Genetic variability of Triatoma brasiliensis (Hemiptera: Reduviidae) populations

Population structure and migration in Triatoma infestans (Hemiptera: Reduviidae) from the Argentine Chaco: An integration of genetic and morphometric data

Genetic and morphological structure of vector populations are useful to identify panmictic groups, reinfestation sources and minimal units for control interventions. Currently, no studies have integrated genetic and morphometric data in Triatoma infestans (Hemiptera: Reduviidae), one of the main vectors of Trypanosoma cruzi. We characterized the genetic and phenotypic structure of T. infestans at a small spatial scale (2-8 km), identified potential migrants and compared flight-related traits among genetic groups and between migrant and non-migrant insects in a well-defined area without insecticide spraying in the previous 12 years. We obtained microsatellite genotypes (N = 303), wing shape and size (N = 164) and body weight-to-length ratios (N = 188) in T. infestans from 11 houses in Pampa del Indio, Argentine Chaco. The uppermost level of genetic structuring partially agreed with the morphological groups, showing high degrees of substructuring. The genetic structure showed a clear spatial pattern around Route 3 and one genetic group overlapped with an area of persistent infestation and insecticide resistance. Females harboured more microsatellite alleles than males, which showed signs of isolation-by-distance. Wing shape discriminant analyses of genetic groups revealed low reclassification scores whereas wing size differed among genetic groups for both sexes. Potential migrants (8%) did not differ from non-migrants in sex, ecotope, wing shape and size. However, male migrants had lower W/L than non-migrants suggesting poorer nutritional state. Our findings may contribute to the understanding of population characteristics, dispersal dynamics and ongoing elimination efforts of T. infestans.

Dynamics of Panstrongylus megistus infestation,the primary vector of Trypanosoma cruzi in Minas Gerais,Brazil

Panstrongylus megistus is considered one of the primary species of epidemiological importance for the transmission of Chagas disease in Brazil due to its wide geographical distribution throughout the national territory, good ability to invade and colonize houses, and high rates of natural infection by the Trypanosoma cruzi. The importance of this species in Minas Gerais has been recognized since the 80s. It is responsible for the high prevalence rates of Chagas disease in the west of the state. Studies conducted in the municipality of Jaboticatubas show that P. megistus is still the most captured vector in the region, even after 40 years of uninterrupted actions of the Chagas Disease Control Program in the municipality. Despite the importance of the species, its population dynamics is little studied. Consequently, crucial genetic information such as genetic diversity and gene flow among environments have not been well characterized yet. In this context, this work presents a population genetic analysis at the microgeographic level using microsatellite markers in samples of P. megistus obtained from Jaboticatubas to better understand the infestation dynamics of the primary vector species of T. cruzi in the region. The observed and expected heterozygosity ranged from 0.26 to 0.47 and 0.47 to 0.65, respectively. Most loci presented Hardy-Weinberg disequilibrium due to the excess of homozygotes. The pairwise Fst ranged from 0.05 to 0.35, with the p-value significant for all comparisons, indicating the absence of gene flow between them. The values of Fis found ranged from 0.25 to 0.52, all values of p ≤ 0.05, probably due to structured populations, inbreeding, or null alleles. The results suggest an extended stay of this species in the colonized environments, with rare dispersal to other locations. Such results differ from that observed for Triatoma brasiliensis and Triatoma dimidiata, species that present constant movement, even after colonization of the artificial environment, and are similar to the behavior of Triatoma infestans. Thus, the study reinforces the importance of continuous entomological surveillance in the areas of occurrence of P. megistus to avoid the installation and formation of new foci of household infestation in the region.

Identification and characterization of microsatellite markers for population genetic studies of Panstrongylus megistus (Burmeister, 1835) (Triatominae: Reduviidae)

Background

Panstrongylus megistus is the most important vector of Chagas disease in Brazil. Studies show that the principal factor hindering the control of triatomines is reinfestation of houses previously treated with insecticides. Studies at the microgeographic level are therefore necessary to better understand these events. However, an efficient molecular marker is not yet available for carrying out such analyses in this species. The aim of the present study was to identify and characterize microsatellite loci for future population genetic studies of P. megistus.

