Microsatellite DNA and isozyme variability in a west African population of Anopheles gambiae

Identification and validation of the first EST-SSR markers based on transcriptome of Anopheles darlingi, the primary transmitter of malaria in Brazil

BACKGROUND

Anopheles darlingi is a monotypic species in terms of its morphological, genetic, and behavioral aspects and is the primary transmitter of human malaria (99%) in Brazil, especially in the Brazilian Amazon. In this pioneering study, 15 expressed sequence tag (EST)-simple sequence repeat (SSR) markers were obtained and characterized in samples from the municipality of São Gabriel da Cachoeira, Amazonas state, Brazil, with polymorphisms that can be used for further genetic research.

METHODS AND RESULTS

The specimens (from egg to larval stage) collected were bred in the insectary at INPA (National Institute for Amazonian Research). The SSR repeats within the contigs of the A. darlingi EST banks were confirmed on the Vector Base site. DNA was extracted and amplified using polymerase chain reaction and then genotyped. Fifteen polymorphic SSR loci were identified and characterized. The number of alleles totaled 76 and ranged from 2 to 9. The observed heterozygosity varied between 0.026 and 0.769, the expected heterozygosity between 0.025 and 0.776, and the mean polymorphism information content was 0.468. Eight loci showed Hardy-Weinberg equilibrium (HWE) after Bonferroni correction (P: (5%) ≤ 0.0033). No linkage disequilibrium was found among the loci.

CONCLUSIONS

The polymorphic SSRs of the loci have been shown to be efficient for investigation of the variability and genetic population structure of A. darlingi.

Special Collection: Highlights of Medical, Urban and Veterinary Entomology. Highlights in Medical Entomology, 2021

Life remained far from normal as we completed the first year of the Covid-19 pandemic and entered a second year. Despite the challenges faced worldwide, together we continue to move the field of Medical Entomology forward. Here, I reflect on parallels between control of Covid-19 and vector-borne disease control, discuss the advantages and caveats of using new genotyping technologies for the study of invasive species, and proceed to highlight papers that were published between 2020 and 2021 with a focus on those related to mosquito surveillance and population genetics of mosquito vectors.

High-resolution species assignment of Anopheles mosquitoes using k-mer distances on targeted sequences

The ANOSPP amplicon panel is a genus-wide targeted sequencing panel to facilitate large-scale monitoring of Anopheles species diversity. Combining information from the 62 nuclear amplicons present in the ANOSPP panel allows for a more senstive and specific species assignment than single gene (e.g. COI) barcoding, which is desirable in the light of permeable species boundaries. Here, we present NNoVAE, a method using Nearest Neighbours (NN) and Variational Autoencoders (VAE), which we apply to k-mers resulting from the ANOSPP amplicon sequences in order to hierarchically assign species identity. The NN step assigns a sample to a species-group by comparing the k-mers arising from each haplotype's amplicon sequence to a reference database. The VAE step is required to distinguish between closely related species, and also has sufficient resolution to reveal population structure within species. In tests on independent samples with over 80% amplicon coverage, NNoVAE correctly classifies to species level 98% of samples within the An. gambiae complex and 89% of samples outside the complex. We apply NNoVAE to over two thousand new samples from Burkina Faso and Gabon, identifying unexpected species in Gabon. NNoVAE presents an approach that may be of value to other targeted sequencing panels, and is a method that will be used to survey Anopheles species diversity and Plasmodium transmission patterns through space and time on a large scale, with plans to analyse half a million mosquitoes in the next five years.

Yearly variations of the genetic structure of Aedes aegypti (Linnaeus) (Diptera: Culicidae) in the Philippines (2017-2019)

Dengue is the fastest emerging arboviral disease in the world, imposing a substantial health and economic burden in the tropics and subtropics. The mosquito, Aedes aegypti, is the primary vector of dengue in the Philippines. We examined the genetic structure of Ae. aegypti populations collected from the Philippine major islands (Luzon, Visayas and Mindanao), each with highland (Baguio city, Cebu city mountains and Maramag, Bukidnon, respectively) and lowland sites (Quezon city; Liloan, Cebu and Cagayan de Oro [CDO] city, respectively) during the wet (2017-2018 and 2018-2019) and dry seasons (2018 and 2019). Mosquitoes (n = 1800) were reared from field-collected eggs and immatures, and were analyzed using 12 microsatellite loci. Generalized linear model analyses revealed yearly variations between highlands and lowlands in the major islands as supported by Bayesian clustering analyses on: 1) stronger selection (inbreeding coefficient, F_IS = 0.52) in 2017-2018 than in 2018-2019 (F_IS = 0.32) as influenced by rainfall, 2) the number of non-neutral loci indicating selection, and 3) differences of effective population size although at p = 0.05. Across sites except Baguio and CDO cities: 1) F_IS varied seasonally as influenced by relative humidity (RH), and 2) the number of non-neutral loci varied as influenced by RH and rainfall indicating selection. Human-mediated activities and not isolation by distance influenced genetic differentiations of mosquito populations within (F_ST = 0.04) the major islands and across sites (global F_ST = 0.16). Gene flow (Nm) and potential first generation migrants among populations were observed between lowlands and highlands within and across major islands. Our results suggest that dengue control strategies in the epidemic wet season are to be changed into whole year-round approach, and water pipelines are to be installed in rural mountains to prevent the potential breeding sites of mosquitoes.

A targeted amplicon sequencing panel to simultaneously identify mosquito species and Plasmodium presence across the entire Anopheles genus

Anopheles is a diverse genus of mosquitoes comprising over 500 described species, including all known human malaria vectors. While a limited number of key vector species have been studied in detail, the goal of malaria elimination calls for surveillance of all potential vector species. Here, we develop a multilocus amplicon sequencing approach that targets 62 highly variable loci in the Anopheles genome and two conserved loci in the Plasmodium mitochondrion, simultaneously revealing both the mosquito species and whether that mosquito carries malaria parasites. We also develop a cheap, nondestructive, and high-throughput DNA extraction workflow that provides template DNA from single mosquitoes for the multiplex PCR, which means specimens producing unexpected results can be returned to for morphological examination. Over 1000 individual mosquitoes can be sequenced in a single MiSeq run, and we demonstrate the panel's power to assign species identity using sequencing data for 40 species from Africa, Southeast Asia, and South America. We also show that the approach can be used to resolve geographic population structure within An. gambiae and An. coluzzii populations, as the population structure determined based on these 62 loci from over 1000 mosquitoes closely mirrors that revealed through whole genome sequencing. The end-to-end approach is quick, inexpensive, robust, and accurate, which makes it a promising technique for very large-scale mosquito genetic surveillance and vector control.

