or circatidal (~12.4 hour) rhythms. To identify the clock cells of interest, the group used brain imaging and gene expression mapping techniques to visualise where and when the specific genes were active in brain tissue. Brains were sampled at regular intervals to allow for the tracking of gene expression over time and the detection of rhythmic patterns. They identified approximately 30 ‘clock’ cells per brain hemisphere and grouped these based on location in the organ.
Interestingly, both species demonstrated distinct cell groups with different rhythms – some were circadian and some circatidal. These cell groups were anatomically and functionally separate in both crustaceans, yet all showed rhythmic expression of the same TTFL genes. Changes in the light-dark cycles did not impact tidal expressions, and similarly when physical tidal cues were introduced to the animals, cells exhibiting circadian rhythms were not diverted from their light-dark cycles, underlining the independence of these mechanisms. This supports the hypothesis that two types of oscillating clock can co-exist in the same organism.
This study reveals how animals which live in coastal regions are able to thrive in some of the most challenging environments on Earth. That the genes used for circatidal rhythms are unchanged from those demonstrated in circadian rhythms, points to the possibility that these tidal clocks are an evolutionary by-product of the environmental pressures experienced in these habitats. Looking more widely, this work sheds light on an under-studied element of internal time keeping. It also pushes the field of circadian models to focus more widely and raises the possibility that these dual clocks also feature in instances of circalunar or seasonal rhythms.
This work was funded by UKRI MRC and Boehringer Ingelheim Fonds.
Further references
Expression of clock genes tracks daily and tidal time in brains of intertidal crustaceans Eurydice pulchra and Parhyale hawaiensis. Oliphant, A., Sia, C.Y., Kyriacou, C.P., Wilcockson D.C., Hastings, M.H. Current Biology
Michael’s group page
David Wilcockson – Aberystwyth University
Revealing the cells behind the biological clocks of intertidal animals – Aberystwyth University
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