Abstract
After a slow start, the comparative analysis of clock genes in insects has developed into a mature area of study in recent years. Brain transplant or surgical interventions in larger insects defined much of the early work in this area, before the cloning of clock genes became possible. We discuss the evolution of clock genes, their key sequence differences, and their likely modes of regulation in several different insect orders. We also present their expression patterns in the brain, focusing particularly on Diptera, Lepidoptera, and Orthoptera, the most common non-genetic model insects studied. We also highlight the adaptive involvement of clock molecules in other complex phenotypes which require biological timing, such as social behaviour, diapause and migration.
Highlights
Our fascination with the 24 h clock is probably due to our own personal experiences when we disrupt it, unpleasantly vivid after intercontinental travel, shift work or a good night out with friends
Such a design is a common feature of all circadian clocks (Dunlap, 1999) and it underlines the mechanistic constraints encountered in the evolution of circadian systems
This study reveals that the housefly has considerable similarities in clock gene expression to Drosophila, and some notable differences
Summary
Our fascination with the 24 h (circadian) clock is probably due to our own personal experiences when we disrupt it, unpleasantly vivid after intercontinental travel, shift work or a good night out with friends. CLOCKS IN OTHER DIPTERA The clock genes of a number of other flies have been compared, the most comprehensive study so far is probably that in Musca domestica, the housefly (Codd, Dolezel, Stehlik et al, 2007) In this species, the RNAs for Musca per, tim, and vri, are cycling and peak early in the subjective night, whereas the Clk transcript cycles with an opposite phase. Clock gene expression In the three species mentioned above, when per and tim mRNA cycling has been studied, cycles have been detected in moth/butterfly heads in LD and DD, with higher mRNA levels during the subjective dark phase (Reppert, Tsai, Roca et al, 1994; Froy, Gotter, Casselman et al, 2003; Iwai, Fukui, Fujiwara et al, 2006; Zhu, Sauman, Yuan et al, 2008). This is related to the social structuring demands of the colony, so that older bees that take on foraging roles have robust behavioural and molecular cycles whereas younger nurses have their circadian cycles largely suppressed within the hive (Toma, Bloch, Moore et al, 2000; Bloch, Toma, Robinson, 2001)
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