Characterisation of Species and Diversity of Anopheles gambiae Keele Colony

Anopheles gambiae sensu stricto was recently reclassified as two species, An. coluzzii and An. gambiae s.s., in wild-caught mosquitoes, on the basis of the molecular form, denoted M or S, of a marker on the X chromosome. The An. gambiae Keele line is an outbred laboratory colony strain that was developed around 12 years ago by crosses between mosquitoes from 4 existing An. gambiae colonies. Laboratory colonies of mosquitoes often have limited genetic diversity because of small starting populations (founder effect) and subsequent fluctuations in colony size. Here we describe the characterisation of the chromosomal form(s) present in the Keele line, and investigate the diversity present in the colony using microsatellite markers on chromosome 3. We also characterise the large 2La inversion on chromosome 2. The results indicate that only the M-form of the chromosome X marker is present in the Keele colony, which was unexpected given that 3 of the 4 parent colonies were probably S-form. Levels of diversity were relatively high, as indicated by a mean number of microsatellite alleles of 6.25 across 4 microsatellites, in at least 25 mosquitoes. Both karyotypes of the inversion on chromosome 2 (2La/2L+^a) were found to be present at approximately equal proportions. The Keele colony has a mixed M- and S-form origin, and in common with the PEST strain, we propose continuing to denote it as an An. gambiae s.s. line.

Seasonal Genetic Changes of Aedes aegypti (Diptera: Culicidae) Populations in Selected Sites of Cebu City, Philippines

Aedes aegypti (L.) is the primary vector of dengue virus in the Philippines, where dengue is endemic. We examined the genetic changes of Ae. aegypti collected from three selected sites in Cebu city, Philippines, during the relatively wet (2011-2012) and dry seasons (2012 and 2013). A total of 493 Ae. aegypti adults, reared in the laboratory from field-collected larvae, were analyzed using 11 microsatellite loci. Seasonal variation was observed in allele frequencies and allelic richness. Average genetic differentiation (DEST=0.018; FST=0.029) in both dry seasons was higher, due to reduced Ne, than in the wet season (DEST=0.006; FST=0.009). Thus, average gene flow was higher in the wet season than in the dry seasons. However, the overall FST estimate (0.02) inclusive of the two seasons showed little genetic differentiation as supported by Bayesian clustering analysis. Results suggest that during the dry season the intense selection that causes a dramatic reduction of population size favors heterozygotes, leading to small pockets of mosquitoes (refuges) that exhibit random genetic differentiation. During the wet season, the genetic composition of the population is reconstituted by the expansion of the refuges that survived the preceding dry season. Source reduction of mosquitoes during the nonepidemic dry season is thus recommended to prevent dengue re-emergence in the subsequent wet season.

Colonization of malaria vectors under semi-field conditions as a strategy for maintaining genetic and phenotypic similarity with wild populations

Background

Malaria still accounts for an estimated 207 million cases and 627,000 deaths worldwide each year. One proposed approach to complement existing malaria control methods is the release of genetically-modified (GM) and/or sterile male mosquitoes. As opposed to laboratory colonization, this requires realistic semi field systems to produce males that can compete for females in nature. This study investigated whether the establishment of a colony of the vector Anopheles arabiensis under more natural semi-field conditions can maintain higher levels of genetic diversity than achieved by laboratory colonization using traditional methods.

Methods

Wild females of the African malaria vector An. arabiensis were collected from a village in southern Tanzania and used to establish new colonies under different conditions at the Ifakara Health Institute. Levels of genetic diversity and inbreeding were monitored in colonies of An. arabiensis that were simultaneously established in small cage colonies in the SFS and in a large semi-field (SFS) cage and compared with that observed in the original founder population. Phenotypic traits that determine their fitness (body size and energetic reserves) were measured at 10^th generation and compared to founder wild population.

Results

In contrast to small cage colonies, the SFS population of An. arabiensis exhibited a higher degree of similarity to the founding field population through time in several ways: (i) the SFS colony maintained a significantly higher level of genetic variation than small cage colonies, (ii) the SFS colony had a lower degree of inbreeding than small cage colonies, and (iii) the mean and range of mosquito body size in the SFS colony was closer to that of the founding wild population than that of small cage colonies. Small cage colonies had significantly lower lipids and higher glycogen abundances than SFS and wild population.

Conclusions

Colonization of An. arabiensis under semi-field conditions was associated with the retention of a higher degree of genetic diversity, reduced inbreeding and greater phenotypic similarity to the founding wild population than observed in small cage colonies. Thus, mosquitoes from such semi-field populations are expected to provide more realistic representation of mosquito ecology and physiology than those from small cage colonies.

Towards mosquito sterile insect technique programmes: exploring genetic, molecular, mechanical and behavioural methods of sex separation in mosquitoes

When considering a mosquito release programme, one of the first issues to be addressed is how to eliminate/separate the females. The greatest number of options might eventually be available for those who can use transgenic mosquitoes, but the inherent characteristics of the target species may also provide possibilities for interim measures until more efficient methods can be developed. Differences in intrinsic size, in behaviour and in development rate between females and males are often available and useful for sexing. Efficient species-specific systems for eliminating females at the embryo stage have been developed, but most have since been discarded due to lack of use. Ideal systems specifically kill female embryos using some treatment that can be manipulated during production. Such killing systems are far more efficient than using intrinsic sexual differences, but they systems require selectable genetic markers and sex-linkage created by rare random chromosomal rearrangements. While intrinsic sexual differences should not be considered as long-term candidates for the development of robust and efficient sexing approaches, in the absence of these, the accessibility and integration of less efficient systems can provide a stop-gap measure that allows rapid start up with a minimum of investment. The International Atomic Energy Agency is funding over a 5 year period (2013-2018) a new Coordinated Research Project on "Exploring Genetic, Molecular, Mechanical and Behavioural Methods of Sex Separation in Mosquitoes" to network researchers and to address the critical need of genetic sexing strains for the implementation of the sterile insect technique (using radiation-sterilised or transgenic male mosquitoes) and for insect incompatibility technique programmes against disease-transmitting mosquitoes.

Population genetic structure of the malaria vector Anopheles nili in sub-Saharan Africa

Background

Anopheles nili is a widespread efficient vector of human malaria parasites in the humid savannas and forested areas of sub-Saharan Africa. Understanding An. nili population structure and gene flow patterns could be useful for the development of locally-adapted vector control measures.

Methods

Polymorphism at eleven recently developed microsatelitte markers, and sequence variation in four genes within the 28s rDNA subunit (ITS2 and D3) and mtDNA (COII and ND4) were assessed to explore the level of genetic variability and differentiation among nine populations of An. nili from Senegal, Ivory Coast, Burkina Faso, Nigeria, Cameroon and the Democratic Republic of Congo (DRC).

Results

All microsatellite loci successfully amplified in all populations, showing high and very similar levels of genetic diversity in populations from West Africa and Cameroon (mean Rs = 8.10-8.88, mean He = 0.805-0.849) and much lower diversity in the Kenge population from DRC (mean Rs = 5.43, mean He = 0.594). Bayesian clustering analysis of microsatellite allelic frequencies revealed two main genetic clusters in the dataset. The first one included only the Kenge population and the second grouped together all other populations. High Fst estimates based on microsatellites (Fst > 0.118, P < 0.001) were observed in all comparisons between Kenge and all other populations. By contrast, low Fst estimates (Fst < 0.022, P < 0.05) were observed between populations within the second cluster. The correlation between genetic and geographic distances was weak and possibly obscured by demographic instability. Sequence variation in mtDNA genes matched these results, whereas low polymorphism in rDNA genes prevented detection of any population substructure at this geographical scale.

