The geographic range of Anopheles gambiae sensu stricto, the major malaria vector in sub-Saharan Africa, includes semi-arid regions characterized by strong seasonal variations in temperature, relative humidity and rainfall [1, 2]. In West Africa, seasonality has been associated with changes in the relative abundance of certain chromosomal inversion arrangements thereby underlining the role of some chromosomal inversions in conferring resistance to desiccation . The Mopti chromosomal form of An. gambiae is characterized by high frequencies of a and bc inversions which are thought to confer resistance to drought. It also predominates in drier parts of the country and increases in relative abundance during the dry season [4, 5]. In contrast, the chromosomal arrangements of the Bamako and Savannah forms lack this combination of inversions. These forms are better adapted to the wetter parts of Mali and their relative abundance and geographical distribution increases in the rainy season [4, 5]. Thus drought and the selection pressures associated with hydric stress (stress induced by limited water availability) are important for the evolution and maintenance of genetic polymorphism in An. gambiae's populations and for determining the respective distribution and abundance of chromosomal forms [4, 6].
In addition to seasonal droughts, mosquito populations in the Sahelian belt are subjected to strong daily fluctuations in temperature and relative humidity. These persistent environmental fluctuations make adaptations for coping with hydric stress and maintaining an adequate body water balance, some of the most important aspects of mosquito behaviour and physiology . In contrast to an extensive body of literature focusing on those aspects in Drosophila melanogaster [8–16], only very few studies have experimentally studied resistance to desiccation in adult An. gambiae sensu lato. Using laboratory maintained colonies, Gray and Bradley  showed that Anopheles arabiensis was more desiccation resistant than its sister species An. gambiae s.s. In Mali, the Mopti chromosomal form is characterized by the M-type of rDNA and the Savanna or Bamako chromosomal forms by the S type . Using offspring from wild-caught blood-fed females, Lee et al  recently demonstrated, that desiccation resistance was higher in individuals of the M molecular form than in those of the S molecular form, thereby corroborating the correlation found between desiccation resistance and the spatial and temporal distribution of chromosomal forms in that region. Finally, a recent study showed using a heterogeneous laboratory colony of the Forest chromosomal form from Cameroon which was split and selected either for the a inversion or the standard arrangement showed that young adults homozygote for a better resisted desiccation than those without the inverted arrangement .
In natural mosquito populations, genetic polymorphism is probably one of many factors that can possibly affect the desiccation resistance of an individual mosquito. The impact of hydric stress on an individual is also likely to depend on factors affecting its phenotypic quality - i.e. its morphological, developmental, behavioural, biochemical and physiological properties. For example, in An. gambiae, it has been amply demonstrated that larval growth conditions influence adult body size and condition, which in turn affects other important traits such as immune response to infection and blood meal utilization [19, 20]. Similarly, larval growth conditions through their effect on teneral reserves and generally body size and condition [21, 22, 20], could impact the ability of adult mosquitoes to cope with desiccation . Although a few studies have examined the effects of body size on fitness in mosquitoes [19–23], none of these studies sought to clarify the contribution of physiological reserves in relation to phenotypic quality and desiccation resistance in An. gambiae s.s. Indeed, almost all the studies investigating the mechanisms of resilience to desiccation are based on selection experiments conducted on Drosophila species [8, 10, 15, 16, 24].
There are three recognized physiological mechanisms by which an insect may overcome or cope with desiccation: (i) increasing water storage either in the form of bulk water (water molecules obtainable from sources other than catabolism) or metabolic water (water molecules obtainable directly from catabolism) or both, (ii) regulating water loss through respiratory and trans-cuticular transpiration, and (iii) being tolerant of water loss . In Drosophilids, adaptations such as lower cuticular permeabilities and water loss rates, as well as higher dry and wet mass have all been shown to be associated with desiccation resistance [7, 25–27]. Archer et al  demonstrated that increased bulk water content and lower water loss rate were the most important physiological mechanism for desiccation resistance in Drosophila melanogaster. Gray and Bradley  found that differences in bulk water content rather than water loss rates explained the higher desiccation resilience of An. arabiensis than its sister species An. gambiae s.s. Metabolic water is generated through the catabolism of glycogen and lipids. Glycogen has the ability to bind the equivalent of three to five times its mass of water and to release it during glycogenolysis , making it a significant source of water during desiccation stress [15, 29]. Lipids can proportionally yield the highest amounts of water per unit of mass and may thus also serve as an important source of metabolic water . However, lipids have so far not been associated with desiccation resistance in Drosophila [24, 29, 31].
To date, the physiological mechanisms responsible for desiccation resistance in An. gambiae s.s. remain to be determined. In the present study, the effect of variation in phenotypic quality and water availability on survival of females of the Mopti chromosomal form of An. gambiae s.s. under desiccation stress were experimentally investigated and changes in wet and dry body mass as well as water, glycogen, cuticular lipid and full-body lipid contents recorded. The results emphasize the general importance of glycogen metabolism for desiccation resistance as well as the impact of variation in phenotypic quality and water availability on resistance via their effects on body water and body lipid reserves.