Abstract
BackgroundThroughout large parts of sub-Saharan Africa, seasonal malaria transmission follows mosquito density, approaching zero during the dry season and peaking during the wet season. The mechanisms by which malaria mosquitoes survive the long dry season, when no larval sites are available remain largely unknown, despite being long recognized as a critical target for vector control. Previous work in the West African Sahel has led to the hypothesis that Anopheles coluzzii (formerly M-form Anopheles gambiae) undergoes aestivation (dry-season diapause), while Anopheles gambiae (s.s.) (formerly S-form An. gambiae) and Anopheles arabiensis repopulate each wet season via long-distance migration. The environmental cues used by these species to signal the oncoming dry season have not been determined; however, studies, mostly addressing mosquitoes from temperate zones, have highlighted photoperiod and temperature as the most common token stimuli for diapause initiation. We subjected newly established colonies of An. coluzzii and An. arabiensis from the Sahel to changes in photoperiod to assess and compare their responses in terms of longevity and other relevant phenotypes.ResultsOur results showed that short photoperiod alone and to a lesser extent, lower nightly temperature (representing the early dry season), significantly increased longevity of An. coluzzii (by ~30%, P < 0.001) but not of An. arabiensis. Further, dry season conditions increased body size but not relative lipid content of An. coluzzii, whereas body size of An. arabiensis decreased under these conditions.ConclusionsThese species-specific responses underscore the capacity of tropical anophelines to detect mild changes (~1 h) in photoperiod and thus support the role of photoperiod as a token stimulus for An. coluzzii in induction of aestivation, although, these responses fall short of a complete recapitulation of aestivation under laboratory conditions.
Highlights
Throughout large parts of sub-Saharan Africa, seasonal malaria transmission follows mosquito density, approaching zero during the dry season and peaking during the wet season
These coordinated changes suggest trade-offs elsewhere in the life-cycle under dry season conditions, which are likely manifest as reduced reproductive output, consistent with the lack of dry season reproduction in nature [21, 32]. These results suggest that An. coluzzii has entered a diapause initiation phase in our experiments, at least partly. These environmental changes alone failed to manifest a bona fide aestivating adult recognized by a longevity greater than 3 months, raising the question of what additional conditions must be met if aestivation is to be manifested?
We compared the responses of An. coluzzii and An. arabiensis to changes in photoperiod and temperature and aimed to induce aestivation in An. coluzzii under shorter photophase and lower nightly temperature, typical of the early dry season
Summary
Throughout large parts of sub-Saharan Africa, seasonal malaria transmission follows mosquito density, approaching zero during the dry season and peaking during the wet season. The mechanisms by which malaria mosquitoes survive the long dry season, when no larval sites are available remain largely unknown, despite being long recognized as a critical target for vector control. Previous work in the West African Sahel has led to the hypothesis that Anopheles coluzzii (formerly M-form Anopheles gambiae) undergoes aestivation (dry-season diapause), while Anopheles gambiae (s.s.) (formerly S-form An. gambiae) and Anopheles arabiensis repopulate each wet season via long-distance migration. In the Sahel (latitudes 12–18°), it is hypothesized that An. coluzzii (previously M-form An. gambiae; [24]) survive the dry season by aestivating locally, while An. gambiae (previously the S-form) and An. arabiensis persist via annual long-distance migration [8, 20, 25,26,27,28]. Uncovering the factors which stimulate mosquitoes to begin, maintain, and terminate aestivation could lead to better prediction of disease-transmission seasons and may provide novel opportunities for vector control [21, 34, 52]
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