Conclusion

Overall, high genetic homogeneity of the An. nili gene pool was found across its distribution range in West and Central Africa, although demographic events probably resulted in a higher level of genetic isolation in the marginal population of Kenge (DRC). The role of the equatorial forest block as a barrier to gene flow and the implication of such findings for vector control are discussed.

Population structuring of the tsetse Glossina tachinoides resulting from landscape fragmentation in the Mouhoun River basin, Burkina Faso

The impact of landscape fragmentation resulting from human- and climate-mediated factors on the structure of a population of Glossina tachinoides Westwood (Diptera: Glossinidae) in the Mouhoun River basin, Burkina Faso, was investigated. Allele frequencies at five microsatellite loci were compared in four populations. The average distance between samples was 72 km. The sampling points traversed an ecological cline in terms of rainfall and riverine forest ecotype, along a river loop that enlarged from upstream to downstream. Microsatellite DNA demonstrated no structuring among the groups studied (F(ST) = 0.015, P = 0.07), which is contrary to findings pertaining to Glossina palpalis gambiensis Vanderplank in the same geographical area. The populations of G. tachinoides showed complete panmixia (F(IS) = 0, P = 0.5 for the whole sample) and no genetic differentiation among populations or global positioning system trap locations. This is in line with the results of dispersal studies which indicated higher diffusion coefficients for G. tachinoides than for G. p. gambiensis. The impact of these findings is discussed within the framework of control campaigns currently promoted by the Pan African Tsetse and Trypanosomosis Eradication Campaign.

Population structure and geographical subdivision of the Leishmania major vector Phlebotomus papatasi as revealed by microsatellite variation

Multi-locus microsatellite typing (MLMT) has been employed to infer the population structure of Phlebotomus papatasi (Scopoli) (Diptera: Psychodidae) sandflies and assign individuals to populations. Phlebotomus papatasi sandflies were collected from 35 sites in 15 countries. A total of 188 P. papatasi individuals were typed using five microsatellite loci, resulting in 113 different genotypes. Unique microsatellite signatures were observed for some of the populations analysed. Comparable results were obtained when the data were analysed with Bayesian model and distance-based methods. Bayesian statistic-based analyses split the dataset into two distinct genetic clusters, A and B, with further substructuring within each. Population A consisted of five subpopulations representing large numbers of alleles that were correlated with the geographical origins of the sandflies. Cluster B comprised individuals collected in the Middle East and the northern Mediterranean area. The subpopulations B1 and B2 did not, however, show any further correlation to geographical origin. The genetic differentiation between subpopulations was supported by F statistics showing statistically significant (Bonferroni-corrected P < 0.005) values of 0.221 between B2 and B1 and 0.816 between A5 and A4. Identification of the genetic structure of P. papatasi populations is important for understanding the patterns of dispersal of this species and to developing strategies for sandfly control.

Population genetic structure of the malaria vector Anopheles moucheti in south Cameroon forest region

We used recently developed microsatellite DNA markers to explore the population genetic structure of the malaria vector, Anopheles moucheti. Polymorphism at 10 loci was examined to assess level of genetic differentiation between four A. moucheti populations from South Cameroon situated 65-400 km apart. All microsatellite loci were highly polymorphic with a number of distinct alleles per locus ranging from 9 to 17. Fst estimates ranging from 0.0094 to 0.0275 (P < 0.001) were recorded. These results suggest a very low level of genetic differentiation between A. moucheti populations. The recently available microsatellite loci revealed useful markers to assess genetic differentiation between geographical populations of A. moucheti in Cameroon.

Higher genetic variation estimated by microsatellites compared to isoenzyme markers in Aedes aegypti from Rio de Janeiro

Aedes aegypti populations from five districts in Rio de Janeiro were analyzed using five microsatellites and six isoenzyme markers, to assess the amount of variation and patterns of gene flow at local levels. Microsatellite loci were polymorphic enough to detect genetic differentiation of populations collected at small geographic scales (e.g. within a city). Ae. aegypti populations were highly differentiated as well in the city center as in the outskirt. Thus, dengue virus propagation by mosquitoes could be as efficient in the urban area as in the outskirt of Rio de Janeiro, the main entry point of dengue in Brazil.

Genetic and morphometric evidence for population isolation of Glossina palpalis gambiensis (Diptera: Glossinidae) on the Loos islands, Guinea

Allele frequencies at four microsatellite loci, and morphometric features based on 11 wing landmarks, were compared among three populations of Glossina palpalis gambiensis (Diptera: Glossinidae) in Guinea. One population originated from the Loos islands separated from the capital Conakry by 5 km of sea, and the two others originated from the continental mangrove area close to Dubreka, these two groups being separated by approximately 30 km. Microsatellites and wing geometry data both converged to the idea of a separation of the Loos island population from those of the mangrove area. Although occasional contacts cannot be excluded, our results support the hypothesis of the Loos population of tsetse flies being a completely isolated population. This situation will favor a sequenced intervention against human African trypanosomosis and the possibility of an elimination of tsetse from this island.

Water quality and immatures of the M and S forms of Anopheles gambiae s.s. and An. arabiensis in a Malian village

INTRODUCTION

The associations between the immatures of Anopheles gambiae s.s. (Diptera: Culicidae), its M and S forms, and Anopheles arabiensis among and within larval breeding habitats in Banambani, Mali were investigated under varying conditions of water quality and rainfall. The intent was to elucidate on niche partitioning of these taxa.

METHODS

Immatures of An. arabiensis, An. gambiae s.s., and its M and S forms were sampled every alternate day for a month in mid-rainy season from three sampling sites in each of the larval breeding habitats (rock pools, swamp, and puddles). Water quality was characterized by alkalinity, conductivity, dissolved oxygen (D.O.), nitrate, orthophosphate, pH, temperature, total dissolved solids (TDS), and turbidity. A type 3 analysis of the GENMOD model was used to examine the associations between the proportional frequencies of young (first and second instar larvae) and old (third and fourth instar larvae and pupae) or total immatures of species or forms among sampling sites within and among larval breeding habitats during a category of rainfall as influenced by water quality.

RESULTS

Of the 4,174 immatures sampled, 1,300 were molecularly identified to species and forms. Significant association between the proportional frequencies of young larvae of An. arabiensis, An. gambiae s.s., its M and S forms was found among sampling sites within habitats but not among larval breeding habitats. The proportional frequencies of young larvae of M and S forms varied daily perhaps due to recruitment, mortality, and dispersal within habitats. Conductivity and TDS had significant effects when the proportional frequencies of young larvae of M and S forms among sampling sites within habitats were significantly associated. Alkalinity, D.O., orthophosphate, pH, nitrate, temperature and turbidity had no effects on niche partitioning of species and forms among sampling sites within habitats. Rainfall did not affect the frequencies of these immatures.

