Origin of the Animal Circadian Clock: Diurnal and Light-Entrained Gene Expression in the Sponge Amphimedon queenslandica

Abstract

The circadian clock is a molecular network that coordinates organismal behavior and physiology with daily environmental changes in the day-night cycle. In eumetazoans (bilaterians + cnidarians), this network appears to be largely conserved, yet different from other known eukaryotic circadian networks. To determine if the eumetazoan circadian network has an older origin, we ask here whether orthologs comprising this network are expressed in a manner consistent with a role in regulating circadian patterns in a representative of an earlier-branching animal lineage, the sponge Amphimedon queenslandica. The A. queenslandica genome encodes orthologs of many eumetazoan circadian genes, including two cryptochrome genes that encode flavoproteins, three Timeout genes, and two PAR-bZIP and seven bHLH-PAS transcription factor genes.There is no apparent Cycle ortholog, although we can identify three closely related ARNT genes. Of the putative circadian genes, only AqPARa and AqCry2 have a consistent oscillating diurnal expression profile, and the rhythmic expression of both these genes is partially lost when the animals are exposed to constant light or darkness. Expression of the other putative circadian genes, in particular AqClock, is neither diurnally-oscillating nor light-dependent. AqPARa and AqCry2 are also temporally and spatially co-expressed throughout embryonic and larval development. Transcripts of these genes are enriched first in cells comprising the larval posterior pigment ring, which is a simple photosensory organ that is responsible for the negative phototactic behavior displayed by larvae, and subsequently in the larval epithelial and subepithelial layers. The combined findings of no clear Cycle ortholog and of PAR-bZIP and cryptochrome being the only orthologs expressed in a pattern consistent with a circadian role suggests that either (i) the ancestral metazoan circadian network was simpler than the eumetazoan network, or (ii) that this sponge has lost some components, as has occurred in some other animals such as Hydra.

Interdependent photo- and chemosensory systems regulate larval settlement in a marine sponge

“…Cryptochromes are blue-light sensing flavoproteins (53) that have a well-characterised role in regulating circadian rhythms in plants (54), insects (55) and mammals (56). The apparent important role for a cryptochrome in light detection and response presented here is consistent with previous findings in this sponge (12) and in other marine invertebrates (45,(47)(48)(49)57). In A. queenslandica, AqCry2 is spatially expressed in the photo-responsive posterior pigment ring (15) that possesses long cilia that act as phototactic rudders, steering larvae away from light (14).…”

“…Indeed, sponge larvae are known to prefer shaded surfaces for settlement (13) and this is consistent with A. queenslandica larvae being negatively phototactic immediately post-emergence (14, but see 3). The phototaxis is regulated by a blue-light flavoprotein, a cryptochrome, which is localised to cells of the posterior photosensory organ in precompetent larvae, prior to their emergence from the adult (12,15). These observations raise the hypothesise that two interdependent sensory systemsa photosensory cryptochrome and a chemosensory systemorchestrate the pelago-benthic transition in this species.…”

“…1C), suggesting that entrained circadian rhythmicity may be governing this behaviour. The temporal and spatial expression of molecular homologs of core circadian genes, including a blue-light flavoprotein, the cryptochrome (AqCry2), provides direct support for the role of circadian rhythms in larval development in this sponge (12,15).…”

“…Competent A. queenslandica larvae do not settle unless they encounter an appropriate biochemical cue (9), such as the articulated coralline algae Amphiroa fragilissima (10). A. queenslandica larvae emerge from maternal brood chambers throughout the early to mid afternoon (11,12). The majority are competent to respond to biochemical cues by 4-6 hours post emergence (3), which coincides with twilight, suggesting that these larvae naturally settle in the dark.…”

Transcriptional remodelling upon light removal in a model cnidarian: Losses and gains in gene expression

“…Species from early diverging animal phyla have been studied to characterize the mechanisms of photoreception and signal transduction as well as the potential for a circadian clock (Plachetzki, Fong, & Oakley, ). Research with sponges has shown that various species have light‐dependent behaviour (Leys, Cronin, Degnan, & Marshall, ) as well as the molecular components for photoreception (cryptochromes; Rivera et al, ), components of the classic bilaterian circadian clock (Jindrich et al, ; Simionato et al, ) and cyclical oscillations in gene expression of these clock genes under diel lighting conditions (Jindrich et al, ). Similarly, light has a significant impact on the behaviour of ctenophores and previous studies have shown they have various light sensing proteins encoded in their genome (Schnitzler et al, ), but the potential for a circadian clock has not yet been studied (Reitzel et al, ).…”