Daily Rhythms in Mosquitoes and Their Consequences for Malaria Transmission

Abstract

The 24-h day involves cycles in environmental factors that impact organismal fitness. This is thought to select for organisms to regulate their temporal biology accordingly, through circadian and diel rhythms. In addition to rhythms in abiotic factors (such as light and temperature), biotic factors, including ecological interactions, also follow daily cycles. How daily rhythms shape, and are shaped by, interactions between organisms is poorly understood. Here, we review an emerging area, namely the causes and consequences of daily rhythms in the interactions between vectors, their hosts and the parasites they transmit. We focus on mosquitoes, malaria parasites and vertebrate hosts, because this system offers the opportunity to integrate from genetic and molecular mechanisms to population dynamics and because disrupting rhythms offers a novel avenue for disease control.

Keywords: Anopheles; Plasmodium; chronobiology; circadian; diel; diurnal; nocturnal.

Figure 1

Some of the environmental rhythms a mosquito may be exposed to across the 24-h day. These include physical environmental changes, such as ambient light, UV radiation, temperature and humidity. Mosquitoes also experience the biological rhythms of their hosts, such as when humans are inside or outside of their dwelling, under or not under a bed net and awake or resting (only alert hosts display defensive behavior); and rhythms in predators, such as dragonflies (diurnal) or bats (nocturnal). Parasites may also have rhythms in the activities they undertake in mosquitoes that impact mosquito fitness.

Figure 2

Flight activity rhythm of a single mated An. stephensi female mosquito continuously monitored for eleven days in a 12:12 light:dark cycle with 1 h-long dawn and dusk transitions, followed by a further eight days in constant dark conditions. Note, under the entraining conditions of a light:dark cycle, flight activity begins at approximately the same time each day. Under constant dark:dark conditions (where there are no light cues to entrain the clock), flight activity maintains a rhythm driven by its non-entrained, free running circadian clock. In the An. stephensi mosquito, the non-entrained clock free runs at a period longer than 24 h as evidenced by flight activity beginning slightly later each day. Recording was performed using a locomotor activity monitor (L.A.M.) from TriKinetics, Inc. (Waltham, MA, USA) at 26 °C and 60% relative humidity. See Rund et al. 2012 for the methods [67].