Genetic relationships among Aedes aegypti (Diptera: Culicidae) populations from Argentina using random amplified polymorphic DNA polymerase chain reaction markers

The Symbiotic Continuum Within Ticks: Opportunities for Disease Control

Among blood-sucking arthropods, ticks are recognized as being of prime global importance because of their role as vectors of pathogens affecting human and animal health. Ticks carry a variety of pathogenic, commensal, and symbiotic microorganisms. For the latter, studies are available concerning the detection of endosymbionts, but their role in the physiology and ecology of ticks remains largely unexplored. This review paper focuses on tick endosymbionts of the genera Coxiella, Rickettsia, Francisella, Midichloria, and Wolbachia, and their impact on ticks and tick-pathogen interactions that drive disease risk. Tick endosymbionts can affect tick physiology by influencing nutritional adaptation, fitness, and immunity. Further, symbionts may influence disease ecology, as they interact with tick-borne pathogens and can facilitate or compete with pathogen development within the vector tissues. Rickettsial symbionts are frequently found in ticks of the genera of Ixodes, Amblyomma, and Dermacentor with relatively lower occurrence in Rhipicephalus, Haemaphysalis, and Hyalomma ticks, while Coxiella-like endosymbionts (CLEs) were reported infecting almost all tick species tested. Francisella-like endosymbionts (FLEs) have been identified in tick genera such as Dermacentor, Amblyomma, Ornithodoros, Ixodes, and Hyalomma, whereas Wolbachia sp. has been detected in Ixodes, Amblyomma, Hyalomma, and Rhipicephalus tick genera. Notably, CLEs and FLEs are obligate endosymbionts essential for tick survival and development through the life cycle. American dog ticks showed greater motility when infected with Rickettsia, indirectly influencing infection risk, providing evidence of a relationship between tick endosymbionts and tick-vectored pathogens. The widespread occurrence of endosymbionts across the tick phylogeny and evidence of their functional roles in ticks and interference with tick-borne pathogens suggests a significant contribution to tick evolution and/or vector competence. We currently understand relatively little on how these endosymbionts influence tick parasitism, vector capacity, pathogen transmission and colonization, and ultimately on how they influence tick-borne disease dynamics. Filling this knowledge gap represents a major challenge for future research.

Genome-Wide Screening of Aedes aegypti (Culicidae: Diptera) Populations From Northwestern Argentina: Active and Passive Dispersal Shape Genetic Structure

Aedes aegypti is the primary vector of arboviruses of great impact on human health. Our goal was to assess the spatial genetic structure of Ae. aegypti at the regional and local levels in Northwestern Argentina, an area with high prevalence of dengue fever. We analyzed 59 Ae. aegypti individuals collected from six locations in Northwestern Argentina using nuclear genome-wide Single Nucleotide Polymorphisms (SNPs) generated with double digest Restriction-site Associated DNA Sequencing. We also performed an entomological survey in 70 households in the cities of Orán and Tartagal. An analysis at the regional level indicated that the populations of Ae. aegypti in Northwestern Argentina are spatially structured and present a significant IBD pattern. Our results suggest that passive transport of eggs/immature stages, in both northward and southward directions, plays an important role in structuring Ae. aegypti populations at a regional scale and also as a source for the introduction of novel genetic variants through migration events into established populations. At a local level, we found neither spatial genetic structure nor significant isolation by distance (IBD) in Tartagal, indicating high gene flow within the city and active dispersal. In contrast, samples from Orán formed two clusters with a significant IBD pattern, although weaker than that at a regional level. Both populations showed signs of recent bottleneck events, probably coincident with past eradication campaigns. The entomological survey revealed a high prevalence of Ae. aegypti in both cities, although significantly higher in Tartagal.

Urban Populations of Aedes aegypti (Diptera: Culicidae) From Central Argentina: Dispersal Patterns Assessed by Bayesian and Multivariate Methods

Aedes aegypti (L.), the main vector of dengue and other arboviruses, was declared eradicated from Argentina in 1964; however, in 1987, it was detected again and nowadays it occurs in most of the country territory. To understand the transmission of vector-borne diseases, knowledge of the dispersal of vector populations is essential to evaluate the risk of pathogen transmission. We conducted a population genetic analysis of Ae. aegypti in 20 neighborhoods from Córdoba, the second largest city in Argentina, using 10 microsatellite loci. High genetic differentiation and the absence of an isolation by distance pattern was found using Weir and Cockerham's θ. Bayesian and multivariate clustering analyses showed that the studied sites included individuals with high membership coefficients (Q) in their populations, individuals with membership in another cluster, and admixed individuals. Individuals with high Q in clusters different from the population in which they were collected strongly suggests that passive transport is important in shaping the Ae. aegypti dispersal pattern in Córdoba city. Knowing the genetic structure of Ae. aegypti populations and their dispersal patterns would contribute to the implementation of vector control programs.