CONCLUSION

Conductivity and TDS have significant effects on niche partitioning of young larvae of M and S forms among sampling sites within habitats in Banambani, Mali.

Multilevel analyses of genetic differentiation in Anopheles gambiae s.s. reveal patterns of gene flow important for malaria-fighting mosquito projects

Malaria control projects based on the introduction and spread of transgenes into mosquito populations depend on the extent of isolation between those populations. On the basis of the distribution of paracentric inversions, Anopheles gambiae has been subdivided into five subspecific chromosomal forms. Estimating gene flow between and within these forms of An. gambiae presents a number of challenges. We compared patterns of genetic divergence (F(ST)) between sympatric populations of the Bamako and Mopti forms at five sites. We used microsatellite loci within the j inversion on chromosome 2, which is fixed in the Bamako form but absent in the Mopti form, and microsatellites on chromosome 3, a region void of inversions. Estimates of genetic diversity and F(ST)'s suggest genetic exchanges between forms for the third chromosome but little for the j inversion. These results suggest a role for the inversion in speciation. Extensive gene flow within forms among sites resulted in populations clustering according to form despite substantial gene flow between forms. These patterns underscore the low levels of current gene flow between chromosomal forms in this area of sympatry. Introducing refractoriness genes in areas of the genome void of inversions may facilitate their spread within forms but their passage between forms may prove more difficult than previously thought.

Population genetic structure of Anopheles gambiae mosquitoes on Lake Victoria islands, west Kenya

Background

Understanding the genetic structure of island Anopheles gambiae populations is important for the current tactics in mosquito control and for the proposed strategy using genetically-modified mosquitoes (GMM). Genetically-isolated mosquito populations on islands are a potential site for testing GMM. The objective of this study was to determine the genetic structure of A. gambiae populations on the islands in Lake Victoria, western Kenya.

Methods

The genetic diversity and the population genetic structures of 13 A. gambiae populations from five islands on Lake Victoria and six villages from the surrounding mainland area in the Suba District were examined using six microsatellite markers. The distance range of sampling sites varied between 2.5 and 35.1 km.

Results

A similar level of genetic diversity between island mosquito populations and adjacent mainland populations was found. The average number of alleles per locus was 7.3 for the island populations and 6.8 for the mainland populations. The average observed heterozygosity was 0.32 and 0.28 for the island and mainland populations, respectively. A low but statistically significant genetic structure was detected among the island populations (F_ST = 0.019) and between the island and mainland populations (F_ST = 0.003). A total of 12 private alleles were found, and nine of them were from the island populations.

Conclusion

A level of genetic differentiation between the island and mainland populations was found. Large extent of gene flow between the island and mainland mosquito populations may result from wind- or human-assisted dispersal. Should the islands on Lake Victoria be used as a trial site for the release program of GMM, mosquito dispersal between the islands and between the island and the mainland should be vigorously monitored.

Issues in public health entomology

Public health entomology focuses on the population biology of vector-borne infections, seeking to understand how such pathogens perpetuate over time and attempting to devise methods for reducing the burden that they impose on human health. As public health entomology passes its centennial, a series of pervasive research themes and spirited debates characterize the discipline, many reflecting a tension between field and laboratory research. In particular, institutional support for population-based research and training programs has fallen behind that for those using modern lab-based approaches. Discussion of modes of intervention against vector-borne infections (such as deployment of genetically modified vectors, the role of DDT in malaria control, host-targeted acaricides for Lyme disease risk reduction, and truck-mounted aerosol spraying against West Nile virus transmission) illustrates the discipline's need for strengthening population-based research programs. Even with the advent of molecular methods for describing population structure, the basis for anophelism without malaria (or its eastern North American counterpart, ixodism without borreliosis) remains elusive. Such methods have not yet been extensively used to examine the phylogeography and geographical origins of zoonoses such as Lyme disease. Basic ecological questions remain poorly explored: What regulates vector populations? How may mixtures of pathogens be maintained by a single vector? What factors might limit the invasion of Asian mosquitoes into North American sites? Putative effects of "global warming" remain speculative given our relative inability to answer such questions. Finally, policy and administrative issues such as the "no-nits" dictum in American schools, the Roll Back Malaria program, and legal liability for risk due to vector-borne infections serve to demonstrate further the nature of the crossroads that the discipline of public health entomology faces at the start of the 21st Century.

Population genetics of Caenorhabditis elegans: the paradox of low polymorphism in a widespread species

Caenorhabditis elegans has become one of the most widely used model research organisms, yet we have little information on evolutionary processes and recent evolutionary history of this widespread species. We examined patterns of variation at 20 microsatellite loci in a sample of 23 natural isolates of C. elegans from various parts of the world. One-half of the loci were monomorphic among all strains, and overall genetic variation at microsatellite loci was low, relative to most other species. Some population structure was detected, but there was no association between the genetic and geographic distances among different natural isolates. Thus, despite the nearly worldwide occurrence of C. elegans, little evidence was found for local adaptation in strains derived from different parts of the world. The low levels of genetic variation within and among populations suggest that recent colonization and population expansion might have occurred. However, the patterns of variation are not consistent with population expansion. A possible explanation for the observed patterns is the action of background selection to reduce polymorphism, coupled with ongoing gene flow among populations worldwide.

Genetic markers for study of the anopheline vectors of human malaria

Human malaria is truly a disease of global proportions and is one of the most broadly distributed vector-borne infections. Anopheline mosquitoes are the exclusive vectors of human malaria. A handful of species predominate as the most notorious malaria vectors, but the species and forms involved in the transmission of human malaria world-wide are incredibly diverse. Many of the anophelines that vector malaria exist as members of species complexes that often contain vector and non-vector species. Additionally, single anopheline species often exhibit significant heterogeneity across the species' range. This phenotypic and genotypic plasticity exacerbates the difficulties in identification of vector populations and implementation of effective surveillance and control strategies. Polytene chromosome investigations were among the first to provide researchers with tangible genetic markers that could be used to differentiate between what are now recognised as species and chromosomal forms of anopheline mosquitoes. The advent of the polymerase chain reaction gave access to the molecular genetics of genomes and the techniques that followed have facilitated investigation of the genetics of individual specimens or population size samples. The variety and number of genetic markers available for the study of malaria vectors has literally exploded in the last 10 years. Markers have expanded from the 'traditional tools' to include a vast array of molecular markers. Contemporary markers range from what are now referred to as 'classical genetic markers' to methods used to detect and identify single nucleotide polymorphisms and finally to highly polymorphic markers. One of the greatest advantages of this wide variety of genetic markers is that researchers may choose to utilise any combination of markers or techniques to address multifaceted questions relating to malaria transmission. These molecular markers have proven useful in a wide variety of applications including molecular taxonomy, evolutionary systematics, population genetics, genetic mapping, and investigation of defined phenotypes.

