Dry season reproductive depression of Anopheles gambiae in the Sahel

Abstract

The African malaria mosquito, Anopheles gambiae, is widespread south of the Sahara including in dry savannahs and semi-arid environments where no surface water exists for several months a year. Adults of the M form of An. gambiae persist through the long dry season, when no surface waters are available, by increasing their maximal survival from 4 weeks to 7 months. Dry season diapause (aestivation) presumably underlies this extended survival. Diapause in adult insects is intrinsically linked to depressed reproduction. To determine if reproduction of the Sahelian M form is depressed during the dry season, we assessed seasonal changes in oviposition, egg batch size, and egg development, as well as insemination rate and blood feeding in wild caught mosquitoes. Results from xeric Sahelian and riparian populations were compared. Oviposition response in the Sahelian M form dropped from 70% during the wet season to 20% during the dry season while the mean egg batch size among those that laid eggs fell from 173 to 101. Correspondingly, the fraction of females that exhibited gonotrophic dissociation increased over the dry season from 5% to 45%, while a similar fraction of the population retained developed eggs despite having access to water. This depression in reproduction the Sahelian M form was not caused by a reduced insemination rate. Seasonal variation in these reproductive parameters of the riparian M form population was less extreme and the duration of reproductive depression was shorter. Blood feeding responses did not change with the season in either population. Depressed reproduction during the dry season in the Sahelian M form of An. gambiae provides additional evidence for aestivation and illuminates the physiological processes involved. The differences between the Sahelian and riparian population suggest an adaptive cline in aestivation phenotypes between populations only 130 km apart.

Seasonal variation in metabolic rate, flight activity and body size of Anopheles gambiae in the Sahel

“…In contrast, the smaller mosquitoes that predominate from April onwards survive the full 7months of the dry season and employ the strategy of 'strong aestivators', and found the next wet season population. Heterogeneity in reproductive strategy of Sahelian Mform A. gambiae was recently noted in our study area (Yaro et al, 2012). Therefore, we hypothesize that mosquitoes may utilize alternative strategies for dry season survival and that the choice of strategy may be size dependent.…”

“…The climate of N'Gabakoro is similar to that of the southern Sahel, although annual precipitation is higher and the first rains fall an average of 4weeks earlier (Yaro et al, 2012).…”

“…Under aestivation, adults that emerge at the end of one wet season cease reproduction, enter protective shelters, and remain in a quiescent state until the next season's rains, possibly with occasional feeding bouts to replenish resources (Yaro et al, 2012). Alternatively, under migration, mosquito populations persist year-round in areas with permanent water and ongoing reproduction, and migrate to seasonal locations once the rainy season begins each year.…”

Aedes aegypti Pharate First Instar Aseasonal Quiescence Cues Anticipatory Plasticity with Implications for Urban Vector Ecology and Control

“…The costs of reproduction involving trade-offs between egg production and mortality are well documented in insects and other animals with trade-offs imposing constraints among survival, growth, resource acquisition and reproduction; nutrient limitation being the central mechanism underlying these trade-offs (Boggs, 2009;Chippindale, et al, 1993). Seasonal variation in the reproductive parameters of Anopheles gambiae have also been demonstrated (Yaro, et al, 2012).…”

“…In the case of anticipatory plasticity, phenotypic changes are induced in response to cues which signal future environmental conditions prior to the actual onset of these conditions (Esperk et al, 2012). In tropical and subtropical regions mosquitoes are often exposed to a favorable rainy season with abundant breeding sites and vegetation, and a stressful drier winter Denlinger, 1986;Mori et al, 1981) during which resources may be less plentiful (Gary and Foster, 2001;Foster, 1995;Yaro, et al, 2012). Regardless of the season fluctuations in moisture are however, unpredictable.…”