Methods

This study work consisted of five stages: (i) sequencing of genomic DNA; (ii) assembly and selection of contigs containing microsatellites; (iii) validation of amplification and evaluation of polymorphic loci; (iv) standardization of the polymorphic loci; and (v) verification of cross-amplification with other triatomine species.

Results

Sequencing of males and females generated 7,908,463 contigs with a total length of 2,043,422,613 bp. A total of 2,043,690 regions with microsatellites in 1,441,091 contigs were obtained, with mononucleotide repeats being the most abundant class. From a panel of 96 loci it was possible to visualize polymorphisms in 64.55% of the loci. Of the 20 loci genotyped, the number of alleles varied from two to nine with an average of 4.9. Cross-amplification with other species of triatomines was observed in 13 of the loci.

Conclusions

Due to the high number of alleles encountered, polymorphism and the capacity to amplify from geographically distant populations, the microsatellites described here show promise for utilization in population genetic studies of P. megistus.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04771-w.

Morphological and Chromatic Variation in Four Populations of Triatoma mexicana (Hemiptera: Reduviidae)

Triatoma mexicana is an endemic species of Mexico and is distributed in the states of Hidalgo, Queretaro, Guanajuato, and San Luis Potosi, being naturally infected with Trypanosoma cruzi, which increases its importance in the region. The species description was made in 1848, but there are only a few studies on its morphology, biology, and behavior. The present manuscript shows the presence of morphological and chromatic variations among populations of T. mexicana from the states of Hidalgo (Valle del Mezquital and Meztitlan), Guanajuato and Queretaro. The study employed 136 specimens collected in four locations. Scanning electron microscopy was used to study the morphological characteristics of the head, pronotum, and scutellum; also, we measured the width of the abdomen and the total length in the specimens of each population. The morphometric analysis considered 19 variables in the previous structures. Significant differences were found in the dimensions of the head and pronotum, but not in the scutellum; there is clear discrimination among the four proposed populations. The chromatic patterns observed in the connexivum go from yellow to brown and show some significant differences related to geographical origin. The set of evaluated characters showed a higher degree of difference in the population of Guanajuato, clearly separating from the rest of the populations, indicating the possibility of a divergence process. The characteristics observed in the remaining populations could be adaptive responses to their habitat.

Assessing the mitochondrial DNA diversity of the Chagas disease vector Triatoma sordida (Hemiptera: Reduviidae)

Triatoma sordida is a species that transmits Trypanosoma cruzi to humans. In Brazil, T. sordida currently deserves special attention because of its wide distribution, tendency to invade domestic environments and vectorial competence. For the planning and execution of control protocols to be effective against Triatominae, they must consider its population structure. In this context, this study aimed to characterise the genetic variability of T. sordida populations collected in areas with persistent infestations from Minas Gerais, Brazil. Levels of genetic variation and population structure were determined in peridomestic T. sordida by sequencing a polymorphic region of the mitochondrial cytochrome b gene. Low nucleotide and haplotype diversity were observed for all 14 sampled areas; π values ranged from 0.002-0.006. Most obtained haplotypes occurred at low frequencies, and some were exclusive to only one of the studied populations. Interpopulation genetic diversity analysis revealed strong genetic structuring. Furthermore, the genetic variability of Brazilian populations is small compared to that of Argentinean and Bolivian specimens. The possible factors related to the reduced genetic variability and strong genetic structuring obtained for studied populations are discussed in this paper.

Simulating population genetics of pathogen vectors in changing landscapes: guidelines and application with Triatoma brasiliensis

Background

Understanding the mechanisms that influence the population dynamics and spatial genetic structure of the vectors of pathogens infecting humans is a central issue in tropical epidemiology. In view of the rapid changes in the features of landscape pathogen vectors live in, this issue requires new methods that consider both natural and human systems and their interactions. In this context, individual-based model (IBM) simulations represent powerful yet poorly developed approaches to explore the response of pathogen vectors in heterogeneous social-ecological systems, especially when field experiments cannot be performed.

