Contents

Biological Clocks in Mosquitoes - Section 2
The underlying mechanisms - how many clocks and why are they circadian?

Evidence from studies of dark-active mosquitoes

The concept developed and described above is of a clock mechanism composed of oscillations in the form of simple sine waves. When the sine waves fall across a point of transition from high light to low light, the actual activity is manifested as asymmetric peaks. The asymmetry is suggested to be the result of the differential action of above- and below-threshold light intensity. Thus, in so-called day-active species the observed level of activity drops when light is below the threshold. Pragmatism suggests that above-threshold light actually promotes activity, particularly as it is quite common for day-active species to display activity in DD (albeit at a low level). It seems logical, therefore, to propose that so-called dark-active species simply respond in an obverse manner, with the observed level of activity being higher when light is below the threshold. For such species the effect of above-threshold light can be suggested as a simple inhibition of activity, and the available evidence is that such inhibition is so strong that activity may appear to cease in constant above-threshold light. For the dark-active species, the four sine waves are plotted with the amplitude changing from an arbitrary maximum of 6.0 in D to 3.0 in L.
(a) Late crepuscular (very low light) mosquitoes

Anopheles atroparvus, Anopheles farauti, Anopheles stephensi and Culex pipiens pipiens were studied, with summer and winter populations of the latter.

An. atroparvus is a European species found between latitudes 30°N and 60°N.

Figure 42 shows the histograms of activity in a range of LD regimes, with the four sine waves plotted with Dt = 22.5h and Lt = 30h. In all the LD regimes, the E activity underlain by the OFF SINE is strong, but the effect of the ON SINE seems to contribute to the lower level activity following light-off. The M peak seems to be determined principally by the ON ANTISINE, with little involvement of the OFF ANTISINE.

Figure 42
Anopheles atroparvus

An. farauti is a truly tropical species, from Australasia and found only between latitudes 0° and 17°30'S.

Figure 43 shows the histograms of activity in a range of LD regimes, with the four sine waves plotted with Dt = 22.5h and Lt = 24.5h. Examining the five LD regimes shows that all four clocks could be having a controlling effect.

Figure 43
Anopheles farauti

An. stephensi is a sub-tropical species, primarily from South Asia and found between latitudes 10°N and 35°N.

Figure 44 shows the histograms of activity in a range of LD regimes, with the four sine waves plotted with Dt = 22.5h and Lt = 26h. The two clocks waves appearing to exert the most influence are the OFF SINE, underlying a large E peak in all five LD regimes, and the ON ANTISINE. Especially in LD 20:4 (see Figure 44e), however, the other two clocks may be leading to the low upsurges in activity. In LD 4:20 the effect of ON SINE is shown by low level activity in D at 8-12h.

Figure 44
Anopheles stephensi

Cx.p.pipiens. When taken as a single entity, this species has quite a wide geographical range, being found between latitudes 30°N and 63°N. The specimens used in the experiments were all from southern England (latitude about 52°N) but comprised two sets of individuals, one obtained by rearing early summer larvae and the second of adults taken from hibernation.

First the histograms of the activity of mosquitoes of the summer generation (normal LD 16:8) are shown in Figure 45, with sine waves with Lt = 30h and Dt = 22.5h overlaid. Both OFF SINE and ON SINE have clear effects. OFF SINE gives rise to the sharp E immediately following light-off and the effect of ON SINE is seen in the wider activity shifting from after light-off in LD 4:20 to before light-off in LD 20:4.

Figure 45
Culex pipiens pipiens

The results of the winter generation, which normally would become active when the natural LD was about 12:12, are shown in Figure 46, with the sine wave Lt = 24.5h. The effect of the clocks is similar to that shown in the Summer generation, with separation of peaks due to the four clocks shown across the spectrum of LD regimes.

Figure 46
Culex pipiens pipiens

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©1998, 2010 - Brian Taylor CBiol FSB FRES
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