Morphological variability in the malaria vector, Anopheles moucheti, is not indicative of speciation: evidences from sympatric south Cameroon populations

Anopheles moucheti is a major human malaria vector in the vicinity of slow moving rivers in the tropical forests of Central Africa. Morphological variations in natural populations of A. moucheti led to the designation of three morphological forms named A. moucheti moucheti, A. moucheti nigeriensis and A. moucheti bervoetsi. Using allozyme markers, we investigated to which extent morphological and/or geographical populations of A. moucheti were genetically differentiated. Mosquitoes were collected from four villages 20-200 km distant apart in south Cameroon, where specimens from each morphological form were found in sympatry. All populations appeared highly homogenous across both morphological type and geographic location. Significant genetic differentiation was only observed between two locations 150 km apart (F(st)=0.029; P=0.006), while no pairwise F(st) estimate between morphological forms reached statistical significance. Further evidence against any taxonomic value of this morphological classification was provided by direct observation of morphological variation within the progeny of field-collected females from all three types. Single female offspring always belonged to at least two morphologically recognised types and most often, a mixture of all three forms was observed. Our results therefore demonstrate that morphological variability within A. moucheti natural populations is not indicative of speciation. With this respect, restricted migration of individuals across river systems may be a more important factor in shaping population genetic structure of A. moucheti.

Genetic differentiation and diagnostic loci among Anopheles (Nyssorhynchus) rangeli, An. (Nys.) nuneztovari, and An. (Nys.) dunhami (Diptera: Culicidae) in the Brazilian Amazon

Genetic variability and divergence were estimated for populations of Anopheles rangeli Gabaldón, Cova-Garcia & Lopes, Anopheles nuneztovari Gabald6n cytotype A, and Anophels dunhami Causey from the Brazilian Amazon using isozyme electrophoresis. These species are included in the Oswaldoi subgroup, subgenus Nyssorhynchus (Diptera: Culicidae). Thirteen enzymes yielded a total of 22 loci, of which 14 were monomorphic in the three species. Three diagnostic loci (Gpi-1, Hk-1, and Me) and a strong differentiation in the Mdh locus were found between An. rangeli and An. nuneztovari. Five diagnostic loci (Mdh, Gpi-l, Hk-1, Gpd, and Me) separated An. rangeli from An. dunhami, whereas one diagnostic locus (Gpd) separated An. nuneztovari from An. dunhami. Moderate differentiation was observed in the Est-5 and Pgm loci between An. rangeli and An. nuneztovari, and between An. nuneztovari and An. dunhami. Anopheles dunhami had the highest values for three indices of genetic variability, whereas An. rangeli showed the lowest values for mean number of alleles per locus and mean heterozygosity. Nei's genetic distance was highest between An. rangeli and An. dunhami (0.280) and lowest between An. nuneztovari and An. dunhami (0.072). Between An. rangeli and An. nuneztovari the genetic distance was 0.237. Anopheles dunhami and An. nuneztovari are sister species very closely related and may have a recent evolutionary origin. Anopheles rangeli probably diverged before the other two species separated. This is the first record of An. dunhami in Coari (Amazonas, Brazil) and only the third collection site of this species since its description in 1945.

Molecular systematics of Anopheles: from subgenera to subpopulations

The century-old discovery of the role of Anopheles in human malaria transmission precipitated intense study of this genus at the alpha taxonomy level, but until recently little attention was focused on the systematics of this group. The application of molecular approaches to systematic problems ranging from subgeneric relationships to relationships at and below the species level is helping to address questions such as anopheline phylogenetics and biogeography, the nature of species boundaries, and the forces that have structured genetic variation within species. Current knowledge in these areas is reviewed, with an emphasis on the Anopheles gambiae model. The recent publication of the genome of this anopheline mosquito will have a profound impact on inquiries at all taxonomic levels, supplying better tools for estimating phylogeny and population structure in the short term, and ultimately allowing the identification of genes and/or regulatory networks underlying ecological differentiation, speciation, and vectorial capacity.

Microsatellite markers for population genetic studies in Aedes aegypti (Diptera: Culicidae) from Côte d'Ivoire: evidence for a microgeographic genetic differentiation of mosquitoes from Bouaké

In West Africa, Aedes aegypti (Diptera: Culicidae) (Linnaeus, C., 1762. Zweyter Theil, enhalt Beschreibungen veschiedener wichtiger Naturalien. In: Hasselquist, F. (Ed.), Reise nach Palastina in den Jahren von 1749 bis 1752, Rostock, Germany, pp. 267-606) represents the principal vector of yellow fever. This study reports the use of microsatellite markers to characterise various A. aegypti populations from Côte d'Ivoire according to a north-south transect, and to perform a temporal genetic survey of the mosquitoes. Three microsatellite loci were used to analyse individuals from four different places: Kabolo, Bouaké, and two different districts of Abidjan. We found that the four populations are genetically distinct except the two Abidjan populations. In the Bouaké population, the coexistence of two cryptic species, not morphologically distinguishable, seems to account for the extensive heterozygote deficiency observed. Comparison of mosquitoes from Bouaké 1 year apart indicated that a dramatic change occurred in the structuring of this population over time. Taken together these results indicate that microsatellite markers could be useful for identifying various populations of A. aegypti on a microgeographic scale and to assess for temporal variation within mosquito populations.

Spatial and habitat distribution of Anopheles gambiae and Anopheles arabiensis (Diptera: Culicidae) in Banambani village, Mali

We studied the larval distribution and composition of Anopheles arabiensis Patton, An. gambiae s.s. Giles, and its forms, among local habitats; and their association with the adults between these habitats in Banambani village, Mali during the mid-rainy seasons of 1997-1999. For species and form identification we used polymerase chain reaction (PCR) and PCR-restriction fragment-length polymorphism (RFLP). Differences among species in the distribution of larvae were observed in 1998, but not in 1997 or 1999, although they were on the borderline of statistical significance. Differences among the M and S molecular forms were statistically significant in 1999 when rainfall was high, but not in the two prior, drier sampling periods. Combining all information into the Fisher multiple comparisons test, there were statistically significant differences between species and molecular forms during the 3-yr study period. Hybrid larvae between the M and S forms were observed (0.57%), the first such observation to our knowledge. In spite of differences among larval distribution, no differences of adult species composition were observed among habitats. Factors that influence the distributions of An. gambiae larval populations are discussed.

New molecular marker for Trypanosoma (Duttonella) vivax identification

Trypanosoma vivax is a widespread hemoparasite in tropical areas and is pathogenic to ruminant domestic livestock as well as wild ruminants. The accurate identification of parasites in both hosts and vectors is crucial for epidemiological studies and disease control programs. We describe here the development of molecular markers specific for T. vivax identification. These markers were used to identify mouthpart infections in field-collected tsetse flies from Cameroon. The markers target the genomic sequence of a species-specific antigen from the bloodstream stages. No cross amplification with other trypanosome species was observed, which makes the markers a reliable tool to detect T. vivax infections, both in hosts and vectors. The PCR-amplified sequence contains a (CA)(n) microsatellite repeat for which 11 different alleles were identified. This microsatellite, which showed high polymorphism, provides a suitable marker for population genetic studies.