Methodology/Principal Findings

We first present guidelines for the use of a spatially explicit IBM, to simulate population genetics of pathogen vectors in changing landscapes. We then applied our model with Triatoma brasiliensis, originally restricted to sylvatic habitats and now found in peridomestic and domestic habitats, posing as the most important Trypanosoma cruzi vector in Northeastern Brazil. We focused on the effects of vector migration rate, maximum dispersal distance and attraction by domestic habitat on T. brasiliensis population dynamics and spatial genetic structure. Optimized for T. brasiliensis using field data pairwise fixation index (FST) from microsatellite loci, our simulations confirmed the importance of these three variables to understand vector genetic structure at the landscape level. We then ran prospective scenarios accounting for land-use change (deforestation and urbanization), which revealed that human-induced land-use change favored higher genetic diversity among sampling points.

Conclusions/Significance

Our work shows that mechanistic models may be useful tools to link observed patterns with processes involved in the population genetics of tropical pathogen vectors in heterogeneous social-ecological landscapes. Our hope is that our study may provide a testable and applicable modeling framework to a broad community of epidemiologists for formulating scenarios of landscape change consequences on vector dynamics, with potential implications for their surveillance and control.

Worldwide, humans are modifying landscapes at an unprecedented rate. These modifications have an influence on the ecology of pathogen vectors, yet this issue has received relatively little input from modeling research. The current study presents guidelines for the use of a modeling framework for the representation of the dynamics and spatial genetic structure of pathogen vectors. It allows considering spatiotemporal landscape modifications explicitly, to represent human-altered modifications and consequences. We applied this modeling framework to Triatoma brasiliensis, vector of the pathogen Trypanosoma cruzi responsible for the Chagas disease, in the semi-arid Northeastern Brazil. Using field data of pairwise fixation index (FST) from microsatellite loci, we found that migration rate, maximum dispersal distance and attraction by domestic habitat were all key parameters to understand vector spatial genetic structure at the landscape level. At the interface across disciplines, this study provides to the community of epidemiologists a testable and applicable framework to foresee landscape modification consequences on vector dynamics and genetic structure, with potential implications for their surveillance and control.

Ecotope effect in Triatoma brasiliensis (Hemiptera: Reduviidae) suggests phenotypic plasticity rather than adaptation

Triatoma brasiliensis (Hemiptera: Reduviidae) is an important vector of Chagas' disease in both sylvatic and peridomestic ecotopes. Discriminating between these populations of Triatominae has been proposed as a means of investigating re-infestation rates of human dwellings. Geometric morphometrics have been widely applied in the study of Triatominae polymorphisms at species and population levels. This study characterizes morphometric differences between sylvatic and peridomestic populations, as well as between sexes in T. brasiliensis specimens from Jaguaruana, Ceará, in northeastern Brazil. No differences in either the shape or size of the cephalic capsule were apparent between sexes or ecotopes. However, the wings showed differentiation in shape and size. Sexual dimorphism was detected, with females presenting significantly higher values and conformations. Size differentiation was also evident, with sylvatic specimens being generally larger than peridomestic examples. These results indicate that differences in the wings of T. brasiliensis may be related to the existence of phenotypic plasticity, and variations in size and shape may be associated with different ecotopes, possibly as a result of conditions in each micro-habitat, such as temperature, relative humidity, food supply and density.

