From Neurogenetic Studies in the Fly Brain to a Concept in Circadian Biology

Abstract

This paper is dedicated to Karl-Friedrich Fischbach, who has always shared with me the interest in the function of the fly brain, especially that of its optic lobes. He has accompanied me during my first steps in scientific research. The paper tells the story how our first common attempts to localize the circadian clock in the fly brain finally helped in phrasing the two-oscillator principle of circadian clocks that seems to be valid far beyond the fly circadian system. I hope that Karl-Friedrich will take this story as praise for his generosity in supporting younger scientists outside his own lab, even without the reward of a common paper.

Keywords::

• circadian clock
• Drosophila melanogaster
• dual-oscillator model
• photoperiod
• pigment-dispersing factor

Acknowledgments

I thank Dr. Chun-Fang Wu for the invitation to write this review, Dr. Monika Stengl and Dr. Taishii Yoshi for helpful comments on the manuscript, and Dr. Pamela Menegazzi and Christiane Hermann for discussions. The work of my group was supported by several grants from the “Deutsche Forschungsgemeinschaft” (DFG; currently by the SFB1047 Insect Timing), as well as from the European Community (the 6th framework project EUCLOCK no. 018741).

Declaration of interest: The author reports no conflicts of interest. The author alone is responsible for the content and writing of the paper.

Notes

¹β-PDHs are a family of octadecapeptides that have been isolated from several crustacean species (Rao & Riehm, 1993). β-PDHs provoke the distal migration of retinal screening pigment in the compound eyes of crustaceans, a process that occurs during the day and protects the photoreceptor cells from light. Peptides with high similarity to the β-PDHs exist also in insects, although the latter do not have hormonal controlled pigment migrations. When applied to the optic stalk of crabs, the insect “PDHs” cause dispersion of the retinal pigment as do the crustacean PDHs. Therefore, they are called pigment-dispersing factors (PDFs) and are supposed to play a role as neuromodulators in the insect nervous system (reviewed in Helfrich-Förster, 2009b).

²The midnight activity peak is so far barely understood, but it seems to be especially pronounced in flies that lack parts of the circadian clock network, as, for example, functional M cells, but it may also be present in per0 mutants. Most probably by mistake, the midnight peak was classified as early M peak in several of our papers (Helfrich & Engelmann, 1987; Helfrich-Förster, 2001; Rieger et al., 2009). We are currently investigating the nature of the midnight peak in more detail.