Genetic variation in arthropod vectors of disease-causing organisms: obstacles and opportunities

Virulence potential of Rickettsia amblyommatis for spotted fever pathogenesis in mice

Rickettsia amblyommatis belongs to the spotted fever group of Rickettsia and infects Amblyomma americanum (Lone Star ticks) for transmission to offspring and mammals. Historically, the geographic range of A. americanum was restricted to the southeastern USA. However, recent tick surveys identified the progressive northward invasion of A. americanum, contributing to the increased number of patients with febrile illnesses of unknown etiology after a tick bite in the northeastern USA. While serological evidence strongly suggests that patients are infected with R. amblyommatis, the virulence potential of R. amblyommatis is not well established. Here, we performed a bioinformatic analysis of three genome sequences of R. amblyommatis and identified the presence of multiple putative virulence genes whose products are implicated for spotted fever pathogenesis. Similar to other pathogenic spotted fever rickettsiae, R. amblyommatis replicated intracellularly within the cytoplasm of tissue culture cells. Interestingly, R. amblyommatis displayed defective attachment to microvascular endothelial cells. The attachment defect and slow growth rate of R. amblyommatis required relatively high intravenous infectious doses to produce dose-dependent morbidity and mortality in C3H mice. In summary, our results corroborate clinical evidence that R. amblyommatis can cause mild disease manifestation in some patients.

Vector Competence for DENV-2 Among Aedes albopictus (Diptera: Culicidae) Populations in China

Aedes albopictus is a vector of over 20 arboviruses that has spread throughout the world, mainly in the second half of the twentieth century. Approximately 50–100 million people are infected with dengue virus (DENV) transmitted by Aedes mosquitoes each year, leading to heavy economic burdens for both governments and individuals, among countless other negative consequences. Understanding the vector competence of vector species is critical for effectively preventing and controlling vector-borne diseases. Accordingly, in this study, vector competence was evaluated by quantitative analysis of DENV-2 loads in mosquito tissues (midguts, heads, and salivary glands) and whole mosquitoes through real-time quantitative polymerase chain reaction (RT-qPCR) analysis. Wolbachia and the expression of immune-associated genes (Rel1, Rel2, Dicer2, and STAT) in mosquitoes were also detected by RT-qPCR to explore their impact on vector competence. The amount of DENV-2 in the mosquito midguts, heads, and salivary glands from southern-western China were found to be lower than those from eastern-central-northern China. The DENV-2 loads in whole mosquitoes showed a negative correlation with Rel1 gene (r = -0.285, P = 0.011) and STAT gene expression levels (r = -0.289, P = 0.009). In terms of Wolbachia strains, the density of the wAlbB strain was found to be significantly higher than that of the wAlbA strain in the eight Ae. albopictus populations, and the relative density of the wAlbB strain in mosquitoes from southern-western China was higher than those from eastern-central-northern China. The relative density of the wAlbB strain showed a negative correlation with the mean loads of DENV-2 in the heads (r = -0.729, P = 0.040), salivary glands (r = -0.785, P = 0.021), and whole mosquitoes (r = -0.909, P = 0.002). Thus, there are lower DENV-2 loads in the mosquitoes from southern-western China, which may be related to the innate immunity of mosquitoes as affected by Rel1 in the Toll pathway, STAT in the JAK-STAT pathway, and the relative density of the wAlbB strain.

Strong genetic structure among populations of the tick Ixodes ricinus across its range

Ixodes ricinus is the most common and widely distributed tick species in Europe, responsible for several zoonotic diseases, including Lyme borreliosis. Population genetics of disease vectors is a useful tool for understanding the spread of pathogens and infection risks. Despite the threat to the public health due to the climate-driven distribution changes of I. ricinus, the genetic structure of tick populations, though essential for understanding epidemiology, remains unclear. Previous studies have demonstrated weak to no apparent spatial pattern of genetic differentiation between European populations. Here, we analysed the population genetic structure of 497 individuals from 28 tick populations sampled from 20 countries across Europe, the Middle-East, and northern Africa. We analysed 125 SNPs loci after quality control. We ran Bayesian and multivariate hierarchical clustering analyses to identify and describe clusters of genetically related individuals. Both clustering methods support the identification of three spatially-structured clusters. Individuals from the south and north-western parts of Eurasia form a separated cluster from northern European populations, while central European populations are a mix between the two groups. Our findings have important implications for understanding the dispersal processes that shape the spread of zoonotic diseases under anthropogenic global changes.