Molecular studies with Aedes (Stegomyia) aegypti (Linnaeus, 1762), mosquito transmitting the dengue virus

Dengue is an infectious viral disease, which can present a wide clinical picture, ranging from oligo or asymptomatic forms, to bleeding and shock, and can progress to death. The disease problem has increased in recent years, especially in urban and suburban areas of tropical and subtropical regions. There are five dengue viruses, called serotypes (DEN-1, DEN-2, DEN-3, DEN-4, and DEN-5), which belong to the Flaviviridae family and are transmitted to humans through infected mosquito bites, with the main vector the Aedes aegypti mosquito (Linnaeus, 1762). Studies performed with Ae. aegypti, aimed at their identification and analysis of their population structure, are fundamental to improve understanding of the epidemiology of dengue, as well for the definition of strategic actions that reduce the transmission of this disease. Therefore, considering the importance of such research to the development of programs to combat dengue, the present review considers the techniques used for the molecular identification, and evaluation of the genetic variability of Ae. aegypti.

Inter- and intraspecific identification of the screwworm, Cochliomyia hominivorax, using random amplified polymorphic DNA-polymerase chain reaction

The screwworm, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), is one of the most devastating arthropod pests of livestock in the Western Hemisphere. Early instars are very difficult to distinguish morphologically from several closely related blow fly species. Random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) markers were developed for identifying C. hominivorax from other wound inhabiting species. Forty decameric primers were screened; nine showed clear reproducible RAPD profiles suitable for distinguishing all life stages of C. hominivorax from 7 other species, including C. macellaria (Fabricius). The results from RAPD-PCR with field-collected samples of unknown first instars agreed with morphological identification that the samples were not C. hominivorax. Three different primers showed DNA polymorphisms (intraspecific) for samples originating from Mexico, Costa Rica, Panama, Jamaica, and Brazil. Therefore, RAPD-PCR may be useful for determining the geographic origin of C. hominivorax samples. Comparing products from these primers, used with known and unknown screwworm samples from an outbreak in Mexico, clearly showed that the outbreak did not originate from the mass rearing facility. Accurate identification of suspected C. hominivorax samples is possible using RAPD-PCR. Further development to identify the geographic origin of samples would benefit the ongoing surveillance programs against C. hominivorax and the decision process during suspected outbreaks of this important pest.

Population genetic structure of Culex quinquefasciatus in India by ISSR marker

OBJECTIVE

To characterize the genetic structure of various populations of Culex quinquefasciatus (Cx. quinquefasciatus) from India representing different geoclimatic locations.

METHODS

Inter simple sequence repeat (ISSR) markers were used. A set of 20 primers were screened with the laboratory populations of mosquito species. Finally the IS 40 primer was chosen based on the scorable banding pattern showing 100 percent polymorphism among the various populations. The statistical analysis was done using POPGENE 1.31 software. The consensus tree was generated based on UPGMA modified from NEIGHBOR procedure of PHYLIP Version 3.5.

RESULTS

The cluster analysis shows the main cluster which is divided into two sub cluster representing all the populations separated as per their phylogeographic and geoclimatic condition.

CONCLUSIONS

The findings will be helpful in understanding the population variation under different ecological conditions and development of effective vector management strategies.

Bionomics of Aedes aegypti subpopulations (Diptera: Culicidae) from Argentina

Differences in biological features of immature and adult Aedes aegypti, as well as variability in vector competence, seem consistent with the existence of genetic variation among subpopulations and adaptation to local conditions. This work aims to compare the bionomics of four Ae. aegypti subpopulations derived from different geographical regions reared under temperate conditions. Life statistics of three Ae. aegypti subpopulations from the provinces of Córdoba, Salta, and Misiones were studied based on horizontal life tables. The Rockefeller strain was used as a control. The development time required to complete the larva and pupa stages varied from 6.91 to 7.95 and 1.87 to 2.41 days, respectively. Significant differences were found in mean larval development time between the Córdoba and Orán subpopulations. The larva-pupa development time was similar in all the subpopulations. However, survival values varied significantly between the Orán and San Javier subpopulations. The proportion of emergent males did not differ from females within each subpopulation nor among them. Adult longevity was similar among the subpopulations. The average number of eggs laid by each female was significantly different. The Rockefeller strain laid a significantly greater number of eggs (463.99 eggs/female) than the rest of the subpopulations. Moreover, differences in the demographic growth parameter R(o) were detected among the four subpopulations. The differences obtained in larval development time, larva-pupa survival values, and net reproductive rates among the subpopulations might reflect underlying genetic differences as a result of colonization from different regions that probably involve adaptations to local conditions.

