Contents

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

A new circadian system

The foregoing interpretation of activity data can be summarised as a new unified circadian system. This, as it operates in LD 12:12, is shown visually in Figure 50 (a-h), and has the following main characteristics -

1. The basic mechanism is made up of four clocks, or oscillators. Each clock exhibits a simple sinewave form of transition from minimal stimulation of activity to maximal stimulation, with a period, frequency or circadian rhythm t @ 24h (Figures 50a & 50b).

2. All the clocks run at one of two speeds; a high-light speed (Lt) and a low-light speed (Dt). The transition in speed appears to be determined by the same threshold as affects the manifestation of activity (see below). One consequence of this, even in median LD cycles, is that the two pairs of clocks are not usually in full coincidence (Figure 50a).

Figure 50a
Full Model A
Figure 50b
Full Model B

3. The observed rises and falls in activity measured in behavioural experiments reflect the underlying sinewaves but are not necessarily direct manifestations of single clocks (for example, the activity often does not exhibit a smooth, roundly curved, pattern).

4. The clocks are reset by either dawn or dusk (upward or downward transitions of overall light intensity). The clocks reset by dusk are those termed OFF SINE and OFF ANTISINE, and those reset by dawn are termed ON SINE and ON ANTISINE. In stable LD regimes each resetting appears to be effected by a shift to the apogee (SINE) or the perigee (ANTISINE). In median LD cycles, the maxima of the SINE clocks fall around dusk and those of the ANTISINE clocks fall around dawn and, so, their separate effects are not readily discerned. In more extreme LD cycles, however, the effects can be separately observed with some ease (see below).

5. In most, if not all, organisms light and dark have differential effects on the amount of activity that can be manifested at any point of the sine wave. In its simplest form, this separates organisms as being light-active (Figure 50c) or dark-active (Figure 50h) and it is common for there to be a total cessation of activity in the unfavourable light condition.
Figure 50c
Full Model C
Figure 50h
Full Model H

6. Activity in the favourable light condition, however, is not necessarily manifest as uniform throughout the period, and complete uniformity is uncommon. For many organisms, activity is concentrated at the beginning and end of the day. In insects, at least, actual activity appears to be related to a threshold level of illumination and the sine waves are manifest only for part of the upper sector of the wave. This is illustrated by the sequence of crepuscular patterns (Figures 50 d-g).
Figure 50d
Full Model D
Figure 50e
Full Model E
Figure 50f
Full Model F
Figure 50g
Full Model G
7. In insects, it seems common for Dt @ 22.5h. In contrast, L varies from species to species and may be related to the natural origin of the species, with t, at least in summer generations, matching the daylength at the summer solstice.

8.The separation of clocks in extreme LD regimes is illustrated in the examples suggested for light-active and dark-active organisms in LD 20:4 (Figures 50 i-j) and LD 4:20 (Figures l-k).

Figure 50i
Full Model I
Figure 50j
Full Model J
Figure 50k
Full Moel k
Figure 50l
Full Model L

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©1998, 2013 - Brian Taylor CBiol FSB FRES
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.
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