Population dynamics of Triatoma vitticeps (Stål, 1859) in Itanhomi, Minas Gerais, Brazil

Taxonomic markers (head structure morphometry, isoenzymes and randon amplified polymorphism of DNA - RAPD) were used to understand the population dynamics of Triatoma vitticeps, predominant triatomine species in Itanhomi district, using samples obtained from domestic, peridomiciliary and sylvatic habitats. Morphometric analysis revealed sexual dimorphism within the three samples although specimens could not be separated according to the habitat in which they were captured. Forty-two bands were analyzed from RAPD profiles generated using four primers. A dendrogram constructed from Dice's similarity coefficient values showed that migration of the insects between the habitats has occurred, without structuring of populations. Moreover, the dendrogram obtained from the genetic distance values showed an important gene flow between the sylvatic and domestic habitats. No polymorphism was found in the electrophoretic mobility of proteins for the ten enzymes studied. Our results revealed movement of triatomines between the three habitats, suggesting that the presence of T. vitticeps in houses should not be ignored. As invasion of houses by sylvatic insects is frequent and the natural infection indices of this species are among the highest known, epidemiological vigilance studies may reveal possible changes in T. vitticeps behaviour which could present future risks to public health.

Biogeography of Brazilian populations of Panstrongylus megistus (Hemiptera, Reduviidae, Triatominae) based on molecular marker and paleo-vegetational data

In epidemiological terms, Panstrongylus megistus is one of the most important species of triatomine bug in Brazil. Samples from 11 localities were studied using the random amplification of polymorphic DNA (RAPD) technique, which was able to differentiate the study populations clearly. Biogeographical data indicate that these populations could already have arisen 18,000 years ago (C(14)), it being possible to differentiate insects from the Brazilian states of Santa Catarina (SC) in the south, Ceará (CE) in the northeast and another large intermediate block containing the remaining eight populations from five other states. These results agree with those obtained by phenograms constructed from RAPD data, in which the SC population lies opposite those of CE, consistent with the greatest geographical distance between these localities. The other eight populations (Alagoas (AL), Bahia (BA), Goiás (GO), Minas Gerais (MG) and São Paulo (SP)) are closer genetically and originated in areas whose vegetational characteristics have remained similar to each other during the last 18,000 years, thus allowing greater contact between them. The greatest divergence of this group of insects and those of Ceará appears to have occurred 8000 years ago. This more humid period gave rise to other landscape changes, allowing greater differentiation of the vegetation and consequent expansion of P. megistus populations. Formation of the Serras do Mar and Mantiqueira probably created geographical barriers that favored a certain degree of isolation and greater differentiation of the SC population. Atlantic forest remnants within the caatinga domain (created between 25 and 17,000 years ago), where the CE populations originated probably constitute ecological refugia produced by successive amplification and retraction of the most suitable habitats for this species.

Identification and characterization of microsatellite markers in the Chagas disease vector Triatoma infestans (Heteroptera: Reduviidae)

Triatoma infestans, the main vector of Chagas disease in the southern cone countries, is the principal target of a regional elimination program. A better understanding of its dispersal, sources of reinfestation, and insecticide resistance is key to an effective control program. To address such problems, we identified and characterized 13 microsatellite loci of T. infestans. For each locus, primer sequences and PCR conditions are presented. Allele variability and frequency were analyzed in 59 T. infestans specimens from different rural communities in northwestern Argentina; nine loci were considered suitable for population genetic studies. Departure from Hardy-Weinberg equilibrium was detected in 10/13 loci with F(IS) values ranging from 0.04 to 0.91, indicating heterozygote deficit and a possible grade of sub-structure in the sample analyzed. Presence of null alleles in some loci cannot be discarded. The present work provides a promising tool to develop a population genetic study of natural populations of T. infestans in tandem with field studies and analyses of bug dispersal and the reinfestation process.