DNA analysis of transferred sperm reveals significant levels of gene flow between molecular forms of Anopheles gambiae

Anopheles gambiae populations in west Africa are complex, being composed of multiple, sympatric subpopulations. Recent studies have failed to reveal significant genetic differences among subpopulations, stimulating a debate regarding the levels of gene flow among them. The observed homogeneity may be the consequence of substantial contemporary gene flow or it may be that reproductive isolation is complete, but too recent for the accumulation of significant levels of genic divergence. Here, we report the results of a study estimating contemporary levels of gene flow between An. gambiae subpopulations by analysing females and transferred sperm removed from their reproductive systems. A total of 251 female and associated sperm extracts was analysed from a single site in Mali. Two molecular forms of An. gambiae, the M- and S-forms, occurred in sympatry at this site. Overall, we found very strong positive assortative mating within forms, however, we did observe significant hybridization between forms. In the M subpopulation 2/195 females (1.03%) contained sperm from S-form males and in 55 S-form females we found one female containing M-form sperm (1.82%). We also identified a mated M xS hybrid adult female. From mating frequencies, we estimate the Nem between the M- and S-form at 16.8, and from the adult hybrid frequency at 5.6. These values are consistent with our earlier estimate, based on FST for 21 microsatellite loci in which Nem = 5.8. We conclude that the general lack of genetic divergence between the M and S subpopulations of An. gambiae can be explained entirely by contemporary gene flow.

SSCP analysis of cDNA markers provides a dense linkage map of the Aedes aegypti genome

An intensive linkage map of the yellow fever mosquito, Aedes aegypti, was constructed using single-strand conformation polymorphism (SSCP) analysis of cDNA markers to identify single nucleotide polymorphisms (SNPs). A total of 94 A. aegypti cDNAs were downloaded from GenBank and primers were designed to amplify fragments <500 bp in size. These primer pairs amplified 94 loci, 57 (61%) of which segregated in a single F(1) intercross family among 83 F(2) progeny. This allowed us to produce a dense linkage map of one marker every 2 cM distributed over a total length of 134 cM. Many A. aegypti cDNAs were highly similar to genes in the Drosophila melanogaster genome project. Comparative linkage analysis revealed areas of synteny between the two species. SNP polymorphisms are abundant in A. aegypti genes and should prove useful in both population genetics and mapping studies.

Microsatellite loci are not abundant in all arthropod genomes: analyses in the hard tick, Ixodes scapularis and the yellow fever mosquito, Aedes aegypti

Plasmid libraries enriched for microsatellites were generated in the tick, Ixodes scapularis and in the mosquito Aedes aegypti. Libraries were enriched for genomic DNA containing (AC)n, (AG)n, (ATG)n, (CAG)n, (TAG)n, (AAT)n, (CTGY)n or (GATA)n motifs. Clones containing each motif were sequenced in both species for PCR primer design. In I. scapularis, most primers amplified a single locus and alleles varied in the number of microsatellite repeats and segregated as codominant markers. In contrast (AC)n, (TAG)n and (GATA)n microsatellite loci extracted from Ae. aegypti appeared to be members of multigene families. A primer pair designed to amplify a particular TAG locus instead amplified many independently segregating loci, some of which did not contain TAG microsatellites. Alleles at the TAG loci segregated as dominant markers and there was limited evidence for length variation among alleles. These results suggest that microsatellite loci are not universally abundant in arthropod genomes nor do alleles always segregate as codominant markers.

A preliminary study of the population genetics of Aedes aegypti (Diptera: Culicidae) from Mexico using microsatellite and AFLP markers

Dengue fever recently reemerged in the Americas. Because vaccines are still under development, dengue prevention depends entirely on vector control. Since Aedes aegypti (Linnaeus, 1762) is the principal vector of this arbovirus, knowledge of the genetic structure of the insect is therefore required to maintain effective vector control strategies and to estimate levels of gene flow from which movement can be inferred. This preliminary study uses microsatellite and amplified fragment length polymorphism (AFLP) markers, to provide insights into genetic diversity of A. aegypti populations from different districts of two towns, located in the north-west of Mexico, Hermosillo and Guaymas. Although the microsatellites used were found to display limited polymorphism, they allowed discrimination between mosquitoes from the northern and the southern districts of Hermosillo. Using AFLP markers, clustering of individuals from the same town and from the same district was observed. Data from microsatellite and AFLP markers analysis both suggest that reinvasion of A. aegypti probably occurs from Guaymas to Hermosillo.

Microsatellite DNA polymorphism and heterozygosity among field and laboratory populations of Anopheles gambiae ss (Diptera: Culicidae)

We compared microsatellite polymorphism at nine loci located on chromosome 3 among two colonies and a field population of Anopheles gambiae sensu stricto Giles mosquitoes. Numbers of microsatellite alleles observed at each locus and mean heterozygosities were drastically reduced among laboratory colonies. Genetic analysis of the field population used in this study revealed an unprecedented frequency of rare alleles (<0.05). In contrast, colony samples revealed large numbers of alleles with frequencies >0.50. Partitioning of field data to assess the impact of rare alleles, null alleles, and sample size on estimates of mean heterozygosity revealed the plasticity of this measurement and suggests that heterozygosity may be reliably estimated from relatively small collections using microsatellites.

Genetic and physical mapping in mosquitoes: molecular approaches

The genetic background of individual mosquito species and populations within those species influences the transmission of mosquito-borne pathogens to humans. Technical advances in contemporary genomics are contributing significantly to the detailed genetic analysis of this mosquito-pathogen interaction as well as all other aspects of mosquito biology, ecology, and evolution. A variety of DNA-based marker types are being used to develop genetic maps for a number of mosquito species. Complex phenotypic traits such as vector competence are being dissected into their discrete genetic components, with the intention of eventually using this information to develop new methods to prevent disease transmission. Both genetic- and physical-mapping techniques are being used to define and compare genome architecture among and within mosquito species. The integration of genetic- and physical-map information is providing a sound framework for map-based positional cloning of target genes of interest. This review focuses on advances in genome-based analysis and their specific applications to mosquitoes.