Genetic relationships among populations of Aedes aegypti from Uruguay and northeastern Argentina inferred from ISSR-PCR data

Aedes aegypti (L.) (Diptera: Culicidae), the main vector of yellow fever and dengue viruses, was eradicated from Argentina between 1955 and 1963, but reinvaded the country in 1986. In Uruguay, the species was reintroduced in 1997. In this study we used highly polymorphic inter-simple sequence repeats (ISSR) markers to analyse the genetic structure of Ae. aegypti populations from Uruguay and northeastern Argentina to identify possible colonization patterns of the vector. Overall genetic differentiation among populations was high (F(ST) = 0.106) and showed no correlation with geographic distance, which is consistent with the short time since the reintroduction of the species in the area. Differentiation between pairs of Argentine populations (F(ST) 0.072 to 0.221) was on average higher than between Uruguayan populations (F(ST)-0.044 to 0.116). Bayesian estimation of population structure defined four genetic clusters and most populations were admixtures of two of them: Mercedes and Treinta y Tres (Uruguay) were mixtures of clusters 1 and 3; Salto (Uruguay) and Paraná (Argentina) of clusters 1 and 4; Fray Bentos (Uruguay) of clusters 2 and 3, and Gualeguaychú (Argentina) of clusters 2 and 3. Posadas and Buenos Aires in Argentina were fairly genetically homogeneous. Our results suggest that Ae. aegypti recolonized Uruguay from bordering cities in Argentina via bridges over the Uruguay River and also from Brazil.

Two different routes of colonization of Aedes aegypti in Argentina from neighboring countries

Aedes aegypti L. (Diptera, Culicidae) is the main vector of dengue and yellow fever. In Argentina, the species was apparently eradicated approximately in 1964; by 1986, it was reintroduced. To identify different gene pools in geographical populations of the species and to ascertain the possible routes of colonization, we analyzed the diversity of mitochondrial DNA haplotypes in 572 specimens from Argentina and neighboring countries. We found that the restriction fragment length polymorphism-polymerase chain reaction screening of a large DNA fragment including the A+T-rich region was the best strategy to reconstruct the colonization pattern ofAe. aegypti in Argentina. Twenty haplotypes were recognized; levels of genetic similarity varied among populations from different geographical locations. The haplotype network constructed on the basis of genetic distances showed three well differentiated groups. Two of them exhibited a well defined spatial distribution and populations in these groups presented an isolation-by-distance pattern. The persistence of relictual populations after the last eradication campaigns would explain the high levels of haplotype diversity and the presence of exclusive haplotypes in urban centers from northwestern Argentina. Eastern Argentine populations showed one prevalent haplotype, also predominant in Brazil and Paraguay. Our results highlight the need for efficient surveys and control campaigns, given the strong effect of land trade on genetic exchange among mosquito populations from Argentina and neighboring countries where dengue is endemic.

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.

Genetic variability in geographical populations of Lutzomyia whitmani elucidated by RAPD-PCR

Unequivocal identification of phlebotomine sand flies is of crucial importance in epidemiological studies of leishmaniasis, because certain species may act as vectors, depending on behavior and physiology. For Lutzomyia whitmani, a major vector of American human cutaneous leishmaniasis in Brazil, an increasing number of studies have suggested the existence of a species complex. In the present work, we evaluated the genetic variability of L. whitmani populations from four Brazilian foci of that disease: Corte de Pedra, Ilhéus, Martinho Campos, and Serra de Baturité. Computational analysis of 85 characters, generated by RAPD-polymerase chain reaction, demonstrated high intrapopulational variability. Those characters led to sex discrimination in three of the populations, with the exception of Martinho Campos individuals, in which sex distinction was not complete. One and two interpopulational phenograms were obtained for females and males, respectively. A higher similarity was observed among the specimens from Ilhéus, Corte de Pedra, and Serra de Baturité, whereas the Martinho Campos population remained external to that cluster. These results, which are in partial accordance with a previous morphometric survey of L. whitmani from the same regions, provide additional evidence to support the existence of at least two spatial clusters of biogeographical populations of L. whitmani in Brazil.

Genetic differentiation of Aedes aegypti (Diptera: Culicidae), the major dengue vector in Brazil

In 2000, Brazil reported 180,137 cases of dengue, approximately 80% of the total in the Americas. However, little is known about gene flow among the vector populations in Brazil. Random amplified polymorphic DNA (RAPD) was used to study the genetic structure of Aedes aegypti in 15 populations from five states, with a range extending 2,800 km. An analysis of 47 polymorphic RAPD loci estimated gene flow at the macro- (different states) and micro- (different cities) geographical levels. Genetic polymorphism was high (H(S) = 0.274), and high levels of genetic differentiation existed both between different states (G(ST) = 0.317) and between cities or neighborhoods in each state (G(ST) = 0.085-0.265). These values are higher than those described for any other populations of A. aegypti.