Biological Clocks in Mosquitoes - Section 2
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Why has such a model not been developed before? Perhaps the simplest answer is that the majority of investigators have used organisms that are either strongly light-active or strongly dark-active, and display little or no activity during the negative photophase. Thus, LLt is often unobservable, although total inactivity in DD is less common and DDt usually can be determined. Additionally, most of the organisms display what appears, or at least is assumed, to be unimodal activity. An unfortunate truth is that the selection of organisms seems to be a consequence of funding, that is the research has to have overt economic or humanitarian "importance", hence the emphasis, for instance, on tsetse flies (Brady, 1988a, b), or Drosophila (now a major component of genetic research and originally of some importance as an agricultural pest, Saunders et al., 1994).
In the latter sense it was fortunate that Aedes aegypti has pest status. What was even more fruitful is that the species has a clear bimodal activity pattern (in median LD cycles) and that, although essentially a light-active species, not only is there activity in DD but also the activity remains bimodal. The use of more extreme LD cycles (for example LD 20:4 and LD 4:20) showed that each of the two normal peaks could be further separated into two components, and this separation continued in DD and LL (see Figure 31 and Figure 34). It should be borne in mind that the activity may well be only a manifestation of the clocks. Thus, the form of any one peak (or related series of peaks) may reflect a combination of stimulation by the related clock and the threshold for activity. For instance, in Figure 34, the peaks are sharper than the sinewaves, and it appears also that the ON ANTISINE clock has the least effect on the intensity of activity. This example serves also to undermine the oft-used interpretation of "unexpected" peaks as being due to the transient effects of phase-setting following changes in the experimental LD regime. Activity in Figure 34a is of mosquitoes reared and observed in LD 4:20 and thus there were no transients. The activity pattern, moreover, is very similar to that found when mosquitoes were reared and initially recorded in LD 12:12 before a change to LD 4:20, either by delaying light-on (as in Figure 34b) or by advancing light-off (as in Figure 34c).
The concept of Lt being specific to the species-origin can be seen as a development of the earlier concept of how the phase-setting effects of light-on and light-off were in balance in appropriate LD regimes (Taylor, 1969b). In addition to the Ae. aegypti evidence, where LLt = 26h and DDt = 22.5h, supporting evidence has come from Chiba & Tomioka (1992). They found the more northern mosquito species, Cx. p. molestus, has an LLt of around 30-31h (although this t is my reinterpretation of their observation of a period of some 15h between the persistent peaks of a bimodal activity pattern, see Figure 36).
In terms of geographic origin, the significance remains the same but the potential for a multi-clock system as permitting time-measurement in photoperiodism is given an added mechanism (see below). The limited evidence is that Lt appears never to exceed about 30-31h. For Arctic species (see Figure 39 and Figure 40), however, this has a positive benefit in that the enforced splitting of peaks in long L cycles provides a framework for activity around noon (the warmest time of day). The variation in Lt, moreover, may be more than species-specific, as latitudinal races are well known to photoperiodism researchers.
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©1998, 2010 - Brian Taylor CBiol FSB FRES 11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K. Comments to dr.b.taylor@ntlworld.com |