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Search Results: 1 - 10 of 299119 matches for " Nora J Besansky "
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Adaptation to Aridity in the Malaria Mosquito Anopheles gambiae: Chromosomal Inversion Polymorphism and Body Size Influence Resistance to Desiccation
Caroline Fouet, Emilie Gray, Nora J. Besansky, Carlo Costantini
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0034841
Abstract: Chromosomal inversions are thought to confer a selective advantage in alternative habitats by protecting co-adapted alleles from recombination. The frequencies of two inversions (2La and 2Rb) of the afro-tropical malaria mosquito Anopheles gambiae change gradually along geographical clines, increasing in frequency with degree of aridity. Such clines can result from gene flow and local selection acting upon alternative karyotypes along the cline, suggesting that these inversions may be associated with tolerance to xeric conditions. Since water loss represents a major challenge in xeric habitats, it can be supposed that genes inside these inversions are involved in water homeostasis. To test this hypothesis, we compared the desiccation resistance of alternative karyotypes from a colonised 2Rb/2La polymorphic population of A. gambiae from Cameroon. The strain included only the molecular form S, one of the genetic units marking incipient speciation in this taxon. Day-old mosquitoes of both sexes were assayed individually for time to death in a dry environment and the karyotype of each was determined post-mortem using molecular diagnostic assays for each inversion. In agreement with expectations based on their eco-geographical distribution, we found that 2La homokaryotypes survived significantly longer (1.3 hours) than the other karyotypes. However, there was weak support for the effect of 2Rb on desiccation resistance. Larger mosquitoes survived longer than smaller ones. Median survival of females was greater than males, but the effect of sex on desiccation resistance was weakly supported, indicating that differential survival was correlated to differences between sexes in average size. We found weak evidence for a heterotic effect of 2La karyotype on size in females. These results support the notion that genes located inside the 2La inversion are involved in water balance, contributing towards local adaptation of A.?gambiae to xeric habitats, beyond the adaptive value conferred by a larger body size.
2La chromosomal inversion enhances thermal tolerance of Anopheles gambiae larvae
Kyle AC Rocca, Emilie M Gray, Carlo Costantini, Nora J Besansky
Malaria Journal , 2009, DOI: 10.1186/1475-2875-8-147
Abstract: A laboratory colony of A. gambiae that is polymorphic for 2La but standard for all other known inversions was used to create 2 homokaryotypic populations (2L+a and 2La). The survival of 4th instar larvae and pupae from both populations was then tested following exposure to thermal stress with and without prior heat hardening.Larvae responded identically to a 40°C heat stress, with about 50% of larvae dying after 1.5–2 h and few larvae surviving a 3 h stress. When heat hardened prior to the thermal stress, thermo-tolerance of both larval populations increased, with 2La 24 h survival significantly exceeding that of 2L+a. Pupae were generally more thermo-tolerant than larvae, although 2La pupae were less so than 2L+a. Heat hardening had no positive effect on pupal thermo-tolerance.The increased thermo-tolerance observed in 2La larvae following heat hardening suggests higher responsiveness (i.e., thermal sensitivity) of the inverted karyotype. By responding more drastically to the heat shock, 2La larvae are better equipped to resist the potentially lethal temperatures that occur in arid habitats. The lower survival of 2La pupae compared with 2L+a may reflect the cost of this sensitivity, whereby the thermal resistance mechanisms prevent successful completion of metamorphosis. The costs and benefits of thermal resistance are discussed in light of the climates characterizing either end of the 2La frequency cline.The pervasiveness of malaria throughout sub-Saharan Africa is linked to that of its vector Anopheles gambiae, who has successfully invaded and adapted to most ecosystems found on the continent [1]. The impressive geographic and seasonal distribution of this species is hypothesized to originate in local adaptations facilitated by inversion polymorphisms [2]. Indeed, extensive cytogenetic studies have found that A. gambiae is not a genetically homogeneous species, but is composed of a mosaic of populations distinguished by different combinations of chromosomal inver
Effective population size of Anopheles funestus chromosomal forms in Burkina Faso
Andrew P Michel, Olga Grushko, Wamdaogo M Guelbeogo, N'Fale Sagnon, Carlo Costantini, Nora J Besansky
Malaria Journal , 2006, DOI: 10.1186/1475-2875-5-115
