Dehydration tolerance in terrestrial arthropods is defined by two factors: an individual’s ability to maintain water balance and capability to respond to the stress generated during fluctuations in the water content. To maintain water balance, an arthropod has to balance water loss and water gain. This is accomplished by reducing water lost through cuticular and respiratory route, improving water re-absorption by the alimentary canal or by increasing water uptake by drinking or absorbing water vapor. Water stress is alleviated by increasing the internal concentrations of protective sugars and polyols and up-regulating stress-related proteins that repair damaged proteins, reduce oxidative stress and maintain cellular integrity. In this thesis, select underlying molecular and physiological changes during dehydration in blood feeding arthropods were examined.
Fully hydrated Aedes aegypti, Anopheles gambiae and Culex pipiens females contained nearly the same amount of water (66-68%), but water loss rates differed among the species, with A. aegypti having the lowest water loss rate (2.6%/h), followed by C. pipiens (3.3%/h), and A. gambiae (5.1%/h). In all three species water could be replaced only by drinking water (or blood). Diapause in C. pipiens improved the ability of females to resist dehydration. Multiple dehydration bouts reduce the nutritional reserves of mosquitoes, likely due to the cost of responding to dehydration stress, leading to reduced survival and reduced egg production. Dehydration elicited expression of hsp70, and hsp90 was constitutively expressed in A. gambiae, A. aegypti, and C. pipiens. Injection of dsRNA to knock down expression of hsp70 and hsp90 in A. aegypti did not alter water content or water loss rates, but the dehydration tolerance was lower. Instead of surviving a 36% water loss, females were able to survive only a 28% water loss in response to RNAi directed against hsp70 and a 26% water loss when RNAi was directed against hsp90.
The bed bug, Cimex lectularius, and the seabird tick, Ixodes uriae, are much more resistant to dehydration than mosquitoes. Both arthropods have low water loss rates and they further reduce water loss by forming aggregations. Bed bugs are incapable of absorbing water vapor from the air and rely solely on blood for liquid water. In contrast, the seabird tick absorbs water from the atmosphere but cannot drink free water. Bed bug water loss rates increase in response to alarm pheromone components, (E)-2-hexenal and (E)-2-octenal, presumably due to the increase in bed bug activity elicited by the alarm pheromones. When these chemical were applied in combination with insecticidal desiccant dust, the effectiveness of this control method increased by nearly 50%.
Overall, these experiments define the water balance characteristics of mosquitoes, the common bed bug, and seabird ticks. Establishing the water balance profiles of these arthropod vectors is a critical aspect for determining their possible distribution and impact on public health.