Optimization of random amplified polymorphic DNA techniques for use in genetic studies of Cuban triatominae

Random amplified polymorphic DNA (RAPD) technique is a simple and reliable method to detect DNA polymorphism. Several factors can affect the amplification profiles, thereby causing false bands and non-reproducibility of assay. In this study, we analyzed the effect of changing the concentration of primer, magnesium chloride, template DNA and Taq DNA polymerase with the objective of determining their optimum concentration for the standardization of RAPD technique for genetic studies of Cuban Triatominae. Reproducible amplification patterns were obtained using 5 pmoL of primer, 2.5 mM of MgCl2, 25 ng of template DNA and 2 U of Taq DNA polymerase in 25 µL of the reaction. A panel of five random primers was used to evaluate the genetic variability of T. flavida. Three of these (OPA-1, OPA-2 and OPA-4) generated reproducible and distinguishable fingerprinting patterns of Triatominae. Numerical analysis of 52 RAPD amplified bands generated for all five primers was carried out with unweighted pair group method analysis (UPGMA). Jaccard's Similarity Coefficient data were used to construct a dendrogram. Two groups could be distinguished by RAPD data and these groups coincided with geographic origin, i.e. the populations captured in areas from east and west of Guanahacabibes, Pinar del Río. T. flavida present low interpopulation variability that could result in greater susceptibility to pesticides in control programs. The RAPD protocol and the selected primers are useful for molecular characterization of Cuban Triatominae.

Molecular research and the control of Chagas disease vectors

Chagas disease control initiatives are yielding promising results. Molecular research has helped successful programs by identifying and characterizing introduced vector populations and by defining intervention targets accurately. However, researchers and health officials are facing new challenges throughout Latin America. Native vectors persistently reinfest insecticide-treated households, and sylvatic triatomines maintain disease transmission in humid forest regions (including Amazonia) without colonizing human dwellings. In these scenarios, fine-scale vector studies are essential to define epidemiological risk patterns and clarify the involvement of little-known triatomine taxa in disease transmission. These eco-epidemiological investigations, as well as the planning and monitoring of control interventions, rely by necessity on accurate taxonomic judgments. The problems of cryptic speciation and phenotypic plasticity illustrate this need - and how molecular systematics can provide the fitting answers. Molecular data analyses also illuminate basic aspects of vector evolution and adaptive trends. Here we review the applications of molecular markers (concentrating on allozymes and DNA sequencing) to the study of triatomines. We analyze the suitability, strengths and weaknesses of the various techniques for taxonomic, systematic and evolutionary investigations at different levels (populations, species, and higher taxonomic categories).

Molecular characterization, relatedness of Haematobia irritans (horn fly) populations, by RAPD-PCR

Haematobia irritans is a hematophagous parasite of cattle that causes significant economic losses in many parts of the world, including Brazil. In the present work, one American and four Brazilian populations of this species were studied by Random Amplified Polymorpht DNA (RAPD) to assess basically genetic variability within and between populations. Ten different decamer random primers were employed in the genomic DNA amplification, yielding 117 fragments in the five H. irritans populations. In Drosophila prosaltans, used as an outgroup, 81 fragments were produced. Forty-three of these fragments were shared by both species. Among the H. irritans samples, that from Rio Branco (Acre State, Brazil) produced the smallest numbers of fragments and polymorphic bands. This high genetic homogenity may be ascribed to its geographic origin (in the Northwest of Brazil), which causes high isolation and low gene flow, unlike the other Brazilian populations, from the South Central region, in which cattle trade is very intensive. Marker fragments (exclusive bands) detected in every sample enabled the population origin to be characterized, but they are also potentially useful for further approaches such as the putative origin of Brazilian populations from North America. Similarity indices [Nei & Li, 1979, Proc. Natl. Acad. Sci. USA 76: 5269-5273] and phylogenetic trees, rooted by using the outgroup and produced by the Phylogenetic Analysis using Parsimony (PAUP 4.0-Swofford, 2001) program showed the closest relationships between flies from Sao Jose do Rio Preto and Turúba (both from São Paulo State, Brazil) while flies from the geographically distant Rio Branco showed the greatest differentiation relative to the others.

