Why does Anopheles Arabiensis predominate over An. Gambiae Sensu Stricto in hot and arid conditions?

Abstract

The mosquitoes Anopheles arabiensis má An. gambiae s. s. are two sibling species that are the most efficient vectors of malaria across Africa. Although they occursympatrically over much of the continent, An. arabiensis predominates in hotter and drier conditions whilst An. gambiae s. s. is more abundant in wetter conditions. This study explored the physiological and behavioural factors responsible for the different spatial and temporal distributions of these species. In the laboratory adult An.arabiensis exhibit a three-fold longer survival than Art. gambiae s.s. at high temperatures and low humidity. Moreover An. arabiensis is tolerant of high temperatures that are actively avoided by An. gambiae S.S. This behavioural response to high temperatures was also observed in adult populations of both species in the field տ The Gambia. The interspecific difference observed in survival rates is not apparent in the 24 hour activity levels. Neither species could sustain periods of activity at temperatures above 30 c. An. arabiensis loses water at a proportionately lower rate at 40 c and 30%RH, and contained a disproportionately greater reserve of accessible water, namely haemolymph, than An. gambiae ร.ร. Although heat-shocked groups of both species expressed higher levels of heat shock proteins when compared to control groups, this did not relate to improved knock-down resistance. There were less obvious differences between the species in the larval stage. In the laboratory An. arabiensis larvae exhibited a greater survival to adulthood at 35 c than An, gambiae ร.ร., when reared separately and together. However, in the field An. arabieหsis did not dominate the hottest breeding sites in The Gambia in the dry season. Larvae of both species dived away from 40 c water surface temperatures, though for short periods only. It is argued that a better tolerance of hot and drying resting conditions in An. Arabiensis may result in longer-term improvements in survival rates compared to An. Gambiae S.S.