Genetic Diversity of Culicoides stellifer (Diptera: Ceratopogonidae) in the Southeastern United States Compared With Sequences From Ontario, Canada

Much of the bluetongue (BT) and epizootic hemorrhagic disease (EHD) research in North America focuses on white-tail deer and Culicoides sonorensis (Wirth & Jones) (Diptera: Ceratopogonidae), though several other biting midge species have been suggested as vectors. Culicoides stellifer (Coquillett) has been associated with hosts susceptible to hemorrhagic disease (HD), and more recently, specimens from Florida have tested positive for EHD and BT viral RNA. If C. stellifer is acting as a vector, this could have an impact on the distribution of HD in North America. To determine if gene flow is occurring across the range of C. stellifer within the southeast United States, a mitochondrial haplotype analysis was performed using the COI gene. Our haplotype network showed no population structure in C. stellifer from Florida, Texas, and South Carolina, as the overall genetic divergence between these sites was equal to the genetic divergence within each. We also compared these haplotypes to published sequences of C. stellifer collected in Ontario, Canada. Surprisingly, the genetic diversity of the flies from Ontario was two times greater than what was observed between the southeast U.S. collection sites. This considerable divergence could be evidence of a cryptic species. A better understanding of the connectivity between C. stellifer populations across all of North America will give insight into the distribution of HD. Our results show that gene flow is occurring between sites in the southeastern United States and potentially throughout the eastern distribution of the species.

Mitochondrial phylogeography and population structure of the cattle tick Rhipicephalus appendiculatus in the African Great Lakes region

Background

The ixodid tick Rhipicephalus appendiculatus is the main vector of Theileria parva, wich causes the highly fatal cattle disease East Coast fever (ECF) in sub-Saharan Africa. Rhipicephalus appendiculatus populations differ in their ecology, diapause behaviour and vector competence. Thus, their expansion in new areas may change the genetic structure and consequently affect the vector-pathogen system and disease outcomes. In this study we investigated the genetic distribution of R. appendiculatus across agro-ecological zones (AEZs) in the African Great Lakes region to better understand the epidemiology of ECF and elucidate R. appendiculatus evolutionary history and biogeographical colonization in Africa.

Methods

Sequencing was performed on two mitochondrial genes (cox1 and 12S rRNA) of 218 ticks collected from cattle across six AEZs along an altitudinal gradient in the Democratic Republic of Congo, Rwanda, Burundi and Tanzania. Phylogenetic relationships between tick populations were determined and evolutionary population dynamics models were assessed by mismach distribution.

Results

Population genetic analysis yielded 22 cox1 and 9 12S haplotypes in a total of 209 and 126 nucleotide sequences, respectively. Phylogenetic algorithms grouped these haplotypes for both genes into two major clades (lineages A and B). We observed significant genetic variation segregating the two lineages and low structure among populations with high degree of migration. The observed high gene flow indicates population admixture between AEZs. However, reduced number of migrants was observed between lowlands and highlands. Mismatch analysis detected a signature of rapid demographic and range expansion of lineage A. The star-like pattern of isolated and published haplotypes indicates that the two lineages evolve independently and have been subjected to expansion across Africa.

