The expression of the clock gene cycle has rhythmic pattern and is affected by photoperiod in the moth Sesamia nonagrioides

Time measurement in insect photoperiodism: external and internal coincidence

The identity and nature of the photoperiodic photoreceptors are now quite well known, as is the nature of the endocrine regulation of the resulting diapauses. The central problem of time measurement-how the photoperiodic clock differentiates long from short days-however, is still obscure, known only from whole-animal experiments and abstract models, although it is clearly a function of the insect circadian system. This review describes some of these experiments in terms of oscillator entrainment and two widely applicable photoperiodic clock models, external and internal coincidence, mainly using data from experiments on flesh flies (Sarcophaga spp) and the parasitic wasp, Nasonia vitripennis.

Circadian clock genes and photoperiodic diapause in the moth Sesamia nonagrioides

Insects, like most organisms, have an internal circadian clock that oscillates with a daily rhythmicity, and a timing mechanism (photoperiodic clock) that mediates seasonal events, including diapause. It has been argued that there is a connection between the two clocks. The Mediterranean corn stalk borer moth, Sesamia nonagrioides, undergoes facultative diapause governed by photoperiod. To obtain clues to the link between the molecular mechanism of circadian and photoperiod clocks, we cloned and investigated the expression profiles of the clock genes Snper, Sntim, Sncyc and Sncry1 in the aforementioned moth species. Our previous results suggested that these genes might be implicated in the regulation of the diapause programming in S. nonagrioides. Here we studied the expression patterns of these four clock genes in larvae reared under abnormal non-24 h light-dark cycles (L10:D62 and L10:D14:L10:D62) in order to assess whether disruption of circadian clock would have any effect in the photoperiodic regulation of diapause. In the L10:D14:L10:D62 cycle abnormal expression patterns of the Sntim/Sncry1 and Snper/Sncyc, pairs were found, compared to normal 24 h light-dark photoperiods suggesting that individual clock genes are acting independently in the molecular diapause program of S. nonagrioides. Photoperiod therefore appears to be the crucial signal for the regulation of these four genes.

Characterizing the relative abundance of circadian transcription factors in diapausing and nondiapausing Northern house mosquitoes

The Northern house mosquito (Culex pipiens) is a major vector of West Nile virus. To survive harsh conditions in winter adult females of Cx. pipiens enter a state of arrested reproductive development called diapause. Diapause is triggered by the short daylengths of late summer and early fall. The methods by which Cx. pipiens measures daylength are still unknown. However, it is suspected that clock genes, which provide information on daylength, may also regulate diapause. The proteins produced by these genes often cycle in abundance throughout the day in diapausing and nondiapausing insects. Two clock genes suspected to control diapause are cycle (cyc) and Par domain protein1 (Pdp1) as they encode circadian transcription factors that may regulate genes that are involved in diapause. Using Western blotting we measured the relative protein abundance of CYC and PDP1 throughout the day in the whole bodies and the heads of Cx. pipiens reared under either long-day, diapause-averting conditions or short-day, diapause-inducing conditions. We found that in whole bodies there was no significant oscillation of CYC or PDP1 abundance in both long day and short day-reared mosquitoes. In the heads of long day-reared mosquitoes both CYC and PDP1 cycled. In contrast, only PDP1 abundance showed diel differences in abundance in the heads of short day-reared mosquitoes. These data bring us one step closer to understanding the role that CYC and PDP1 may play in regulating diapause and other biological processes.

Dormancy, Diapause, and the Role of the Circadian System in Insect Photoperiodism

Whole-animal experiments devised to investigate possible association between photoperiodic time measurement and the circadian system (Bünning's hypothesis) are compared with more recent molecular investigations of circadian clock genes. In Sarcophaga argyrostoma and some other species, experimental cycles of light and darkness revealed a photoperiodic oscillator, set to constant phase at dusk and measuring night length repeatedly during extended periods of darkness. In some species, however, extreme dampening revealed an unrepetitive (i.e., hourglass-like) response. Rhythms of clock gene transcript abundance may also show similar phase relationships to the light cycle, and gene silencing of important clock genes indicates that they play a crucial role in photoperiodism either alone or in concert. However, the multiplicity of peripheral oscillators in the insect circadian system indicates that more complex mechanisms might also be important.