Circadian Expression of Clock Genes in Two Mosquito Disease Vectors: cry2 Is Different

Abstract

Different mosquito species show a full range of activity patterns, including diurnal, crepuscular, and nocturnal behaviors. Although activity and blood-feeding rhythms are controlled by the circadian clock, it is not yet known whether such species-specific differences in behavior are controlled directly by core clock genes or instead reflect differences in how the information of the central clock is translated into output signals. The authors have analyzed the circadian expression of clock genes in two important mosquito vectors of tropical diseases, Aedes aegypti and Culex quinquefasciatus . Although these two species show very different locomotor activity patterns and are estimated to have diverged more than 22 million years ago, they show conserved circadian expression patterns for all major cycling clock genes except mammalian-like cryptochrome2 (cry2). The results indicate that different mechanisms for cry2 regulation may exist for the two species. The authors speculate that the correlation between the differences in behavior between Ae. aegypti and Cx. quinquefasciatus and their corresponding cry2 mRNA profiles suggests a potential role for this clock gene in controlling species-specific rhythmic behavior. However, further work is needed to establish that this is the case as the different cry2 expression patterns might reflect differences between the Aedes and Culex lineages that are not directly related to changes in behavior.

Functional circadian clock genes are essential for the overwintering diapause of the Northern house mosquito,Culex pipiens

“…Our observations of per, tim, cyc and cry2 oscillations throughout the diapause program support the circadian basis for diapause initiation and maintenance in C. pipiens and suggest that the rhythmic oscillations of these transcripts may be involved in continually measuring night length throughout diapause. In contrast, Clk and cry1 did not show significant changes in daily expression in the brains of mosquitoes at any of the stages we examined, a result that is consistent with the pattern of Clk and cry1 expression in the heads of the Southern house mosquito, C. quinquefasciatus (Gentile et al, 2009). Although we did not measure the expression profiles of any of the clock genes in constant darkness, the expression that Gentile et al report for non-diapausing C. quinquefasciatus in L:D conditions was unchanged in the absence of light.…”

“…Although we did not measure the expression profiles of any of the clock genes in constant darkness, the expression that Gentile et al report for non-diapausing C. quinquefasciatus in L:D conditions was unchanged in the absence of light. Furthermore, the expression of clock genes in nondiapausing C. pipiens corresponded to those reported in C. quinquefasciatus (Gentile et al, 2009). Therefore, we anticipate that the clock gene oscillations we observed are circadian in nature and would persist in constant darkness.…”

“…A complete reversal of clock gene expression has not been documented, and we are uncertain why this occurred. However, Gentile et al (Gentile et al, 2009) observed that while cry2 expression peaked at lights off in C. quinquefasciatus, cry2 expression peaked at both lights on and lights off in A. aegypti. This dual peak in cry2 expression in A. aegypti likely explains their crepuscular circadian activity; these mosquitoes are active at both dawn and dusk while C. quinquefasciatus are active primarily at dusk.…”

“…Yuan et al (Yuan et al, 2007) demonstrated that CRY2 proteins isolated from the monarch butterfly, the malaria mosquito, the honey bee and the flour beetle are light insensitive, and instead repress CLK:CYC-mediated transcription of per in Drosophila Schneider 2 cells. Since its initial discovery, cry2 has been reported in numerous insects (except the higher Diptera), and its transcripts consistently cycle and reach peak expression early in scotophase (Yuan et al, 2007;Gentile et al, 2009). It is currently unknown whether CRY2 interacts with PER and TIM proteins in vivo in any insect, but it is likely that CRY2 serves as a more potent negative regulator of the circadian clock than either PER or TIM (Zhu et al, 2008).…”

“…In the mosquitoes Culex quinquefasciatus and Aedes aegypti, Gentile et al (Gentile et al, 2009) reported a peak of cyc expression early in photophase and the expected peak of per and tim at the onset of the of scotophase, but transcripts encoding Clk and cry1 remained unchanged throughout the day-night cycle.…”