Biological Clocks in Mosquitoes
|
This website provides the background information underpinning my paper "How many clocks".
The flight activity of fifteen species of mosquitoes was recorded in the laboratory, using an acoustic actograph technique. The patterns of activity in five or more light-dark (LD) regimes were obtained for eight species:- Aedes (Stegomyia) aegypti (Linnaeus), with two strains each of the subspecies aegypti (LSHTM and Ilobi) and formosus (Bwamba and West Nile); Aedes (Ochlerotatus) punctor (Kirby); Anopheles (Anopheles) atroparvus van Thiel; Anopheles (Cellia) farauti Laveran; Anopheles (Anopheles) plumbeus Stephens; Anopheles (Cellia) stephensi Liston; Culex (Culex) pipiens pipiens Linnaeus, for both summer and winter generations; and, Culex (Culex) pipiens quinquefasciatus Say. The other seven species, which were observed in fewer LD regimes are:- Aedes (Aedes) cinereus (Meigen), Aedes (Ochlerotatus) detritus (Haliday); Aedes (Finlaya) geniculatus (Olivier); Aedes (Ochlerotatus) impiger (Walker); Aedes (Ochlerotatus) nigripes (Zetterstedt); Coquillettidia richiardii (Ficalbi); and, Culex (Culex) torrentium Martini. Between them, the species represent different geographical distributions from equatorial to Arctic and show all the characteristics of day-active, early-crepuscular, late-crepuscular, and night-active patterns. A set of Appendices provides details of the known geographic ranges of each of the species, together with the sources of the populations studied in the experiments, and the raw results of the experiments in the form of mean activity over several consecutive days and nights.
In the main body of the text, Section 1, a double plotting method, the photoperiodogram, is used to present comparative displays of the distribution of percentage activity in each half-hour of every 24 h light-dark (LD) cycle in a range of LD regimes. The photoperiodograms reveal how the apparently precise timing of activity, so often seen in mosquitoes in LD 12:12 regimes (and upon which several interpretations of underlying control mechanism have been based), responds to variation in the LD ratio. The occurrence of activity peaks at unexpected times, often apparently contradicting the inhibitory effect of light or dark, led to the development of the multi-clock concept.
In Section 2, how this concept arose is fully illustrated. The evidence presented in Section 1 and the Appendices, together with some further examples, is drawn on to suggest that the apparent clear and uncomplicated daily pattern of unimodal or bimodal activity is actually the end product of complex interactions between endogenous circadian oscillators, or clocks, and the exogenous effects of the LD cycle. The key to the concept is the recognition that it is oscillators in plural and steps up and down in light intensity that interact, and also that the speed, or circadian period (t), of the oscillations above and below a light intensity threshold is different. Moreover, below the light intensity threshold t generally is around 22.5h, whereas above the threshold t is species-specific (in some instances even being specific at a geographical variety or race level).
In median light:dark (LD) regimes, such as a 12h L:12h D regime (LD 12:12 ), unimodal (single peak) activity can be explained as the product of a single oscillator but it, and, even more so, bimodal activity, is better explained by there being two oscillators. One oscillator is entrained by the daily upwards transition of the light threshold (light-on or dawn) and the second by the daily downwards transition (light-off or dusk). In more extreme LD regimes, however, multiple peaks were recorded and some of these peaks occurred at times that did not match the effect of either oscillator. Not only were these peaks repeated on successive days but they also persisted in constant light or dark following extreme LD regimes. This appears to demand at least two pairs of oscillators. The members of each pair run in a counter state, so that one member gives activity around the time of natural summer dusk and the other around the time of summer dawn. The above- and below-threshold variation in t is important because it appears to be related to the maximum day-length experienced by the particular species (or variety, or race) in its normal geographical range. In terms of daily flight activity, inter-specific variations - such as being dark-active or light-active, or even more precisely timed, such as in crepuscular species - are visualised as resulting from a combination of the underlying promotion of activity by oscillators at or near their maximum and the inhibition or stimulation of activity by the presence or absence of light (the threshold effect). Lastly, the profound implications of a combination of at least four internal clocks and the further effect on the sytem that is due to differential clock speeds in light and dark are discussed. If the concept and the combination are correct then conventional circadian theory is cast in serious doubt.
Thus, he alluded to a clock, used the term oscillation and considered both tension and escapement - all commonplace in the contemporary terminology of circadian rhythms. For this, surely Roubaud deserves belated recognition as a true father of circadian rhythm research.
- 1°; Le rhythme du vol qui, réglé sur la marge des oscillations crépusculaire, creé chez le Moustique un état de tension comparable a l'action du ressort dans un mécanisme d' horlogerie;
- 2°; Le réflexe inhibiteure diurne, que retient le départ jusqu'à l'instant précis de la baisse crépusculaire, jouant le rôle de l'échappement dans le mécanisme d' horlogerie.
|
©1998, 2010 - Brian Taylor CBiol FSB FRES 11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K. Comments to dr.b.taylor@ntlworld.com |