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Aestivation and dispersive migration are the two strategies evoked in the literature to explain the way in which malaria vectors Anopheles coluzzii and A. gambiae survive the harsh climatic conditions of the dry season in sub-Saharan Africa. However, the physiological mechanisms regulating these two strategies are unknown. In the present study, mosquito species were exposed to controlled environmental conditions mimicking the rainy and dry seasons of south western Burkina Faso. Survival strategies were studied through morphometric (wing length), ecophysiological (respiratory gas exchanges), biochemical (cuticular hydrocarbons composition) and molecular (AKH mRNA expression levels) parameters, variations of which are usually considered to be hallmarks of aestivation and dispersion mechanisms in various insects. Our results showed that ecophysiological and morphometric adjustments are made in both species to prevent water losses during the dry season. However, the usual metabolic rate modifications expected as signatures of aestivation and migration were not observed, highlighting specific and original physiological mechanisms sustaining survival in malaria mosquitoes during the dry season. Differences in epicuticular hydrocarbon composition and AKH levels of expression were found between the permanent and temporary A. coluzzii populations, illustrating the great phenotypic plasticity of this mosquito species. Altogether, our work underlines the diverse and complex pattern of changes occurring in the two mosquito species and at the population level to cope with the dry season and highlights potential targets of future control tools., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

Background: In West Africa, populations of the malaria vector mosquito, Anopheles coluzzii, are seasonally exposed to strong desiccating conditions during the dry season. Their dynamics strictly follows the pace of the availability of suitable larval development sites (water collections). Accordingly, mosquitoes can reproduce all year long where permanent breeding is possible, or stop reproduction and virtually disappear at the onset of the dry season when surface water dries up, like observed in temporary habitats of dry savannah areas. This highlights the strong adaptive abilities of this mosquito species, which relies at least in part, upon physiological and molecular mechanisms of specific signatures., Methods: Here, we analysed a range of physiological and molecular responses expressed by geographically different populations of An. coluzzii inhabiting permanent and temporary breeding sites from the north and the south-west of Burkina Faso. Four mosquito colonies, namely (i) Oursi, built from females breeding in permanent habitats of the north; (ii) Déou, from temporary northern habitats; (iii) Soumousso from south-western temporary breeding sites; and (iv) Bama, from permanent habitats of the same south-western zone, were reared in climatic chambers under contrasted environmental conditions, mimicking temperature, relative humidity and light regimen occurring in northern Burkina Faso. Female mosquitoes were analysed for the seasonal variation in their amounts of proteins, triglycerides and free-circulating metabolites. The expression level of genes coding for the adipokinetic (AKH-I) and the AKH/corazonin-related peptides (ACP) were also assessed and compared among populations and environmental conditions., Results: Our analysis did not reveal an apparent pattern of physiological and molecular variations strictly correlated with either the larval ecotype or the geographical origin of the mosquitoes. However, specific distinct responses were observed among populations, suggesting that dry season survival may rely on more complex ecological parameters at a micro-habitat scale. Interestingly, the physiological and molecular data support the hypothesis that different aestivation abilities exist among populations of An. coluzzii inhabiting contrasted ecological settings. In particular, the striking metabotypes differentiation and the AKH mRNA expression level observed in females from temporary northern populations may suggest the existence of a "strong" aestivation strategy in these specimens., Conclusion: Our work provides insights into the physiological and molecular basis of dry and rainy season responses in An. coluzzii, and highlights the important diversity of the mechanisms involved. Such results represent key data for understanding the ecophysiological mechanisms underpinning the strong adaptive potential of this malaria vector species, which undoubtedly contributes to the spreading of mosquito distribution areas in space and time.