Conclusions

Two sympatric R. appendiculatus lineages occur in the Great Lakes region. Lineage A, the most diverse and ubiquitous, has experienced rapid population growth and range expansion in all AEZs probably through cattle movement, whereas lineage B, the less abundant, has probably established a founder population from recent colonization events and its occurrence decreases with altitude. These two lineages are sympatric in central and eastern Africa and allopatric in southern Africa. The observed colonization pattern may strongly affect the transmission system and may explain ECF endemic instability in the tick distribution fringes.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2904-7) contains supplementary material, which is available to authorized users.

The quest for a non-vector psyllid: Natural variation in acquisition and transmission of the huanglongbing pathogen 'Candidatus Liberibacter asiaticus' by Asian citrus psyllid isofemale lines

Genetic variability in insect vectors is valuable to study vector competence determinants and to select non-vector populations that may help reduce the spread of vector-borne pathogens. We collected and tested vector competency of 15 isofemale lines of Asian citrus psyllid, Diaphorina citri, vector of ‘Candidatus Liberibacter asiaticus’ (CLas). CLas is associated with huanglongbing (citrus greening), the most serious citrus disease worldwide. D. citri adults were collected from orange jasmine (Murraya paniculata) hedges in Florida, and individual pairs (females and males) were caged on healthy Murraya plants for egg laying. The progeny from each pair that tested CLas-negative by qPCR were maintained on Murraya plants and considered an isofemale line. Six acquisition tests on D. citri adults that were reared as nymphs on CLas-infected citrus, from various generations of each line, were conducted to assess their acquisition rates (percentage of qPCR-positive adults). Three lines with mean acquisition rates of 28 to 32%, were classified as ‘good’ acquirers and three other lines were classified as ‘poor’ acquirers, with only 5 to 8% acquisition rates. All lines were further tested for their ability to inoculate CLas by confining CLas-exposed psyllids for one week onto healthy citrus leaves (6–10 adults/leaf/week), and testing the leaves for CLas by qPCR. Mean inoculation rates were 19 to 28% for the three good acquirer lines and 0 to 3% for the three poor acquirer lines. Statistical analyses indicated positive correlations between CLas acquisition and inoculation rates, as well as between CLas titer in the psyllids and CLas acquisition or inoculation rates. Phenotypic and molecular characterization of one of the good and one of the poor acquirer lines revealed differences between them in color morphs and hemocyanin expression, but not the composition of bacterial endosymbionts. Understanding the genetic architecture of CLas transmission will enable the development of new tools for combating this devastating citrus disease.

Population structure and genetic diversity of Rhipicephalus microplus in Zimbabwe

Recently there was an expansion in the geographic range of Rhipicephalus microplus in Zimbabwe. In order to understand gene flow patterns and population structure in this highly invasive and adaptable cattle tick, a population genetics study was carried out. Eighty-seven R. microplus tick samples drawn from 5 distinct populations were genotyped using eight polymorphic microsatellite loci. Genetic diversity (H_e) was high (0.755-0.802) in all the populations, suggesting high levels of gene flow with 97% of genetic variation found within populations and 3% amongst populations. No isolation by distance was observed with low but significant genetic differentiation amongst the populations (0-0.076). Most of the sampled individuals had admixed genetic backgrounds, except for those from Matabeleland North whose genetic makeup appeared different from the rest. Rhipicephalus microplus was recently recorded in this area and the environmental conditions do not support survival of the tick there. These results confirm recent range expansion of the tick and the lowest genetic diversity recorded in the Matabeleland North population is suggestive of a founder effect, which may lead to genetic drift. Generally, the very low levels of genetic differentiation amongst the populations could be a result of the frequent movement of livestock from one area to another, which will have implications for disease control. This study offers further opportunities to study evolutionary adaptation of R. microplus in Zimbabwe and southern Africa.

Population genetics analysis of Phlebotomus papatasi sand flies from Egypt and Jordan based on mitochondrial cytochrome b haplotypes

BACKGROUND

Phlebotomus papatasi sand flies are major vectors of Leishmania major and phlebovirus infection in North Africa and across the Middle East to the Indian subcontinent. Population genetics is a valuable tool in understanding the level of genetic variability present in vector populations, vector competence, and the development of novel control strategies. This study investigated the genetic differentiation between P. papatasi populations in Egypt and Jordan that inhabit distinct ecotopes and compared this structure to P. papatasi populations from a broader geographical range.

