Hydra: Imaging Nerve Nets in Action

Hydra vulgaris exhibits day-night variation in behavior and gene expression levels

BACKGROUND

Day-night behavioral variation is observed in most organisms, and is generally controlled by circadian clocks and/or synchronization to environmental cues. Hydra species, which are freshwater cnidarians, are thought to lack the core clock genes that form transcription-translation feedback loops in clock systems. In this study, we examined whether hydras exhibit diel rhythms in terms of behavior and gene expression levels without typical clock genes.

RESULTS

We found that the total behavior of hydras was elevated during the day and decreased at night under a 12-h light-dark cycle. Polyp contraction frequency, one component of behavior, exhibited a clear diel rhythm. However, neither total behavior nor polyp contraction frequency showed rhythmic changes under constant light and constant dark conditions. To identify the genes underlying diel behavior, we performed genome-wide transcriptome analysis of hydras under light-dark cycles. Using three different analytic algorithms, we found that 380 genes showed robust diel oscillations in expression. Some of these genes shared common features with diel cycle genes of other cnidarian species with endogenous clock systems.

CONCLUSION

Hydras show diel behavioral rhythms under light-dark cycles despite the absence of canonical core clock genes. Given the functions of the genes showing diel oscillations in hydras and the similarities of those genes with the diel cycle genes of other cnidarian species with circadian clocks, it is possible that diel cycle genes play an important role across cnidarian species regardless of the presence or absence of core clock genes under light-dark cycles.

Optical manipulation of animal behavior using a ruthenium-based phototrigger

A visible-light activatable caged compound based on a ruthenium-polypyridine complex was used to elicit the feeding response of the freshwater cnidarian Hydra vulgaris. The phototrigger delivers l-arginine in a clean reaction under irradiation with blue or green light. The synthesis, characterization and application mode of this caged arginine are described. A combination of fiber-optics setup and a high absorbance medium allows the precise control of uncaging in the submillimetric range, needed to address the zone where activation takes place.