Tick Tock: The Role of Circadian Clocks in Reproductive Shut Down for Northern House Mosquitoes

Date Published: February 1, 2015

Source: The Journal of Experimental Biology

Summary: A study was conducted to investigate how the effects of day length on the Culex pipiens' circadian clock components trigger their diapause.

The Northern house mosquito, Culex pipiens, faces a reproductive shut down during the winter time. This "reproductive shut down" that occurs is also known as diapause and it occurs when the amount of time the day lasts reaches a certain length – which in the case of C. pipiens occurs during the winter time. And after shutting down during the winter, C. pipiens resume their reproductive activity in the spring.

The cause of diapause has been researched in the past in numerous other species. This previous research has proved that circadian clocks, an internal clock that is in rhythm with daylight and darkness of the night, regulates body function within many organisms. In C. pipiens however, no definitive explanation to prove what particular mechanism helps these mosquitoes in measuring the day length and how that data triggers diapause is unknown. Megan Meuti from The Ohio State University and her colleagues: Mary Stone, Tomoko Ikeno, and David Denlinger studied C. pipiens to prove how their circadian clocks are affected by day length. Along with proving the role of circadian clocks, the team noted that C. pipiens are also carriers for deadly diseases. And with the discovery of what mechanisms trigger diapause, the team hopes to prevent disease transmission by controlling the mosquitoes' reproductive activity. 

To prove their hypothesis, the researchers broke down the internal clock into five parts: clock, cycle, period, timeless, and cryptochrome2. They analyzed that each of the levels fluctuate in response to daylight. In the case of the C. pipiens, researchers aimed to discover if during the time of diapause, these parts of the clock continued to still run. By using adult mosquitoes (each of which were exposed to different times and length of daylight) the team discovered that the mRNA levels, even during winter, were uninterrupted. This proved to the team of researchers that without the clock, the insects would never know when and when not to be in diapause. This proves that the clock serves as a mechanism to help measure the length of day and night. The researchers also made note of the cryptochrome, which after diapause began to hit peaks at night time. Thus proving when the mosquitoes were active the most, which was during daytime.

After attesting the influence circadian clocks have on diapause, the team decided to concentrate on the five components mentioned previously. To better understand each of their particular roles, researchers wanted to know how reduced mRNA levels would affect the adult female mosquitoes in their ability to reproduce. In order to do this, an RNA molecule designed to stop mRNA from its continuous cycle and prohibit transcription of particular parts of the circadian clock, was injected. The researchers then observed how females exposed to short daylight failed to regulate diapause when the mRNA levels of the last three regulators of the circadian clock were reduced. However, when another part of the circadian clock not mentioned previously, pigment dispersing factor, was also turned off, it initiated diapause when in fact they should have been fertile. This further substantiated that components of the circadian clock do play a vital role in diapause.

After completing their experiment, the researchers concluded how the results paint a bigger picture in the world of ecology than initially anticipated. Aside from the reproductive aspects of C. pipiens, these mosquitoes, as mentioned previously, are in fact a major carrier for diseases like: the West Nile virus, St. Louis encephalitis, and filariasis. With new knowledge on the components of the internal clock needed to be inactivated in order to initiate and halt diapause, researchers hope to prohibit the spread of these diseases in order to create a safe ecological environment. 

Citation:

Meuti, M. E., Stone, M., Ikeno, T. and Denlinger, D. L. 2015. Functional circadian clock genes are essential for the overwintering diapause of the Northern house mosquito, Culex pipiens. The Journal of Experimental Biology 218: 412-422.