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

Malaria in Africa is vectored primarily by the Anopheles gambiae complex. Although the mechanisms of population persistence during the dry season are not yet known, targeting dry season mosquitoes could provide opportunities for vector control. In the Sahel, it appears likely that M-form A. gambiae survive by aestivation (entering a dormant state). To assess the role of eco-physiological changes associated with dry season survival, we measured body size, flight activity and metabolic rate of wild-caught mosquitoes throughout 1 year in a Sahelian locality, far from permanent water sources, and at a riparian location adjacent to the Niger River. We found significant seasonal variation in body size at both the Sahelian and riparian sites, although the magnitude of the variation was greater in the Sahel. For flight activity, significant seasonality was only observed in the Sahel, with increased flight activity in the wet season when compared with that just prior to and throughout the dry season. Whole-organism metabolic rate was affected by numerous biotic and abiotic factors, and a significant seasonal component was found at both locations. However, assay temperature accounted completely for seasonality at the riparian location, while significant seasonal variation remained after accounting for all measured variables in the Sahel. Interestingly, we did not find that mean metabolic rate was lowest during the dry season at either location, contrary to our expectation that mosquitoes would conserve energy and increase longevity by reducing metabolism during this time. These results indicate that mosquitoes may use mechanisms besides reduced metabolic rate to enable survival during the Sahelian dry season.

Variation in metabolic rate of Anopheles gambiae and A. arabiensis in a Sahelian village.

In the Sahel, the Anopheles gambiae complex consists of Anopheles arabiensis and the M and S molecular forms of A. gambiae sensu stricto. However, the composition of these malaria vectors varies spatially and temporally throughout the region and is thought to be linked to environmental factors such as rainfall, larval site characteristics and duration of the dry season. To examine possible physiological divergence between these taxa, we measured metabolic rates of mosquitoes during the wet season in a Sahelian village in Mali. To our knowledge, this study provides the first measurements of metabolic rates of A. gambiae and A. arabiensis in the field. The mean metabolic rate of A. arabiensis was higher than that of M-form A. gambiae when accounting for the effects of female gonotrophic status, temperature and flight activity. However, after accounting for their difference in body size, no significant difference in metabolic rate was found between these two species (whilst all other factors were found to be significant). Thus, body size may be a key character that has diverged in response to ecological differences between these two species. Alternatively, these species may display additional differences in metabolic rate only during the dry season. Overall, our results indicate that changes in behavior and feeding activity provide an effective mechanism for mosquitoes to reduce their metabolic rate, and provide insight into the possible strategies employed by aestivating individuals during the dry season. We hypothesize that female mosquitoes switch to sugar feeding while in dormancy because of elevated metabolism associated with blood digestion.

The contribution of aestivating mosquitoes to the persistence of Anopheles gambiae in the Sahel.

BACKGROUND: Persistence of African anophelines throughout the long dry season (4-8 months) when no surface waters are available remains one of the enduring mysteries of medical entomology. Recent studies demonstrated that aestivation (summer diapause) is one mechanism that allows the African malaria mosquito, Anopheles gambiae, to persist in the Sahel. However, migration from distant localities - where reproduction continues year-round - might also be involved. METHODS: To assess the contribution of aestivating adults to the buildup of populations in the subsequent wet season, two villages subjected to weekly pyrethrum sprays throughout the dry season were compared with two nearby villages, which were only monitored. If aestivating adults are the main source of the subsequent wet-season population, then the subsequent wet-season density in the treated villages will be lower than in the control villages. Moreover, since virtually only M-form An. gambiae are found during the dry season, the reduction should be specific to the M form, whereas no such difference is predicted for S-form An. gambiae or Anopheles arabiensis. On the other hand, if migrants arriving with the first rain are the main source, no differences between treated and control villages are expected across all members of the An. gambiae complex. RESULTS: The wet-season density of the M form in treated villages was 30% lower than that in the control (P < 10-4, permutation test), whereas no significant differences were detected in the S form or An. arabiensis. CONCLUSIONS: These results support the hypothesis that the M form persist in the arid Sahel primarily by aestivation, whereas the S form and An. arabiensis rely on migration from distant locations. Implications for malaria control are discussed.

Reproduction-longevity trade-off in Anopheles gambiae (Diptera: Culicidae).

Reduced survival and future reproduction due to of current reproduction is a trade-off known as the cost of reproduction. Surprisingly, only a few studies have assessed the cost of reproduction in arthropod disease vectors, despite its effect on longevity, and thus on vectorial capacity. We evaluated the cost of reproduction on survival of Anopheles gambiae Giles by comparing mosquitoes that were denied exposure to the other sex, hereafter named virgins, and those that were allowed exposure to the other sex and mating, hereafter named mated. Merely 6 d of exposure to females with mating activity reduced male survival from a median of 17 d in virgins to 15 d in mated, indicating that male mating cost is substantial. The increase in mortality of mated males began several days after the exposure to females ended, indicating that mating is not associated with immediate mortality risk. Notably, body size was negatively correlated with male mortality in mated males, but not in virgins. The rate of insemination declined after 4 d of exposure to females, indicating that male mating capacity is limited and further supporting the hypothesis that mating is costly for males. Consistent with previous studies, female survival on sugar alone (median=16 d) was shorter than on blood and sugar (median=19 d), regardless if she was mated or virgin. Overall, survival of mated females was lower than that of virgins on a diet of blood and sugar, but no difference was found on a diet of sugar only. However, the cost of reproduction in females remains ambiguous because the difference in survival between virgin and mated females was driven by the difference between virgin (median=19 d) and uninseminated females exposed to males (median=17 d), rather than between virgin and inseminated females (median=19 d). Accordingly, sperm and seminal fluid, egg development, and oviposition have negligible cost in terms of female survival. Only exposure to males without insemination decreased female survival. Nonetheless, if exposure to males under natural conditions is also associated with reduced survival, it might explain why females remain monogamous.