METHODS

A 461 base pair (bp) fragment from the mtDNA cytochrome b (cyt b) gene was PCR amplified and sequenced from 116 individual female sand flies from Aswan and North Sinai, Egypt, as well as Swaimeh and Malka, Jordan. Haplotypes were identified and used to generate a median-joining network, F _ST values and isolation-by-distance were also evaluated. Additional sand fly individuals from Afghanistan, Iran, Israel, Jordan, Libya, Tunisia and Turkey were included as well as previously published haplotypes to provide a geographically broad genetic variation analysis.

RESULTS

Thirteen haplotypes displaying nine variant sites were identified from P. papatasi collected in Egypt and Jordan. No private haplotypes were identified from samples in North Sinai, Egypt, two were observed in Aswan, Egypt, four from Swaimeh, Jordan and two in Malka, Jordan. The Jordan populations clustered separately from the Egypt populations and produced more private haplotypes than those from Egypt. Pairwise F _ST values fall in the range 0.024-0.648.

CONCLUSION

The clustering patterns and pairwise F _ST values indicate a strong differentiation between Egyptian and Jordanian populations, although this population structure is not due to isolation-by-distance. Other factors, such as environmental influences and the genetic variability in the circulating Le. major parasites, could possibly contribute to this heterogeneity. The present study aligns with previous reports in that pockets of genetic differentiation exists between populations of this widely dispersed species but, overall, the species remains relatively homogeneous.

Evolutionary potential of the extrinsic incubation period of dengue virus in Aedes aegypti

Dengue fever is the most common arboviral disease worldwide. It is caused by dengue viruses (DENV) and the mosquito Aedes aegypti is its primary vector. One of the most powerful determinants of a mosquito's ability to transmit DENV is the length of the extrinsic incubation period (EIP), the time it takes for a virus to be transmitted by a mosquito after consuming an infected blood meal. Here, we repeatedly measured DENV load in the saliva of individual mosquitoes over their lifetime and used this in combination with a breeding design to determine the extent to which EIP might respond to the evolutionary forces of drift and selection. We demonstrated that genetic variation among mosquitoes contributes significantly to transmission potential and length of EIP. We reveal that shorter EIP is genetically correlated with reduced mosquito lifespan, highlighting negative life-history consequences for virus-infected mosquitoes. This work highlights the capacity for local genetic variation in mosquito populations to evolve and to dramatically affect the nature of human outbreaks. It also provides the impetus for isolating mosquito genes that determine EIP. More broadly, our dual experimental approach offers new opportunities for studying the evolutionary potential of transmission traits in other vector/pathogen systems.

Population and Evolutionary Genomics of Amblyomma americanum, an Expanding Arthropod Disease Vector

The lone star tick, Amblyomma americanum, is an important disease vector and the most frequent tick found attached to humans in the eastern United States. The lone star tick has recently experienced a rapid range expansion into the Northeast and Midwest, but despite this emerging infectious threat to wildlife, livestock, and human health, little is known about the genetic causes and consequences of the geographic expansion. In the first population genomic analysis of any tick species, we characterize the genetic diversity and population structure of A. americanum across its current geographic range, which has recently expanded. Using a high-throughput genotyping-by-sequencing approach, we discovered more than 8,000 single nucleotide polymorphisms in 90 ticks from five locations. Surprisingly, newly established populations in New York (NY) and Oklahoma (OK) are as diverse as historic range populations in North and South Carolina. However, substantial population structure occurs among regions, such that new populations in NY and OK are genetically distinct from historic range populations and from one another. Ticks from a laboratory colony are genetically distinct from wild populations, underscoring the need to account for natural variation when conducting transmission or immunological studies, many of which utilize laboratory-reared ticks. An F_ST-outlier analysis comparing a recently established population to a long-standing population detected numerous outlier sites, compatible with positive and balancing selection, highlighting the potential for adaptation during the range expansion. This study provides a framework for applying high-throughput DNA sequencing technologies for future investigations of ticks, which are common vectors of diseases.

