Zoophilic and anthropophilic behaviour in the Anopheles gambiae complex

Abstract

The work included in this thesis arose from a long-standing interest in the possible use of genetic control for one of the most important malaria vectors in Africa, Anopheles gambiae sensu stricto. It was hoped that the genes involved in animal biting in the sibling species Anopheles quadriannulatus could be exploited by crossing with An. gambiae s. s. and using the fertile female hybrids for successive backcrosses to An. gambiae s. s.. It was envisaged that this method could be used in an attempt to produce a mosquito stock that is harmless to humans because it is zoophilic but with the genetic background of An. gambiae s. s. so that there would be no barriers to cross-mating in the wild. The innate host preference of mosquitoes can be determined in the laboratory using an olfactometer, or in the field using baits. Host-selection patterns in nature are often affected by external factors, which may or may not mask the host preference of the mosquito species in question (discussed in section 1.3). The main aim of this work was to investigate the host preferences of the two most behaviourally diverse members of the Anopheles gambiae complex (An. gambiae s. s. and An. quadriannulatus) and to attempt to modify the anthropophilic host preference of An. gambiae s. s. by hybridisation experiments with the zoophilic An. quadriannulatus. The behaviour of these two sibling species was assessed in an olfactometer by testing a range of odours. Anopheles gambiae s. s. behaved as expected and was attracted to human odours. However, the behaviour of An. quadriannulatus was somewhat unexpected and a large proportion also selected human odour. Therefore an outdoor host-choice experiment was performed and confirmed the olfactometer findings. A standardised bioassay was developed to evaluate the behaviour of mosquitoes resulting from the crossing experiments. Human and cow skin washings were prepared and tested. Cow skin washing combined with carbon dioxide and tested against a control of clean air was then chosen to evaluate the behaviour of hybrids and backcrossed mosquitoes in the olfactometer. Three backcrosses to both An. gambiae s. s. and An. quadriannulatus were performed, which showed extreme stability of the anthropophilic trait. A field study in Ethiopia investigated the host preference of An. quadriannulatus species B and suggested that this species is more zoophilic than its sibling species An. arabiensis but is also attracted to humans. The impact of these findings on our current understanding of the behaviour of members of the Anopheles gambiae complex is discussed.