Intraspecific variability in Triatoma dimidiata (Hemiptera: Reduviidae) populations from Guatemala based on chemical and morphometric analyses

The intraspecific variability of Triatoma dimidiata Latreille, a major vector of Chagas disease, was studied in four departments of Guatemala. Insects were collected from either domestic and sylvatic habitats, and their cuticular hydrocarbon pattern and head morphology were analyzed using ordination and classification techniques. A significant discrimination was obtained both with morphometric and hydrocarbon analyses. Insects from northern departments were easily differentiated from southern conspecifics. Distinctive hydrocarbon pattern and head shape were detected for insects collected from caves in the north central region of the country, posing concern about their taxonomic status.

Dynamics between sylvatic, peridomestic and domestic populations of Triatoma brasiliensis (Hemiptera: Reduviidae) in Ceara State, Northeastern Brazil

Triatoma brasiliensis is the most important Chagas disease vector in the drier regions of the "Brazilian Caatinga", colonizing both sylvatic and domestic environments, usually forming abundant colonies. Control trials using insecticides against domestic and peridomestic populations suggest that the T. brasiliensis has a high capacity to repopulate treated habitats from the neighboring sylvatic populations, making its elimination more complex. The aim of this work was to determine genetic variability among sylvatic, peridomestic and domestic populations of T. brasiliensis using head morphometry and random amplified polymorphic DNA (RAPD). Both morphometric analysis and RAPD patterns showed a separation between sylvatic and domestic populations, being the peridomestic ones between them. Based on this data, we suggest that there exists a flow between natural and artificial environments, being the peridomestic population mainly responsible for this interchange. It is possible that the peridomestic environment is maintaining the variability on the insects found on artificial habitats, which guarantee T. brasiliensis success on adaptation in both environments and also increase the risk of introduction of new Trypanosoma cruzi strains in the domestic cycle of Chagas disease in this region.

A preliminary assessment of genetic differentiation of Triatoma dimidiata (Hemiptera: Reduviidae) in Guatemala by random amplification of polymorphic DNA-polymerase chain reaction

The population genetics of Triatoma dimidiata (Latreille, 1811) from five different provinces in Guatemala, including three sylvan and three domestic populations, was investigated by random amplification of polymorphic DNA-polymerase chain reaction. There is a high degree of genetic variation in all of the T. dimidiata populations as evidenced by high levels of average expected heterozygosity and polymorphism. Domestic populations are more closely related to each other (D = 0.05-0.085, Nei's genetic distance) than are the sylvan (D = 0.121-0.189). Within the limited sample size of three populations, there was a correlation with geographic and genetic distance for the domestic populations, but not for the sylvan. Surprisingly, one of the sylvan populations was genetically very similar to the domestic populations. The FST demonstrated a high degree of differentiation at the country-wide level (FST = 0.175) and a moderate degree of differentiation within the sylvan (FST = 0.135) or domestic (FST = 0.097) populations. Although these results demonstrated that gene flow is limited between different provinces in Guatemala, hierarchical analysis showed that barriers between the Atlantic and Pacific drainage slopes were not biologically significant limiters of gene flow.

Nested clade and phylogeographic analyses of the Chagas disease vector Triatoma brasiliensis in Northeast Brazil

Triatoma brasiliensis (Hemiptera: Reduviidae: Triatominae) is the most important Chagas disease vector in the semiarid areas of Northeast Brazil. We analyzed mitochondrial cytochrome b sequence variation among 136 individuals representing 16 populations from across the species' distribution. Neighbor-joining and parsimony tree-building methods were used in conjunction with nested clade analysis to describe the systematics and phylogeography of this species. Our results indicate that T. brasiliensis is composed of four genetically distinct chromatic forms (referred to as brasiliensis, macromelasoma, juazeiro, and melanica) that present inter-population divergence values (0.027-0.119, corrected K2-p) and a pattern of haplotype geographic distribution compatible with the existence of a species complex. As a consequence, such forms can be treated as isolated targets in vector control programs. We were unable to infer what is shaping the population structure of the brasiliensis form as we obtained mutually exclusive causes of structure, namely a barrier to gene flow caused by past population fragmentation, and isolation by distance between populations (which would permit gene flow). We found indication of mitochondrial DNA introgression occurring among forms in putative hybrid zones.