Genetic diversity and differentiation between Palearctic and Nearctic populations of Aedimorphus (=Aedes) vexans (Meigen, 1830) (Diptera, Culicidae)

Genetic diversity was studied at allozyme loci in two Palearctic and one Nearctic population of Aedimorphus (=Aedes) vexans, a species of public health and veterinary importance. The population from Serbia was the most polymorphic (P= 35%) with the highest observed heterozygosity (H(o) = 0.027). The lowest observed heterozygosity (H(o) = 0.010) was obtained for the Nearctic population. All analyses based on individual (STRUCTURE analysis) and population level (pairwise F(ST), Nm values, AMOVA, Nei's D value) revealed significant structuring between Nearctic and Palearctic populations, indicating a lack of gene flow and thus, the presence of independent gene pools. Taxon-specific alleles at the diagnostic Ao, Hk-2, Hk-3, Hk-4, Idh-1, and Idh-2 loci were used for identification and separation of Nearctic and Palearctic populations. Population genetics study provided valuable information on the correct distinction of Am. vexans populations and their adaptive potential that could find a future use in the studies of vector competence and development of vector-control strategies.

Wahlund effects and sex-biased dispersal in Ixodes ricinus, the European vector of Lyme borreliosis: new tools for old data

Population genetics can help us better understand species microevolution and population biology, but inferences made from the genetic polymorphisms of field-collected organisms critically rely on sampling design. The population structure of the tick Ixodes ricinus L. (Acari, Ixodidae), a commonly encountered ectoparasite and the principal vector of human Lyme disease in Europe, has been the focus of some study, but many ecological aspects of this species remain poorly understood. Here, we apply a Bayesian clustering approach to observed and simulated data to examine within-population structure in I. ricinus, and to re-analyse patterns of sex-biased dispersal based on this substructure. We found between 18 and 27 distinct clusters within each of the 12 subsamples examined with a significant drop of heterozygote deficits. Parallel analyses on a comparable species, the seabird tick Ixodes uriae, indicated that these clusters can reflect important ecological features of the species (i.e., local host-associations). Analyses that considered the within-population clustering pattern of I. ricinus showed reversed patterns of sex-biased dispersal as compared to raw data (i.e., female-biased instead of male-biased dispersal). Simulated data supported the hypothesis that these scale-dependant patterns could be due to a combination of sex-specific dispersal and mortality. These different results raise new questions on the dispersal and host use strategies of I. ricinus and the potential importance of these ecological features for disease transmission. Furthermore, this work underlines the importance of taking into account patterns of genetic substructure when investigating sex-biased dispersal in natural populations.

Spatial genetic structure of Aedes aegypti mosquitoes in mainland Southeast Asia

Aedes aegypti mosquitoes originated in Africa and are thought to have spread recently to Southeast Asia, where they are the major vector of dengue. Thirteen microsatellite loci were used to determine the genetic population structure of A. aegypti at a hierarchy of spatial scales encompassing 36 sites in Myanmar, Cambodia and Thailand, and two sites in Sri Lanka and Nigeria. Low, but significant, genetic structuring was found at all spatial scales (from 5 to >2000 km) and significant F IS values indicated genetic structuring even within 500 m. Spatially dependent genetic-clustering methods revealed that although spatial distance plays a role in shaping larger-scale population structure, it is not the only factor. Genetic heterogeneity in major port cities and genetic similarity of distant locations connected by major roads, suggest that human transportation routes have resulted in passive long-distance migration of A. aegypti. The restricted dispersal on a small spatial scale will make localized control efforts and sterile insect technology effective for dengue control. Conversely, preventing the establishment of insecticide resistance genes or spreading refractory genes in a genetic modification strategy would be challenging. These effects on vector control will depend on the relative strength of the opposing effects of passive dispersal.

