Circadian rhythms: pervasive, and often times evasive

Area of Science:

• Biology
• Evolutionary Biology
• Chronobiology

Background:

• Circadian clocks exhibit convergent evolution across diverse taxa, from cyanobacteria to humans.
• However, homologs of known clock components are absent in several animal, protist, fungal, and bacterial phyla.
• This raises questions about the evolutionary history and diversity of circadian systems.

Purpose of the Study:

• To reflect on the challenges and open questions in the study of non-model circadian systems.
• To emphasize the importance of investigating organisms lacking canonical clock components or observable rhythms.
• To highlight the need for exploring novel experimental conditions for uncovering hidden circadian functionalities.

Main Methods:

• Literature review and synthesis of current knowledge on circadian clock evolution.
• Analysis of genomic data for core clock elements in diverse taxa.
• Discussion of experimental approaches for studying rhythmicity in non-model organisms.

Main Results:

• Genomic identification of core clock elements does not always correlate with observed rhythmicity.
• Environmental cues (abiotic or nutritional) are often required to elicit rhythmic behavior in non-model systems.
• The absence of known homologs or observable rhythms does not preclude the existence of functional circadian clocks.

Conclusions:

• The study of non-model circadian systems is crucial for a comprehensive understanding of biological timing.
• Further research is needed to elucidate the mechanisms conditioning clock functionality and the evolutionary pathways of circadian regulation.
• Rethinking criteria for identifying functional clocks is essential, moving beyond reliance on canonical components and observable rhythms.