The Nocturnal Activity of Fruit Flies Exposed to Artificial Moonlight Is Partly Caused by Direct Light Effects on the Activity Level That Bypass the Endogenous Clock

Abstract

Artificial moonlight was recently shown to shift the endogenous clock of fruit flies and make them nocturnal. To test whether this nocturnal activity is partly due to masking effects of light, we exposed the clock-mutants per(01), tim(01), per(01);tim(01), cyc(01), and Clk(JRK) to light/dark and light/dim-light cycles and determined the activity level during the day and night. We found that under moonlit nights, all clock mutants shifted their activity significantly into the night, suggesting that this effect is independent of the clock. We also recorded the flies under continuous artificial moonlight and darkness to judge the effect of dim constant light on the activity level. All mutants, except Clk(JRK) flies, were significantly more active under artificial moonlight conditions than under complete darkness. Unexpectedly, we found residual rhythmicity of per(01) and especially tim(01) mutants under these conditions, suggesting that TIM and especially PER retained some activity in the absence of its respective partner. Nevertheless, as even the double mutants and the cyc(01) and Clk(JRK) mutants shifted their activity into the night, we conclude that dim light stimulates the activity of fruit flies in a clock-independent manner. Thus, nocturnal light has a twofold influence on flies: it shifts the circadian clock, and it increases nocturnal activity independently of the clock. The latter was also observed in some primates by others and might therefore be of a more general validity.

Chronobiology by moonlight

“…The locomotor activity of fruitflies can be easily entrained to moonlight-dark cycles, and when artificial moonlight is given during the dark period of a 12 L : 12 D cycle, the flies' usual crepuscular activity patterns turn more nocturnal: the flies shift their activity largely into the night [98]. Fruitflies seems to be active at dim light [99]; thus, part of their nocturnality is due to moonlight-stimulated activity without shifting the circadian clock [100]. Such responses are also called masking effects (see below).…”

Light Intensity Determines Temporal Niche Switching of Behavioral Activity in Deep-WaterNephrops norvegicus(Crustacea: Decapoda)

“…Although temporal niche switches in response to changes in the relative light/dark intensity in the LD cycle had been previously reported (Mrosovsky and Hattar, 2005;Kempinger et al, 2009), our study is the first one to show nocturnality/diurnality switches in a deep-water marine species that encounters dramatic light intensity changes depending on the habitat where each population lives. Nephrops showed predominantly diurnal activity patterns under dim activity under 0.1-lux LD cycles and nocturnal E activity under 10-lux LD cycles.…”

“…1C), the predominance of locomotor activity in either the dark or light phase could be solely determined by positive and/or negative masking, regardless of the oscillator governance. This is the case in Drosophila, in which nocturnal moonlight leads to nocturnal activity (Kempinger et al, 2009), and it may be the case in nocturnal rodents (Cohen et al, 2010), in which…”

Novel masking effects of light are revealed inDrosophilaby skeleton photoperiods

“…The exogenous masking effects of light and the endogenous regulation of activity are both also temperature dependent (Vanin et al 2012;Yoshii et al 2012;Varma et al 2013) and strongly dependent on the amount of light (e.g. Bachleitner et al 2007;Kempinger et al 2009;Menagazzi et al 2012Menagazzi et al , 2013 as well as social interaction (Fujii et al 2007). The standard 12:12 LD laboratory activity pattern, with its strong daytime illumination and no social interaction, is, rather than an artifact, one end of a continuum of masking effects by light and social interaction and is attenuated by temperature.…”

“…In Drosophila melanogaster, endogenous circadian clocks (Konopka & Benzer 1971;Sehgal et al 1994;Dunlap 1999;Ceriani et al 2002;Hall 2003) generate daily rhythms that can be shaped by various exogenous factors such as temperature (Vanin et al 2012;Varma et al 2013), ambient light (Bachleitner et al 2007;Reiger et al 2007;Kempinger et al 2009;Menagazzi et al 2013), and social interaction (Fujii et al 2007). Much of the research on circadian clock function has required that such exogenous factors be held constant; therefore, the mechanisms underlying their influences are not well understood.…”