Abstract: Short-term Ne was estimated by evaluating variation at 16 microsatellite loci across temporal samples collected annually from 2000–2002. Estimates were based on standardized variance in allele frequencies or a maximum likelihood method. Long-term Ne was estimated from genetic diversity estimates using mtDNA sequences and microsatellites.For both forms, short-term and long-term Ne estimates were on the order of 103 and 105, respectively. Long-term Ne estimates were larger when based on loci from chromosome 3R (both inside and outside of inversions) than loci outside of this arm.Ne values indicate that An. funestus is not subject to seasonal bottlenecks. Though not statistically different because of large and overlapping confidence intervals, short-term Ne estimates were consistently smaller for Kiribina than Folonzo, possibly due to exploitation of different breeding sites: permanent for Folonzo and intermittent for Kiribina. The higher long-term Ne estimates on 3R, the arm carrying the two inversions mainly responsible for defining the chromosomal forms, give natural selection broader scope and merit further study.The efficient application of malaria control methods that target the mosquito vector depends upon knowledge of its population genetic structure. This information can improve current insecticide-based strategies and aid in the management of insecticide resistance, but it is also essential to future genetic control or modification strategies that aim to reduce, eliminate or replace vector populations with non-vectors. Unfortunately, present understanding of the population structure of any malaria vector is insufficient to underpin a genetic control programme, and nowhere is this shortfall more critical than in sub-Saharan Africa where three widespread species (Anopheles gambiae, Anopheles arabiensis and Anopheles funestus) are responsible for transmitting most of the 1–3 million fatal cases each year [1].An. funestus, one of the most anthropophilic vectors k
Inversion 2La is associated with enhanced desiccation resistance in Anopheles gambiae
Emilie M Gray, Kyle AC Rocca, Carlo Costantini, Nora J Besansky
Malaria Journal , 2009, DOI: 10.1186/1475-2875-8-215
Abstract: Two homokaryotypic populations of A. gambiae (inverted 2La and standard 2L+a) were created from a parental laboratory colony polymorphic for 2La and standard for all other known inversions. Desiccation resistance, water, energy and dry mass of adult females of both populations were compared at several ages and following acclimation to a more arid environment.Females carrying 2La were significantly more resistant to desiccation than 2L+a females at emergence and four days post-emergence, for different reasons. Teneral 2La females had lower rates of water loss than their 2L+a counterparts, while at four days, 2La females had higher initial water content. No differences in desiccation resistance were found at eight days, with or without acclimation. However, acclimation resulted in both populations significantly reducing their rates of water loss and increasing their desiccation resistance. Acclimation had contrasting effects on the body characteristics of the two populations: 2La females boosted their glycogen stores and decreased lipids, whereas 2La females did the contrary.Variation in rates of water loss and response to acclimation are associated with alternative arrangements of the 2La inversion. Understanding the mechanisms underlying these traits will help explain how inversion polymorphisms permit exploitation of a heterogeneous environment by this disease vector.In tropical environments, water availability can be a major factor limiting insect distribution [1]. As such, the geographic and seasonal range of the mosquito Anopheles gambiae, the principal African vector of malaria, depends on its ability to survive in arid environments [2]. Its small size results in a large surface area to volume ratio over which evaporative water loss can occur. The activity of adult mosquitoes exacerbates the risk of water loss, yet the scale of malaria morbidity and mortality attest to the mosquito's success throughout most of the continent [3].It is thought that A. gambiae owe
Localization of Candidate Regions Maintaining a Common Polymorphic Inversion (2La) in Anopheles gambiae
Bradley J White,Matthew W Hahn,Marco Pombi,Bryan J Cassone,Neil F Lobo,Frederic Simard,Nora J Besansky
PLOS Genetics , 2007, DOI: 10.1371/journal.pgen.0030217
Abstract: Chromosomal inversion polymorphisms are thought to play a role in adaptive divergence, but the genes conferring adaptive benefits remain elusive. Here we study 2La, a common polymorphic inversion in the African malaria vector Anopheles gambiae. The frequency of 2La varies clinally and seasonally in a pattern suggesting response to selection for aridity tolerance. By hybridizing genomic DNA from individual mosquitoes to oligonucleotide microarrays, we obtained a complete map of differentiation across the A. gambiae genome. Comparing mosquitoes homozygous for the 2La gene arrangement or its alternative (2L+a), divergence was highest at loci within the rearranged region. In the 22 Mb included within alternative arrangements, two ~1.5 Mb regions near but not adjacent to the breakpoints were identified as being significantly diverged, a conclusion validated by targeted sequencing. The persistent association of both regions with the 2La arrangement is highly unlikely given known recombination rates across the inversion in 2La heterozygotes, thus implicating selection on genes underlying these regions as factors responsible for the maintenance of 2La. Polymorphism and divergence data are consistent with a model in which the inversion is maintained by migration-selection balance between multiple alleles inside these regions, but further experiments will be needed to fully distinguish between the epistasis (coadaptation) and local adaptation models for the maintenance of 2La.