Enzymatic analysis in Anopheles nuneztovari Gabaldón (Diptera, Culicidae)

Enzymatic analysis in Anopheles nuneztovari was made using four populations from the Brazilian Amazon and two from Colombia. The enzymes ME and XDH presented a monomorphic locus in all of the studied populations. EST and LAP presented a higher number of loci. In EST, genetic variation was observed in the five loci; LAP presented four loci, with allec variation in two loci. In IDH, three activity regions were stained, with genetic variation for locus Idh-1 in the Brazilian Amazon populations. A locus for MDH was observed, with genetic variation in the six populations. A region was verified for ACON, with four alleles in Sitronela and three in the other populations. PGM constituted one locus, with a high variability in the Brazilian Amazon populations. A locus was observed for 6-PGD with allelic variation in all of the populations with the exception of Tibú. Enzyme PGI presented two loci, both with genetic variability in the Tucuruí population. The enzyme alpha-GPD showed an activity region with polymorphism in the Tucuruí, Tibú and Sitronela populations. The phenotypic variations detected for these enzymes suggest that four (EST, LAP, ACON and PGM) possess monomeric structures and five (IDH, MDH, 6-PGD, PGI and alpha-GPD) dimeric structures in their proteins. These enzymes constitute in important markers to estimate variability and genetic divergence in natural populations of A. nuneztovari.

Persistence of Anopheles arabiensis during the severe dry season conditions in Senegal: an indirect approach using microsatellite loci

Variation at nine microsatellite loci was investigated to understand how Anopheles arabiensis populations survive the dry season in the sahelian region of Senegal. Low estimates of genetic differentiation (F(ST) = 0.012, R(ST) = 0.009) between two populations, 250 km apart, suggested extensive gene flow across this distance. Despite extreme seasonal fluctuation in abundance with dry season minima in which mosquitoes virtually disappeared, allele frequencies remained stable over time in the village of Barkedji from August 1994 to December 1997 (including four rainy seasons and three dry seasons). The effective population size (Ne) was estimated to be 601 with 95% CI (281, 1592), providing strong evidence against annual bottlenecks. Differences in measures of genetic diversity and linkage disequilibrium between the dry and the rainy seasons were not detected. These results suggest that despite extreme minima in local density, An. arabiensis maintains large permanent deme spread out over large area.

Genetic differentiation of Anopheles claviger s.s. in France and neighbouring countries

An investigation of polymorphism of 11 autosomal and one sex-linked allozyme loci was made on 18 samples of Anopheles claviger Meigen (Diptera: Culicidae) from localities across France and neighbouring sites in Germany and Switzerland, plus one sample of Anopheles petragnani Del Vecchio from the French Pyrénées. Genetic differentiation between these two sibling species was confirmed (Nei genetic distance 0.33-0.44) and two genetically distinct groups of populations were identified within An. claviger. These two forms of An. claviger showed contiguous geographical distributions, Group I found across western and Central France, Group II in eastern France and nearby parts of Germany and Switzerland. The two groups were in contact in a region near the Rhone Valley where two intermediate samples were found. The taxonomic significance of this finding is discussed in the context of the recent climatic history of Europe and in relation to the vector potential of each member of the An. claviger complex.

Evidence for extensive genetic differentiation among populations of the malaria vector Anopheles arabiensis in Eastern Africa

We describe the geographical population structure of the malaria vector Anopheles arabiensis in Eastern Africa. Allelic variation at eight microsatellite loci was scored in samples from nine localities along a 4500 km transect from Sudan to Mozambique. Highly significant differences in genotype frequencies were found between all populations separated by more than 200 km. Populations within Malawi separated by 191 km were indistinguishable, as were those within Sudan separated by 134 km. FST and rhoST gave significant estimates of isolation by distance. These data, lead us to conclude that there are extensive barriers to gene flow in this region. The high estimates of Nm (9.4 from FST and 5.2 from rhoST) indicate recent range expansion in this species rather than extensive contemporary gene flow.

Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France

Aphis gossypii is an aphid species that is found throughout the world and is extremely polyphagous. It is considered a major pest of cotton and cucurbit species. In Europe, A. gossypii is assumed to reproduce exclusively by apomictic parthenogenesis. The present study investigates the genetic diversity of A. gossypii in a microgeographic, fragmented habitat consisting of eight glasshouses of cucurbit crops. This analysis, which was based on the results from seven microsatellite loci, has confirmed that A. gossypii populations in southern France are primarily asexual, as only 12 nonrecombinant genotypic classes (clones) were identified from 694 aphids. Moreover, a high proportion of the aphids (87%) had one of three common genotypes. No significant correlation was found between genotypic class and host plant species. Within a glasshouse population of A. gossypii, a significant reduction in clonal diversity was observed as the spring/summer season progressed. The final predominance of a clone could result from interclonal competition. At the microgeographic level (i.e. glasshouses within a 500-m radius), significant genetic subdivision was detected and could be attributed to founder effects and the limitation of gene flow imposed by the enclosed nature of the glasshouse structure. Finally, the three common clones of A. gossypii detected in 1996 reappeared in spring 1997 following the winter extinction, together with rare clones that had not previously been seen. The probability that A. gossypii overwinters within refuges at a microgeographic scale from which populations are renewed each spring is discussed.

Intraspecific variability in natural populations of Glossina palpalis gambiensis from West Africa, revealed by genetic and morphometric analyses

Glossina palpalis gambiensis Vanderplank (Diptera: Glossinidae) from West Africa (Senegal and Burkina Faso) were analysed for microsatellite DNA polymorphisms and size of the wings. In the overall sample a strong heterozygote deficiency was found at two polymorphic microsatellite loci. It led to a highly significant value of Fis (within-sample heterozygote deficit) in the western zone of Sideradougou area in Burkina Faso. Genetic differentiation was significant on a macrogeographic scale, i.e. between tsetse coming from Senegal and Burkina Faso. Wing measures also differed between these two countries; flies from Senegal appeared to be smaller. Microsatellite loci further allowed differentiation of populations of G. palpalis gambiensis trapped on the same hydrographic network a few kilometres apart. The results are interpreted as indicating that further investigations will allow the study of genetic variability of tsetse flies in relation to the dynamics of transmission of human and animal trypanosomoses.

Microsatellite markers and genotyping procedures for Anopheles gambiae

The procedures outlined in this article by Rui Wang, Fotis Kafatos and Liangbiao Zheng are well suited to studies of field mosquito populations, and also to the genetic mapping of qualitative and quantitative traits of Anopheles gambiae, a major malaria vector in Africa. An accurate, reproducible and high-throughput microsatellite genotyping procedure has been established.

Complexities in the genetic structure of Anopheles gambiae populations in west Africa as revealed by microsatellite DNA analysis

Chromosomal forms of Anopheles gambiae, given the informal designations Bamako, Mopti, and Savannah, have been recognized by the presence or absence of four paracentric inversions on chromosome 2. Studies of karyotype frequencies at sites where the forms occur in sympatry have led to the suggestion that these forms represent species. We conducted a study of the genetic structure of populations of An. gambiae from two villages in Mali, west Africa. Populations at each site were composed of the Bamako and Mopti forms and the sibling species, Anopheles arabiensis. Karyotypes were determined for each individual mosquito and genotypes at 21 microsatellite loci determined. A number of the microsatellites have been physically mapped to polytene chromosomes, making it possible to select loci based on their position relative to the inversions used to define forms. We found that the chromosomal forms differ at all loci on chromosome 2, but there were few differences for loci on other chromosomes. Geographic variation was small. Gene flow appears to vary among different regions within the genome, being lowest on chromosome 2, probably due to hitchhiking with the inversions. We conclude that the majority of observed genetic divergence between chromosomal forms can be explained by forces that need not involve reproductive isolation, although reproductive isolation is not ruled out. We found low levels of gene flow between the sibling species Anopheles gambiae and Anopheles arabiensis, similar to estimates based on observed frequencies of hybrid karyotypes in natural populations.

