GRAY, E.M.; BRADLEY, T.J.; Univ. of California, Irvine; Univ. of California, Irvine: Water Balance in two Tropical Mosquitoes, Anopheles gambiae and An. arabiensis

Anopheles arabiensis and its sister species An. gambiae occupy distinct ranges in the African Sub-Saharan region: the former is found in more arid habitats than the latter. Small insects face a high risk of desiccation in dry habitats; yet various physiological adaptations exist to reduce or eliminate this stress. Among members of the Anopheles gambiae complex we think that specific physiological adaptations are allowing range expansion of these major malaria vectors in Africa. We have initiated a study to elucidate the physiological traits related to aridity tolerance in these mosquitoes. When the two species are reared from egg to adult under identical conditions, An. arabiensis have a significantly higher desiccation resistance than An. gambiae (p<0.0001). Our initial experiments indicate that An. arabiensis have a higher body water content than An. gambiae before desiccation (p=0.003) but the same body water at death. Their average water loss rates during desiccation do not differ. We measured energy stores and metabolic rate of females of both species between emergence and 10 days of age. These results and the possible implications for desiccation resistance of diurnal activity patterns, fuel an water storage, and fuel utilization during desiccation will be discussed. Given that these two species were reared under common garden conditions we believe that the differences we found are genetic in origin. Evidence accumulated over the past 20 years suggests that some chromosomal inversions fixed in An. arabiensis but polymorphic in An. gambiae might be tied to arid adaptation. This study provides the first physiological evidence of differential resistance to aridity in these species and some clues as to the mechanisms involved.