Spatially Explicit Analyses of Anopheline Mosquitoes Indoor Resting Density: Implications for Malaria Control
Colince Kamdem, Caroline Fouet, Joachim Etouna, Fran?ois-Xavier Etoa, Frédéric Simard, Nora J. Besansky, Carlo Costantini
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0031843
Abstract: Background The question of sampling and spatial aggregation of malaria vectors is central to vector control efforts and estimates of transmission. Spatial patterns of anopheline populations are complex because mosquitoes' habitats and behaviors are strongly heterogeneous. Analyses of spatially referenced counts provide a powerful approach to delineate complex distribution patterns, and contributions of these methods in the study and control of malaria vectors must be carefully evaluated. Methodology/Principal Findings We used correlograms, directional variograms, Local Indicators of Spatial Association (LISA) and the Spatial Analysis by Distance IndicEs (SADIE) to examine spatial patterns of Indoor Resting Densities (IRD) in two dominant malaria vectors sampled with a 5×5 km grid over a 2500 km2 area in the forest domain of Cameroon. SADIE analyses revealed that the distribution of Anopheles gambiae was different from regular or random, whereas there was no evidence of spatial pattern in Anopheles funestus (Ia = 1.644, Pa<0.05 and Ia = 1.464, Pa>0.05, respectively). Correlograms and variograms showed significant spatial autocorrelations at small distance lags, and indicated the presence of large clusters of similar values of abundance in An. gambiae while An. funestus was characterized by smaller clusters. The examination of spatial patterns at a finer spatial scale with SADIE and LISA identified several patches of higher than average IRD (hot spots) and clusters of lower than average IRD (cold spots) for the two species. Significant changes occurred in the overall spatial pattern, spatial trends and clusters when IRDs were aggregated at the house level rather than the locality level. All spatial analyses unveiled scale-dependent patterns that could not be identified by traditional aggregation indices. Conclusions/Significance Our study illustrates the importance of spatial analyses in unraveling the complex spatial patterns of malaria vectors, and highlights the potential contributions of these methods in malaria control.
Phylogenetic analysis and temporal diversification of mosquitoes (Diptera: Culicidae) based on nuclear genes and morphology
Kyanne R Reidenbach, Shelley Cook, Matthew A Bertone, Ralph E Harbach, Brian M Wiegmann, Nora J Besansky
BMC Evolutionary Biology , 2009, DOI: 10.1186/1471-2148-9-298
Abstract: We carried out maximum parsimony and maximum likelihood, including Bayesian, analyses on a data set consisting of six nuclear genes and 80 morphological characters to assess their ability to resolve relationships among 25 genera. We also estimated divergence times based on sequence data and fossil calibration points, using Bayesian relaxed clock methods. Strong support was recovered for the basal position and monophyly of the subfamily Anophelinae and the tribes Aedini and Sabethini of subfamily Culicinae. Divergence times for major culicid lineages date to the early Cretaceous.Deeper relationships within the family remain poorly resolved, suggesting the need for additional taxonomic sampling. Our results support the notion of rapid radiations early in the diversification of mosquitoes.Mosquitoes (Diptera: Culicidae) are a monophyletic group of true flies [1-4], recognizable by their elongate adult mouthparts through which the females of most species feed on vertebrate blood. Mosquitoes occur throughout temperate and tropical regions, and well beyond the Arctic Circle, but are most diverse in tropical forest environments [5]. A bewildering amount of morphological diversity parallels their spectacular radiation into virtually every conceivable collection of water, ranging from a few droplets trapped by plant parts to large bodies of fresh and brackish surface water, making mosquitoes "as ubiquitous as water" [6]. The relationship between human health and those species that are medically important (<200 of 3,524 currently recognized; http://mosquito-taxonomic-inventory.info/ webcite) has driven most mosquito research. Within this small subset of disease vector species, morphological similarities between close relatives (e.g., cryptic or sibling species complexes) continue to pose practical and academic challenges to disease control, conventional taxonomy, and phylogenetic inference. Ironically, it is not the morphological similarity but rather the morphological divers
Seasonal distribution of Anopheles funestus chromosomal forms from Burkina Faso
Wamdaogo M Guelbeogo, N'Fale Sagnon, Olga Grushko, Malgaouende A Yameogo, Daniela Boccolini, Nora J Besansky, Carlo Costantini
Malaria Journal , 2009, DOI: 10.1186/1475-2875-8-239
