Abstract
Cryptochrome (CRY) plays an important role in the input of circadian clocks in various species, but gene copies in each species are evolutionarily divergent. Type I CRYs function as a photoreceptor molecule in the central clock, whereas type II CRYs directly regulate the transcriptional activity of clock proteins. Functions of other types of animal CRYs in the molecular clock remain unknown. The water flea Daphnia magna contains four Cry genes. However, it is still difficult to analyse these four genes. In this study, we took advantage of powerful genetic resources available from Drosophila to investigate evolutionary and functional differentiation of CRY proteins between the two species. We report differences in subcellular localisation of each D. magna CRY protein when expressed in the Drosophila clock neuron. Circadian rhythm behavioural experiments revealed that D. magna CRYs are not functionally conserved in the Drosophila molecular clock. These findings provide a new perspective on the evolutionary conservation of CRY, as functions of the four D. magna CRY proteins have diverse subcellular localisation levels. Furthermore, molecular clocks of D. magna have been evolutionarily differentiated from those of Drosophila. This study highlights the extensive functional diversity existing among species in their complement of Cry genes.
Highlights
The circadian rhythm is a fundamental system for organisms, from prokaryotes to humans[1]
Unlike Drosophila, mammals have a second type of CRY protein that directly suppresses the transcriptional activity of the CLK/BMAL1 heterodimer in the nucleus in a light-independent manner[18,19]
We revealed the subcellular localisation of each Dapma-CRY in the brain of D. melanogaster (Fig. 3): Dapma-CRYA localised to both the nucleus and cytoplasm in the cell soma; Dapma-CRYB localised to the cytosol and nucleus; Dapma-CRYC clearly localised to the nucleus; Dapma-CRYD showed relatively strong localisation to the cytosol rather than the nucleus
Summary
The circadian rhythm is a fundamental system for organisms, from prokaryotes to humans[1]. Unlike Drosophila, mammals have a second type of CRY protein (type II CRY) that directly suppresses the transcriptional activity of the CLK/BMAL1 heterodimer (the mammalian orthologue of CYC) in the nucleus in a light-independent manner[18,19]. Type II CRY and PER proteins heterodimerise in the cytoplasm and move to the nucleus to suppress the transcriptional activity of the CLK/BMAL1 heterodimer, which in turn, activates the transcription of cry and per mRNA These transcription-translation feedback loops generate the cell-autonomous molecular clock in mammals. The molecular functions of and the relationships among these four CRY genes in the circadian clock are still unknown To address this question, we focused on an additional water flea species. Expression of D. magna CRYs failed to rescue the cry mutant flies, suggesting functional differences between D. magna and D. melanogaster CRYs
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