Spatial swarm segregation and reproductive isolation between the molecular forms of Anopheles gambiae.

Anopheles gambiae, the major malaria vector in Africa, can be divided into two subgroups based on genetic and ecological criteria. These two subgroups, termed the M and S molecular forms, are believed to be incipient species. Although they display differences in the ecological niches they occupy in the field, they are often sympatric and readily hybridize in the laboratory to produce viable and fertile offspring. Evidence for assortative mating in the field was recently reported, but the underlying mechanisms awaited discovery. We studied swarming behaviour of the molecular forms and investigated the role of swarm segregation in mediating assortative mating. Molecular identification of 1145 males collected from 68 swarms in Donéguébougou, Mali, over 2 years revealed a strict pattern of spatial segregation, resulting in almost exclusively monotypic swarms with respect to molecular form. We found evidence of clustering of swarms composed of individuals of a single molecular form within the village. Tethered M and S females were introduced into natural swarms of the M form to verify the existence of possible mate recognition operating within-swarm. Both M and S females were inseminated regardless of their form under these conditions, suggesting no within-mate recognition. We argue that our results provide evidence that swarm spatial segregation strongly contributes to reproductive isolation between the molecular forms in Mali. However this does not exclude the possibility of additional mate recognition operating across the range distribution of the forms. We discuss the importance of spatial segregation in the context of possible geographic variation in mechanisms of reproductive isolation.

Assessment of alternative mating strategies in Anopheles gambiae: Does mating occur indoors?

Mating in Anopheles gambiae has been observed only in outdoor swarms. Here we evaluate whether mating also occurs indoors. Mark-release-recapture of virgin males and females in natural houses showed that mating occurred over a single day even when mosquitoes can leave the house through exit traps and without adaptation to laboratory conditions. In these experiments, insemination rate in the M molecular form of An. gambiae (and An. arabiensis) was higher than that of the S form (15 versus 6%). Under these conditions, smaller females of the M form mated more frequently than larger females of that form. Sampling mosquitoes throughout the day showed that both sexes enter houses around sunrise and leave around sunset, staying indoors together from dawn to dusk. In an area dominated by the M form, the daily rate of insemination in samples from exit traps was approximately 5% higher than in those from entry traps, implying that mating occurred indoors. Importantly, frequency of cross mating between the molecular forms was as high as that between members of the same form, indicating that, indoors, assortative mating breaks down. Altogether, these results suggest that indoor mating is an alternative mating strategy of the M molecular form of An. gambiae. Because naturally occurring mating couples have not yet been observed indoors, this conclusion awaits validation.

The distribution of hatching time in Anopheles gambiae.

BACKGROUND: Knowledge of the ecological differences between the molecular forms of Anopheles gambiae and their sibling species, An. arabiensis might lead to understanding their unique contribution to disease transmission and to better vector control as well as to understanding the evolutionary forces that have separated them. METHODS: The distributions of hatching time of eggs of wild An. gambiae and An. arabiensis females were compared in different water types. Early and late hatchers of the S molecular form were compared with respect to their total protein content, sex ratio, development success, developmental time and adult body size. RESULTS: Overall, the distribution of hatching time was strongly skewed to the right, with 89% of the eggs hatching during the second and third day post oviposition, 10% hatching during the next four days and the remaining 1% hatching over the subsequent week. Slight, but significant differences were found between species and between the molecular forms in all water types. Differences in hatching time distribution were also found among water types (in each species and molecular form), suggesting that the eggs change their hatching time in response to chemical factors in the water. Early hatchers were similar to late hatchers except that they developed faster and produced smaller adults than late hatchers. CONCLUSION: Differences in hatching time and speed of development among eggs of the same batch may be adaptive if catastrophic events such as larval site desiccation are not rare and the site's quality is unpredictable. The egg is not passive and its hatching time depends on water factors. Differences in hatching time between species and molecular forms were slight, probably reflecting that conditions in their larval sites are rather similar.

Dry season reproductive depression of Anopheles gambiae in the Sahel.

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.

Aestivation of the African malaria mosquito, Anopheles gambiae in the Sahel.

The African malaria mosquito, Anopheles gambiae, inhabits diverse environments including dry savannas, where surface waters required for larval development are absent for 4-8 months per year. Under such conditions, An. gambiae virtually disappears. Whether populations survive the long dry season by aestivation (a dormant state promoting extended longevity during the summer) or are reestablished by migrants from distant locations where larval sites persist has remained an enigma for over 60 years. Resolving this question is important, because fragile dry season populations may be more susceptible to control. Here, we show unequivocally that An. gambiae aestivates based on a demographic study and a mark release-recapture experiment spanning the period from the end of one wet season to the beginning of the next. During the dry season, An. gambiae was barely detectable in Sahelian villages of Mali. Five days after the first rain, before a new generation of adults could be produced, mosquito abundance surged 10-fold, implying that most mosquitoes were concealed locally until the rain. Four days after the first rain, a marked female An. gambiae s.s. was recaptured. Initially captured, marked, and released at the end of the previous wet season, she has survived the 7-month-long dry season. These results provide evidence that An. gambiae persists throughout the dry season by aestivation and open new questions for mosquito and parasite research. Improved malaria control by targeting aestivating mosquitoes using existing or novel strategies may be possible.