Genetic variability in geographical populations of Culex quinquefasciatus Say (Diptera: Culicidae) from India based on random amplified polymorphic DNA analysis

Genetic variability and environmental factors may influence the refractiveness, propagation of pathogen and transmission of disease. Random amplified polymorphic DNA (RAPD) is one of the widely used molecular markers for population genetic diversity studies. In present study, RAPD is used to ascertain the genetic variability in Culex quinquefasciatus populations collected from various Indian geographical locations. Out of 50 RAPD primers screened, 14 primers exhibited clear, concrete and distinct banding pattern showing up to 100% polymorphism. Primer OPBD3 was tested with DNA of 14 geographical populations from India (including one laboratory population) showed 21 loci representing 14 populations with 100% polymorphism. The genetic diversity among the populations indicated the Shannon index (I) and gene diversity index (H(ST)), 0.48 and 0.31, respectively among the population, displaying rich genetic variation among the Cx. quinquefasciatus populations. Consensus tree showed two clusters indicating the genetic variation among the various geographical populations. The findings of this study may be useful to understand the population variation under different ecological conditions and development of effective vector management strategies.

Tsetse flies: genetics, evolution, and role as vectors

Tsetse flies (Diptera: Glossinidae) are an ancient taxon of one genus, Glossina, and limited species diversity. All are exclusively haematophagous and confined to sub-Saharan Africa. The Glossina are the principal vectors of African trypanosomes Trypanosoma sp. (Kinetoplastida: Trypanosomatidae) and as such, are of great medical and economic importance. Clearly tsetse flies and trypanosomes are coadapted and evolutionary interactions between them are manifest. Numerous clonally reproducing strains of Trypanosoma sp. exist and their genetic diversities and spatial distributions are inadequately known. Here I review the breeding structures of the principle trypanosome vectors, G. morsitans s.l., G. pallidipes, G. palpalis s.l. and G. fuscipes fuscipes. All show highly structured populations among which there is surprisingly little detectable gene flow. Rather less is known of the breeding structure of T. brucei sensu lato vis à vis their vector tsetse flies but many genetically differentiated strains exist in nature. Genetic recombination in Trypanosoma via meiosis has recently been demonstrated in the laboratory thereby furnishing a mechanism of strain differentiation in addition to that of simple mutation. Spatially and genetically representative sampling of both trypanosome species and strains and their Glossina vectors is a major barrier to a comprehensive understanding of their mutual relationships.

Genetic variation affecting host-parasite interactions: different genes affect different aspects of sigma virus replication and transmission in Drosophila melanogaster

In natural populations, genetic variation affects resistance to disease. Knowing how much variation exists, and understanding the genetic architecture of this variation, is important for medicine, for agriculture, and for understanding evolutionary processes. To investigate the extent and nature of genetic variation affecting resistance to pathogens, we are studying a tractable model system: Drosophila melanogaster and its natural pathogen the vertically transmitted sigma virus. We show that considerable genetic variation affects transmission of the virus from parent to offspring. However, maternal and paternal transmission of the virus is affected by different genes. Maternal transmission is a simple Mendelian trait: most of the genetic variation is explained by a polymorphism in ref(2)P, a gene already well known to affect resistance to sigma. In contrast, there is considerable genetic variation in paternal transmission that cannot be explained by ref(2)P and is caused by other loci on chromosome 2. Furthermore, we found no genetic correlation between paternal transmission of the virus and resistance to infection by the sigma virus following injection. This suggests that different loci affect viral replication and paternal transmission.