Polymorphic microsatellite markers for population analysis of a tephritid pest species, Bactrocera tryoni

To obtain a set of microsatellite markers for the Queensland fruit fly Bactrocera tryoni, a genomic library was screened with a number of simple repeat oligonucleotide probes. Sequencing recovered 22 repeat loci. The microsatellite sequences were short, with repeat numbers ranging from five to 11. Of these, 16 polymerase chain reaction (PCR) primer sets yielded amplifiable products, which were tested on 53 flies from five widely separated sites. All loci showed polymorphism in the population sample, with the number of alleles ranging from two to 16. Several dinucleotide repeats showed alleles separated by single-base differences and multiple steps, suggesting a mutation process more complex than the stepwise mutation model.

Microsatellite markers for genetic population studies in Glossina palpalis gambiensis (Diptera: Glossinidae)

Little is known about intraspecific variability in tsetse flies and its consequences for vectorial capacity. Microsatellite markers have been developed for Glossina palpalis gambiensis. Three loci have been identified and showed size polymorphisms for insectarium samples. G. palpalis gambiensis from Burkina Faso were also subjected to PCR to investigate then genetic variability. Amplifications were observed in different species belonging to the palpalis group. These molecular markers will be useful to estimate gene flow within G. palpalis gambiensis populations and analysis could be extended to related species.

Use of short tandem repeats for the analysis of genetic variability in sympatric populations of Anopheles gambiae and Anopheles arabiensis

Anopheles gambiae and An. arabiensis were analysed at 30 short tandem repeat (STR) loci originally developed for use in An. gambiae. All specimens were collected from the same village in Kilifi district, coastal Kenya. All 30 loci were amplified in the An. gambiae specimens, whereas 25 out of 30 loci (83.3%) were successfully amplified in the An. arabiensis specimens. Both species had similar levels of polymorphism for the loci that were amplified (93.3% for An. gambiae and 92% for An. arabiensis). Median FST and RST values between the two species were 0.249 and 0.197, respectively, corresponding to Nm values of 0.75 and 0.51, respectively, and suggesting limited interchange of genes between these species. These, together with the relatively high Nei unbiased genetic distance (0.202) between the two sibling species, are consistent with the occurrence of sympatric species with limited gene flow. FST/RST values for individual loci varied greatly (FST range 0.00-0.87; RST range 0.00-0.73), indicating that the loci differ in their ability to measure levels of differentiation between these two species. Location of loci within paracentric inversions seems to be an important factor affecting levels of differentiation measured by the different loci.

Polymorphic microsatellites in Simulium damnosum s.l. and their use for differentiating two savannah populations: implications for epidemiological studies

In West Africa, Onchocerca volvulus, the cause of human onchocerciasis, is transmitted by sibling species of the Simulium damnosum complex. Little is known about blackfly intraspecific variability and its consequences on vectorial capacity. This study reports the use of microsatellite markers for differentiating populations of S. damnosum s.l. Five microsatellite loci were characterized and used to analyze individuals from two savannah populations in Mali, 120 km apart. Four loci were highly polymorphic, having 8-12 alleles per locus and gene diversities ranging from 77.9 to 88.2%. A significant heterozygote deficiency was observed in the two populations. This may arise from inbreeding, population structure (the Walhund effect), or the presence of null alleles. To test this last hypothesis, new primers were designed for two loci and used to analyze homozygous individuals. After correcting for null alleles, heterozygote deficit persisted. Population subdivision in the two foci remains the most likely explanation. Our results indicate that microsatellite markers could differentiate fly populations, making them valuable tools for the study of population genetic structure.

Four years' entomological study of the transmission of seasonal malaria in Senegal and the bionomics of Anopheles gambiae and A. arabiensis

From 1993 to 1996, an entomological survey was conducted in the village of Ndiop, Senegal, as part of a research programme on malaria epidemiology and the mechanisms of protective immunity. Mosquitoes were captured on human bait and by indoor spraying. Species from the Anopheles gambiae complex were identified using the polymerase chain reaction, and Plasmodium falciparum infections were detected by enzyme-linked immunosorbent assay for circumsporozoite protein. The vector species identified were A. gambiae (33.9%), A. arabiensis (63.2%), A. melas (0.3%) and A. funestus (2.5%). Similar proportions of A. gambiae (74.2%) and A. arabiensis (73.8%) contained human blood; 27.0% of A. gambiae and 28.3% of A. arabiensis had fed on cattle. The sporozoite rates were similar for A. gambiae (3.2%) and A. arabiensis (3.7%). The annual entomological inoculation rates varied greatly depending on the year. There were 63, 17, 37 and 7 infected bites per person per year in 1993, 1994, 1995 and 1996 respectively. Transmission was highly seasonal, from July to October. A. arabiensis was responsible for 66% of malaria transmission, A. gambiae for 31%, and A. funestus for 3%.

Population genetic structure of Ixodes ricinus in Switzerland from allozymic data: no evidence of divergence between nearby sites

Ixodes ricinus is a vector and reservoir of numerous infectious agents, especially Borrelia burgdorferi, the agent of Lyme disease. In Switzerland, its ecology and physiology have been well studied. Moreover, the foci of some infectious agents transmitted by this tick are identified. They can display relatively to extremely small geographical size depending on the diseases considered. In order to understand how the gene flows occur and to characterise the genetic structure of Ixodes ricinus populations, we used an indirect method based on genetic markers: allozymes. The sampling was carried out in 5 localities. Eighteen loci were analysed and 2 appeared polymorphic. This shows the low allozymic variability displayed by Ixodes ricinus. Based on these 2 loci, the populations appeared panmictic in Switzerland. This may be explained by the wide range of vertebrate species this tick can infest, especially birds. However, the result is surprising if we consider the extreme localisation of the foci of some infectious agents. We conclude that more powerful genetic markers could be used in order to better understand the epidemiology of tick-borne diseases in Switzerland.

Microsatellite markers for genetic population studies in Glossina palpalis (Diptera: Glossinidae)

Little is known about tsetse intraspecific variability and its consequences on vectorial capacity. Since isoenzyme analyses revealed little polymorphism, microsatellite markers have been developed for Glossina palpalis gambiensis species. Three loci have been identified and showed size polymorphisms for insectarium samples. Moreover, amplifications were observed in different species belonging to palpalis group. These molecular markers will be useful to estimate gene flow within G. p. gambiensis populations and analyses could be extended to related species.