Persistence in continuous light of a circadian rhythm in the mosquito Culex pipiens fatigans Wied.

Abstract

BRADY¹ has discussed Truman's² hypothesis that circadian clocks can be divided into two types: type 1 in which light seems to act directly on the oscillator, stopping it in constant light (LL); and type 2, in which the photoreceptor and oscillator are separate and the rhythm is free-running in LL. Typically, in insects, developmental events such as eclosion seem to be controlled by the first type, and daily behavioural activities by the second². In Diptera, however, behavioural rhythms seem to be rapidly damped out by LL¹; for example in the mosquito Anopheles gambiae, the ‘clock’ controlling flight activity seems to stop when the light phase is prolonged, and to restart at the next onset of darkness³, thus showing type 1 characteristics. Previously, the only known exception among the Diptera was Aedes aegypti, a light-active mosquito, in which the flight activity rhythm persists, rather weakly, in LL⁴. Here I show that circadian flight activity in the mosquito Culex pipiens fatigans Wied. persists in LL, although in other respects the rhythm is very similar to that in A. gambiae. As in A. gambiae³ the period of the first cycle in constant darkness (DD) is distorted by the previous light period, but, in contrast, the magnitude of the effect depends on a rhythm that evidently persists in LL.