Transmission of two viruses that cause Barley Yellow Dwarf is controlled by different loci in the aphid, Schizaphis graminum

Clonal populations of the aphid, Schizaphis graminum, have been separated into biotypes based on host preference and their ability to overcome resistance genes in wheat. Recently, several biotypes were found to differ in their ability to transmit one or more of the viruses that cause barley yellow dwarf disease in grain crops, and vector competence was linked to host preference. The genetics of host preference has been studied in S. graminum, but how this may relate to the transmission of plant viruses is unknown. Sexual morphs of a vector and nonvector S. graminum genotype were induced from parthenogenetic females and reciprocal crosses made. Eighty-nine hybrids were generated and maintained by parthenogenesis. Each hybrid was evaluated for its ability to transmit Barley yellow dwarf virus-PAV and Cereal yellow dwarf virus-RPV, and for its ability to colonize two wheat genotypes each expressing a different gene that confers resistance to S. graminum. The F1 genotypes were genetically variable for their ability to transmit virus and to colonize the aphid resistant wheat, but these traits were not genetically correlated. Individual F1 genotypes ranged in transmission efficiency from 0-100% for both viruses, although the overall mean transmission efficiency was similar to the transmission competent parent, indicating directional dominance. The direction of the cross did not significantly affect the vector competency for either virus, suggesting that maternally inherited cytoplasmic factors, or bacterial endosymbionts, did not contribute significantly to the inheritance of vector competency in S. graminum. Importantly, there was no genetic correlation between the ability to transmit Barley yellow dwarf virus and Cereal yellow dwarf virus-RPV in the F1 genotypes. These results taken together indicate that multiple loci are involved in the circulative transmission, and that the successful transmission of these closely related viruses is regulated by different sets of aphid genes.

Biometrical genetic analysis of luteovirus transmission in the aphid Schizaphis graminum

The aphid Schizaphis graminum is an important vector of the viruses that cause barley yellow dwarf disease. We studied the genetic architecture of virus transmission by crossing a vector and a non-vector genotype of S. graminum. F1 and F2 hybrids were generated, and a modified line-cross biometrical analysis was performed on transmission phenotype of two of the viruses that cause barley yellow dwarf: Cereal yellow dwarf virus (CYDV)-RPV and Barley yellow dwarf virus (BYDV)-SGV. Our aims were to (1) determine to what extent differences in transmission ability between vectors and non-vectors is due to net additive or non-additive gene action, (2) estimate the number of loci that determine transmission ability and (3) examine the nature of genetic correlations between transmission of CYDV-RPV and BYDV-SGV. Only additive effects contributed significantly to divergence in transmission of both CYDV-RPV and BYDV-SGV. For each luteovirus, Castle-Wright's estimator for the number of effective factors segregating for transmission phenotype was less than one. Transmission of CYDV-RPV and BYDV-SGV was significantly correlated in the F2 generation, suggesting that there is a partial genetic overlap for transmission of these luteoviruses. Yet, 63% of the F2 genotypes transmitted CYDV-RPV and BYDV-SGV at significantly different rates. Our data suggest that in S. graminum, the transmission efficiency of both CYDV-RPV and BYDV-SGV is regulated by a major gene or set of tightly linked genes, and the transmission efficiency of each virus is influenced by a unique set of minor genes.

The tsetse fly Glossina palpalis palpalis is composed of several genetically differentiated small populations in the sleeping sickness focus of Bonon, Côte d'Ivoire

Glossina palpalis is the main vector of human African trypanosomosis (HAT, or sleeping sickness) that dramatically affects human health in sub-Saharan Africa. Because of the implications of genetic structuring of vector populations for the design and efficacy of control campaigns, G. palpalis palpalis in the most active focus of sleeping sickness in Côte d'Ivoire was studied to determine whether this taxon is genetically structured. High and statistically significant levels of within population heterozygote deficiencies were found at each of the five microsatellite loci in two temporally separated samples. Neither null alleles, short allele dominance, nor trap locations could fully explain these deviations from random mating, but a clustering within each of the two samples into different genetic sub-populations (Wahlund effect) was strongly suggested. These different genetic groups, which could display differences in infection rates and trypanosome identity, were composed of small numbers of individuals that were captured together, leading to the observed Wahlund effect. Implications of this population structure on tsetse control are discussed.