For some species, surviving winter is too much of an ordeal to keep everything ticking over. So, as soon as the days reach a certain length, adult Northern house mosquitoes (Culex pipiens) prepare for their seasonal reproductive winter shut down (diapause), ready to re-commence reproduction in the spring. ‘Much is known about the photoperiodic cues, endocrine signals and physiological changes that regulate and accompany diapause’, says Megan Meuti from The Ohio State University, USA. However, little was known about the mechanism that the insects use to measure day length or how they convert the information into the hormone signals that orchestrate the insects' diapause. Although many insects are known to regulate body functions using a light-regulated circadian clock, it was not clear whether this mechanism triggered preparation for diapause in C. pipiens, so Meuti and her colleagues, Mary Stone, Tomoko Ikeno and David Denlinger, began investing the effects of day length on key components of the mosquito's circadian clock (p. 412).
The team explain that the expression levels of the five components of the molecular clock (Clock, cycle, period, timeless and cryptochrome2) rise and fall in relation to the length of daylight exposure over a 24 h period, so they decided to find out whether the clock continued running during diapause. Measuring expression of period, timeless, cycle and cryptochrome2 in the brains of adult female mosquitoes that had been exposed to daylight periods of different length to stimulate the onset and duration of diapause, the team discovered that the mRNA levels continued cycling – even when the insects were plunged into the depths of winter. Meuti explains that the clock has to function throughout the seasons to ensure that the insect knows when to emerge from diapause and says, ‘Our observations… 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’. The team were a little perplexed when they discovered that expression of cryptochrome became reversed (peaking at night) after emergence from diapause; however, they suspect that this reflects that the insects are most active in the early morning.
Having confirmed that the insect's circadian clock plays a role in regulating diapause, the team began investigating the individual role of each of the clock's components. Injecting an RNA molecule designed to lock up the mRNA and prevent transcription of one individual component of the clock into young female adult mosquitoes and then monitoring the effect of the component loss on their fatness and fertility, the team found that females that were experiencing short daylight exposures – which should have induced diapause – failed to enter the condition when period, timeless and cryptochrome mRNA levels were reduced. However, when the team inactivated another component of the molecular clock – pigment dispersing factor – under long daylight conditions, the loss triggered diapause in insects that should have been able to reproduce.
‘Our results implicate the circadian clock in the initiation of diapause in C. pipiens’, says Meuti, who adds that she is keen to understand the complete diapause pathway, from the measurement of day length to the physiological mechanisms that allow the mosquitoes to survive the winter. Explaining that C. pipiens is a carrier of major diseases such as West Nile virus, St Louis encephalitis and filariasis, Meuti is hoping to use this knowledge against the insect to control disease transmission – by either averting diapause in winter or initiating diapause in other seasons.