Abstract: Chromosomal karyotypes were determined from indoor-resting, half-gravid females sampled within and across six breeding seasons, from December 1998 to April 2007.As expected, the pattern of chromosomal polymorphism in An. funestus was consistent with assortatively mating Folonzo and Kiribina forms. When samples were assigned to each chromosomal form, their relative abundance varied within successive breeding seasons in a repeating pattern of temporal variability. Relative abundance of the Folonzo form was correlated with climatic variables related to temperature and rainfall.The relative abundance of Folonzo and Kiribina forms of An. funestus likely reflects different larval ecologies that are linked to varying climatic conditions. Further analysis of the bionomics of these vectors is recommended in light of its relevance to vector control.Malaria remains a major health concern in Africa today. This situation is due to the presence of three efficient vectors in subgenus Cellia--Anopheles gambiae, its sibling species Anopheles arabiensis and Anopheles funestus. These species co-occur geographically across most of sub-Saharan Africa and can inhabit the same villages, shelter in the same houses, and blood-feed on the same individuals. Vector control efforts are complicated not only by the spread of resistance to insecticides, but also by behavioural and genetic variation within and between species. Among the challenges of efficient vector control are cryptic barriers to gene flow among populations of the same species that can arise as a consequence of differential adaptations to heterogeneities in the environment. This challenge applies not only to the introduction and spread of "refractoriness genes" in wild mosquito populations, but also to the use of bed nets whose efficacy could be lessened by exophilic and exophagic vector sub-populations [1]. Sustained vector control efforts across Africa will benefit from knowledge of the extent of genetic diversity in natural po
Genetic structure of Anopheles gambiae populations on islands in northwestern Lake Victoria, Uganda
Jonathan K Kayondo, Louis G Mukwaya, Aram Stump, Andrew P Michel, Mamadou B Coulibaly, Nora J Besansky, Frank H Collins
Malaria Journal , 2005, DOI: 10.1186/1475-2875-4-59
Abstract: Four Islands (from 20–50 km apart) and two surrounding mainland populations (96 km apart) were studied. Samples of indoor resting adult mosquitoes, collected over two consecutive years, were genotyped at microsatellite loci distributed broadly throughout the genome and analysed for genetic structure, effective migration (Nem) and effective population size (Ne).Ne estimates showed island populations to consist of smaller demes compared to the mainland ones. Most populations were significantly differentiated geographically, and from one year to the other. Average geographic pair-wise FST ranged from 0.014–0.105 and several pairs of populations had Ne m < 3. The loci showed broad heterogeneity at capturing or estimating population differences.These island populations are significantly genetically differentiated. Differences reoccurred over the study period, between the two mainland populations and between each other. This appears to be the product of their separation by water, dynamics of small populations and local adaptation. With further characterisation these islands could become possible sites for applying measures evaluating effectiveness of control by genetic manipulation.Malaria kills over a million people annually, most from sub-Saharan Africa [1]. Additionally, malaria mortality is on the rise, largely because of the emergence over the past two decades of widespread Plasmodium resistance to affordable antimalarial drugs [2]. Control approaches such as insecticide impregnated bed nets are also being challenged by the emergence of insecticide resistance in Anopheles gambiae and Anopheles funestus, the two primary malaria vectors in sub-Saharan Africa [3,4].An alternative malaria control strategy being investigated in a number of laboratories is to genetically modify the vectorial capacity of vector populations by driving a genetic construct into the natural population. Genes that influence blood meal host selection, mosquito longevity, or Plasmodium survival ha
Segmental Duplication Implicated in the Genesis of Inversion 2Rj of Anopheles gambiae
Mamadou B. Coulibaly, Neil F. Lobo, Meagan C. Fitzpatrick, Marcia Kern, Olga Grushko, Daniel V. Thaner, Sékou F. Traoré, Frank H. Collins, Nora J. Besansky
PLOS ONE , 2007, DOI: 10.1371/journal.pone.0000849
Abstract: The malaria vector Anopheles gambiae maintains high levels of inversion polymorphism that facilitate its exploitation of diverse ecological settings across tropical Africa. Molecular characterization of inversion breakpoints is a first step toward understanding the processes that generate and maintain inversions. Here we focused on inversion 2Rj because of its association with the assortatively mating Bamako chromosomal form of An. gambiae, whose distinctive breeding sites are rock pools beside the Niger River in Mali and Guinea. Sequence and computational analysis of 2Rj revealed the same 14.6 kb insertion between both breakpoints, which occurred near but not within predicted genes. Each insertion consists of 5.3 kb terminal inverted repeat arms separated by a 4 kb spacer. The insertions lack coding capacity, and are comprised of degraded remnants of repetitive sequences including class I and II transposable elements. Because of their large size and patchwork composition, and as no other instances of these insertions were identified in the An. gambiae genome, they do not appear to be transposable elements. The 14.6 kb modules inserted at both 2Rj breakpoint junctions represent low copy repeats (LCRs, also called segmental duplications) that are strongly implicated in the recent (~0.4Ne generations) origin of 2Rj. The LCRs contribute to further genome instability, as demonstrated by an imprecise excision event at the proximal breakpoint of 2Rj in field isolates.
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