Abstract
BackgroundSurvival to dry season conditions of sub-Saharan savannahs is a major challenge for insects inhabiting such environments, especially regarding the desiccation threat they are exposed to. While extensive literature about insect seasonality has revealed morphologic, metabolic and physiological changes in many species, only a few studies have explored the responses following exposure to the stressful dry season conditions in major malaria vectors. Here, we explored morphological changes triggered by exposure to dry season conditions in An. gambiae s.l. mosquitoes by comparing females reared in climatic chambers reflecting environmental conditions found in mosquito habitats during the rainy and dry seasons in a savannah area of Burkina Faso (West Africa).ResultsUsing scanning electron microscopy (SEM) and confocal imaging, we revealed significant changes in morphological features of the spiracles in females An. gambiae s.l. exposed to contrasted environmental conditions. Hence, the hairs surrounding the spiracles were thicker in the three species when raised under dry season environmental conditions. The thicker hairs were in some cases totally obstructing spiracular openings. Specific staining provided evidence against contamination by external microorganisms such as bacteria and fungi. However, only further analysis would unequivocally rule out the hypothesis of experimental artifact.ConclusionMorphological changes in spiracular features probably help to limit body water loss during desiccating conditions, therefore contributing to insect survival. Differences between species within the An. gambiae complex might therefore reflect different survival strategies used by these species to overcome the detrimental dry season conditions in the wild.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-015-1289-0) contains supplementary material, which is available to authorized users.
Highlights
Survival to dry season conditions of sub-Saharan savannahs is a major challenge for insects inhabiting such environments, especially regarding the desiccation threat they are exposed to
Environmental conditions, mosquito rearing and sample collection Mosquitoes were reared from the egg to the adult stage in programmable climatic chambers (Sanyo MLR 315H, Japan), where the climatic parameters characterizing the rainy and the dry season conditions were defined from hourly averaged records collected in Bama during August 2010 (Additional file 1: Figure S1A) and December 2010 (Additional file 1: Figure S1C) using a Vantage Pro2 monitoring station (Weatherlink; Davis Instruments, Hayward, CA, U.S.A.) As previously described [18, 34], twelve steps cycles were designed to reproduce as close as possible the natural climatic variations monitored in the fields
Our observations showed that in 100 % of the females observed under the scanning electron microscopy (SEM), the mesothoracic spiracles are wide open in females reared in rainy season (RS) conditions whereas the hairs appear wider and thicker in females reared under dry season (DS) conditions, plugging entirely the spiracular aperture in An. coluzzii and An. arabiensis (Figs. 1 and 2)
Summary
Survival to dry season conditions of sub-Saharan savannahs is a major challenge for insects inhabiting such environments, especially regarding the desiccation threat they are exposed to. While extensive literature about insect seasonality has revealed morphologic, metabolic and physiological changes in many species, only a few studies have explored the responses following exposure to the stressful dry season conditions in major malaria vectors. The two molecular forms of An. gambiae s.s., recently named An. coluzzii (former M molecular form) and An. gambiae (former S molecular form) [11] and An. arabiensis are members of the Anopheles gambiae s.l. complex, a group of closely related and morphologically indistinguishable species [12]. Their distribution ranges encompass broad environmental and ecological settings, including arid and semi-arid areas. Migration to/from more favourable localities where reproduction continues year-round might be involved [19]
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