Biological Clocks in Mosquitoes - Section 2
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For the present, at least, and for the following reasons, the new circadian system should be taken as applicable only to insects. When the mosquito experiments were carried out (Taylor, 1969a) the accepted theories for circadian mechanisms were all-embracing, at least for animals, and the finding that for Ae. aegypti t was longer in LL than DD was contrary to the then contemporary "Aschoff's Rule" (Aschoff, 1960; Hoffman, 1965). Later, Aschoff adopted the generalisation that in arthropods t in LL is greater than in DD, and that t in LL > 24h (Aschoff, 1979). From work on illumination thresholds for commencement of activity in nocturnal moths (see below), Dreisig (1980) also concluded that insects differ from birds and mammals, which show a gradual change in their reaction to illumination. A key factor may be that, unlike birds and mammals, there are few species of temperate region insects (or other terrestrial arthropods) which do not cease activity during the cold winter months and, therefore, adjustment of activity timing to accommodate days with L < 10-12h is superfluous.
Orthodox theories regarding entrainment and the mechanisms underlying the observed patterns of daily activity almost all rely on evidence drawn from experiments that appear to show a single change in light intensity, most commonly light-off, as being the zeitgeber. Pittendrigh (1993) wrote how -
"It was clear from the outset (ca. 1956) that to lock on to the daily cycle of light and dark, the oscillator driving the rhythm must be differentially responsive to light at different phases of its cycle".It was this which led to the widely repeated form of experiments in which single light pulses are given in otherwise constant darkness. Similarly he stated the belief that day-active species track dawn and night-active species track dusk. His overall thinking seemed to regard the only real difference in activity patterns as being whether organisms are day-active or night-active. The bimodalism which is common in insects was put down to the influence of environmental factors, such as the adverse effect of high temperatures and low humidity on most insects, and a consequent avoidance behaviour giving resting in the middle of the day. It appears that he disregarded the fact that some species display bimodalism under constant conditions. A feature of the insect research led by Pittendrigh was the use of once-only phenomena, such as the eclosion of Drosophila pseudoobscura, as the baseline for interpretations of complex and otherwise repetitive behaviour patterns (Pittendrigh, 1966). This has clear dangers and, indeed, Smith (1985) considered that several pieces of evidence, including that from the blowfly, Lucilia cuprina, of different free-running periods in flight activity and eclosion, could indicate different clock mechanisms.
For mosquitoes, perhaps the most widely cited of theoretical approaches is the "limit cycle" hypothesis of Peterson (1980a, b, c), which was based on observations of Cx. p. quinquefasciatus, and was drawn on heavily by Winfree (1986) in his extensive review of circadian rhythm theory. However, Peterson took the interval of "dusk" flights as "phase zero" because the dusk peaks were more crisply defined, he assumed that the E peaks were genuinely sharp and he took only light-off as a zeitgeber.
Friesen et al. (1993) clarified the term "limit cycle oscillators" by writing - "Biological oscillators" ....."generate both transient and protracted oscillations; they are all nonlinear. In most instances, perturbations transiently alter both cycle period and amplitude; given sufficient time, the cycle period and activity wave form return to the pre-perturbed values, providing that system parameters remain unchanged. Such systems are described as limit cycle oscillators, wherein the unperturbed activity is stable for all time, and where small perturbations vanish by exponential decay".
The majority of authors have selected a single peak per circadian
period and sought to interpret the underlying clock mechanism on the
basis of the timing of that peak. This can be illustrated by reference
to Friesen et al. (1993), who, in discussing formal approaches
to understanding biological oscillators wrote - "It is becoming
increasingly evident that few behaviours escape some form of modulation
by biological clocks". They also provided some useful, comprehensible,
definitions.
The primary observable is the "cycle period"; defined as the repetition
of some arbitrary reference point, this "phase reference point"
should be chosen on the basis of convenience.
The term "phase" simply designates subsequent points on the cycle with
reference to some reference point. Another use of this term is to
designate the normalised interval between the activity in two system
variables.
The term "phase angle" designates a fraction of the cycle period as °
or normalised circadian hours; for example, phase difference between
the zeitgeber (external timing cue) and an indicator variable
(360° = 24h). As an example, in their examination of the
locomotor activity of honeybees in a wide range of LD cycles, Moore
& Rankin (1993) wrote of seeking to determine how "the phase of the
rhythm" could be accomplished by the actions of discrete signals, such
as lights-on or light-off events, or single pulses of light at these
transitions points. Citing DeCoursey (1972), they seem to have started
with an assumption that little or no phase change should occur for a
wide variety of LD ratios and that the phase of the rhythm would
maintain either a fixed position, with respect to lights-on or
lights-off, or a position indicating a proportional effect between the
two signals. Presumably because of that starting point, they simply
expressed themselves unable to proffer any explanation when the
entrainment pattern in LD 22:2 did not match their assumptions.
In contrast, the concept developed in this paper is founded on accepting the highest point of any upsurge in activity as being a genuine peak. In many instances, especially when the presence or absence of light is inhibitory (i.e. for light-active species in D; and, even more pronounced, for dark-active species in L) this demands acknowledgement that the upsurge may be observable only as a low level of activity. Consequently, the interpretation of links between the postulated sine waves and the observed activity depends largely upon a visual check of whether activity is random or predictable. Arguably, it depends also on an acceptance that the inherent variability of results obtained from living organisms may add an element of inexactitude.
How this is important will be examined later by reference to the very recent paper by Watari & Arai (1997). They studied the day-active Onion Fly, Delia antiqua, in ten LD regimes from LD 2:22 to LD 20:4. In LD 12:12 the activity resembled that of Ae.aegypti, An. plumbeus and Cx.p.molestus in being bimodal with M and E' peaks, the latter coming some 8 h after light-on, plus an E response to light-off. They used fA where I have adopted E'. In their commentary the pattern of multiple peaks was noted but low levels of activity during the scotophase were given no attention.
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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 |