A comparative analysis of the circadian clock in Diptera

The circadian central oscillator of Drosophila melanogaster consists of at least two interlocked negative transcriptional feedback loops. This has been taken to be a general model for higher eukaryotes with the core components conserved but their regulation altered. The work presented here indicates that in Musca domestica, a dipteran closely related to Drosophila, one of these regulatory loops, involving PERIOD (PER) and TIMELESS (TIM), functions in a completely different manner. This study shows that in contrast to Drosophila, Musca PER remains constant in western studies in any lighting condition, whereas like Drosophila TIM cycles in both LD and DD and is constantly degraded in LL. In addition within the central brain immunostaining revealed that even in the small set of cells thought to contain the central pacemaker PER staining was restricted exclusively to the cytoplasm. However following the Drosophila model PER was observed to cycle in the cytoplasm of these cells. Although TIM co-localises with PER in these cells, unlike PER, TIM does become nuclear. This indicates that the negative feedback model illustrated by analysis of the Drosophila is inadequate to explain clock function in Musca. A putative Musca PER nuclear export sequence which functions in other species was tested in GFP constructs but not shown to be involved in altered localisation. In contrast in peripheral tissue such as photoreceptor cells both PER and TIM cycle and both proteins become nuclear late at night as in Drosophila. Stability of Musca PER in LL and an altered relationship between transgenic Musca PER and Drosophila DOUBLETIME indicates an altered relationship between PER and the DBT kinase that may be responsible for PER stability. Thus although it can be seen that a different model is required for other insect species how these proteins act remains to be elucidated.