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

Identification of morphological and chemical markers of dry- and wet-season conditions in female Anopheles gambiae mosquitoes

BACKGROUND: Increased understanding of the dry-season survival mechanisms of Anopheles gambiae in semi-arid regions could benefit vector control efforts by identifying weak links in the transmission cycle of malaria. In this study, we examined the effect of photoperiod and relative humidity on morphologic and chemical traits known to control water loss in mosquitoes. METHODS: Anopheles gambiae body size (indexed by wing length), mesothoracic spiracle size, and cuticular hydrocarbon composition (both standardized by body size) were examined in mosquitoes raised from eggs exposed to short photoperiod and low relative humidity, simulating the dry season, or long photoperiod and high relative humidity, simulating the wet-season. RESULTS: Mosquitoes exposed to short photoperiod exhibited larger body size and larger mesothoracic spiracle length than mosquitoes exposed to long photoperiod. Mosquitoes exposed to short photoperiod and low relative humidity exhibited greater total cuticular hydrocarbon amount than mosquitoes exposed to long photoperiod and high relative humidity. In addition, total cuticular hydrocarbon amount increased with age and was higher in mated females. Mean n-alkane retention time (a measure of cuticular hydrocarbon chain length) was lower in mosquitoes exposed to short photoperiod and low relative humidity, and increased with age. Individual cuticular hydrocarbon peaks were examined, and several cuticular hydrocarbons were identified as potential biomarkers of dry- and wet-season conditions, age, and insemination status. CONCLUSIONS: Results from this study indicate that morphological and chemical changes underlie aestivation of Anopheles gambiae and may serve as biomarkers of aestivation

Evidence for divergent selection between the molecular forms of Anopheles gambiae: role of predation

The molecular forms of Anopheles gambiae are undergoing speciation. They are characterized by a strong assortative mating and they display partial habitat segregation. The M form is mostly found in flooded/irrigated areas whereas the S form dominates in the surrounding areas, but the ecological factors that shape this habitat segregation are not known. Resource competition has been demonstrated between species undergoing divergent selection, but resource competition is not the only factor that can lead to divergence. In a field experiment using transplantation of first instar larvae, we evaluated the role of larval predators in mediating habitat segregation between the forms. We found a significant difference in the ability of the molecular forms to exploit the different larval sites conditioned on the presence of predators. In absence of predation, the molecular forms outcompeted each other in their respective natural habitats however, the developmental success of the M form was significantly higher than that of the S form in both habitats under predator pressure. Our results provide the first empirical evidence for specific adaptive differences between the molecular forms and stress the role of larval predation as one of the mechanisms contributing to their divergence.https://doi.org/10.1186/1471-2148-8-

Australian mosquito assemblages vary between ground and sub-canopy habitats

The surveillance and control of mosquito-borne diseases is dependent upon understanding the bionomics and distribution of the vectors. Most studies of mosquito assemblages describe species abundance, richness and composition close to the ground defined often by only one sampling method. In this study, we assessed Australian mosquito species near the ground and in the sub-canopy using two traps baited with a variety of lures

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

<p>Abstract</p> <p>Background</p> <p>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, <it>Anopheles gambiae</it>, to persist in the Sahel. However, migration from distant localities - where reproduction continues year-round - might also be involved.</p> <p>Methods</p> <p>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 <it>An. gambiae </it>are found during the dry season, the reduction should be specific to the M form, whereas no such difference is predicted for S-form <it>An. gambiae </it>or <it>Anopheles arabiensis</it>. 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 <it>An. gambiae </it>complex.</p> <p>Results</p> <p>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 <it>An</it>. <it>arabiensis</it>.</p> <p>Conclusions</p> <p>These results support the hypothesis that the M form persist in the arid Sahel primarily by aestivation, whereas the S form and <it>An. arabiensis </it>rely on migration from distant locations. Implications for malaria control are discussed.</p

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

Urban malaria may be spreading via the wind: here’s why that’s important

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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

Diversity, composition, altitude, and seasonality of high-altitude windborne migrating mosquitoes in the Sahel: Implications for disease transmission

Recent studies have reported Anopheles mosquitoes captured at high-altitude (40–290 m above ground) in the Sahel. Here, we describe this migration modality across genera and species of African Culicidae and examine its implications for disease transmission and control. As well as Anopheles, six other genera—Culex, Aedes, Mansonia, Mimomyia, Lutzia, and Eretmapodites comprised 90% of the 2,340 mosquitoes captured at altitude. Of the 50 molecularly confirmed species (N = 2,107), 33 species represented by multiple specimens were conservatively considered high-altitude windborne migrants, suggesting it is a common migration modality in mosquitoes (31–47% of the known species in Mali), and especially in Culex (45−59%). Overall species abundance varied between 2 and 710 specimens/species (in Ae. vittatus and Cx. perexiguus, respectively). At altitude, females outnumbered males 6:1, and 93% of the females have taken at least one blood meal on a vertebrate host prior to their departure. Most taxa were more common at higher sampling altitudes, indicating that total abundance and diversity are underestimated. High-altitude flight activity was concentrated between June and November coinciding with availability of surface waters and peak disease transmission by mosquitoes. These hallmarks of windborne mosquito migration bolster their role as carriers of mosquito-borne pathogens (MBPs). Screening 921 mosquitoes using pan-Plasmodium assays revealed that thoracic infection rate in these high-altitude migrants was 2.4%, providing a proof of concept that vertebrate pathogens are transported by windborne mosquitoes at altitude. Fourteen of the 33 windborne mosquito species had been reported as vectors to 25 MBPs in West Africa, which represent 32% of the MBPs known in that region and include those that inflict the heaviest burden on human and animal health, such as malaria, yellow fever, dengue, and Rift Valley fever. We highlight five arboviruses that are most likely affected by windborne mosquitoes in West Africa: Rift Valley fever, O'nyong'nyong, Ngari, Pangola, and Ndumu. We conclude that the study of windborne spread of diseases by migrating insects and the development of surveillance to map the sources, routes, and destinations of vectors and pathogens is key to understand, predict, and mitigate existing and new threats of public health