Similarity of the patterns of sensilla on the antennae of Triatoma melanosoma and Triatoma infestans

Does Trypanosoma cruzi (Chagas, 1909) (Kinetoplastida: Trypanosomatidae) modify the antennal phenotype of Triatoma dimidiata (Latreille, 1811) (Hemiptera: Triatominae)?

Background

Triatoma dimidiata is a vector of the protozoan parasite Trypanosoma cruzi, the etiologic agent of Chagas disease. Phenotypic plasticity allows an organism to adjust its phenotype in response to stimuli or environmental conditions. Understanding the effect of T. cruzi on the phenotypic plasticity of its vectors, known as triatomines, has attracted great interest because of the implications of the parasite–triatomine interactions in the eco-epidemiology and transmission of the etiologic agent of Chagas disease. We investigated if the infection of the vector with T. cruzi may be associated with a change in the antennal phenotype of sylvatic, domestic, and laboratory-reared populations of T. dimidiata.

Methods

The abundance of each type of sensillum (bristles, basiconic, thick- and thin-walled trichoid) on the antennae of T.cruzi-infected and non-infected T.dimidiata reared in the laboratory or collected in sylvatic and domestic ecotopes were measured under light microscopy and compared using Kruskal–Wallis non-parametric tests and permutational multivariate analysis of variance.

Results

We found significant differences between sensilla patterns of infected and non-infected insects within sylvatic and domestic populations. Conversely, we found no significant differences between sensilla patterns of infected and non-infected insects within the laboratory-reared population. Besides, for sylvatic and domestic populations, sexual dimorphism tended to be increased in infected insects.

Conclusion

The differences observed in infected insects could be linked to higher efficiency in the perception of odor molecules related to the search for distant mates and hosts and the flight dispersal in search of new habitats. In addition, these insects could have a positive effect on population dynamics and the transmission of T.cruzi.

Graphical Abstract

Triatoma guazu Lent and Wygodzinsky Is a Junior Synonym of Triatoma williami Galvão, Souza and Lima

Simple Summary

Triatomines are blood-sucking insects, potential vectors of Trypanosoma cruzi, the etiological agent of Chagas disease. Triatoma guazu and Triatoma williami are phylogenetically very close and occur in sympatry. Morphologic, morphometric, and genetic analyses were performed to discuss the taxonomic status of these species. Morphometric and molecular data do not show diagnostic characteristics between species, whereas their different patterns of connexival spots were considered a phenotypic polymorphism, common in triatomines. These results suggest T. guazu as a junior synonym of T. williami. Therefore, the synonym between these species is formally proposed here.

Abstract

Triatoma guazu Lent and Wygodzinsky and Triatoma williami Galvão, Souza, and Lima (Hemiptera: Triatominae) are found in human dwellings and are potential vectors of the protozoan Trypanosoma cruzi, the etiological agent of Chagas disease. Triatoma guazu was described based solely on a single female specimen, from the municipality of Villarica, Guairá Department, Paraguay, and posteriorly, a male from Barra do Garças, Mato Grosso, Brazil was described and designated as the allotype of this species. Triatoma williami is found in the central-west of Brazil between Goiás, Mato Grosso, and Mato Grosso do Sul. However, the taxonomic “status” of these species is questioned. Previous studies indicate the lack of isoenzymatic diagnostic loci, morphometric similarity, low genetic divergence, and close evolutionary relationship of these species. In this study, we compared the morphology, morphometry, and mitochondrial DNA fragments of the populations of the two species. The morphological diagnostic characteristic among these species is the difference in the connexivum spots pattern, which has been recognized as a phenotypic variation that exists among populations resulting from ecological diversity. Furthermore, our analysis also revealed the morphometric similarity and low genetic divergence between these species. Therefore, in the present paper, we formally propose T. guazu as a junior synonym of T. williami.

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.

Antennal phenotype of Mexican haplogroups of the Triatoma dimidiata complex, vectors of Chagas disease

Triatoma dimidiata (Latreille) is a species complex that spans North, Central, and South America and which is a key vector of all known discrete typing units (DTU) of Trypanosoma cruzi, the etiologic agent of Chagas disease. Morphological and genetic studies indicate that T. dimidiata is a species complex with three principal haplogroups (hg) in Mexico. Different markers and traits are still inconclusive regarding if other morphological differentiation may indicate probable behavioral and vectorial divergences within this complex. In this paper we compared the antennae of three Mexican haplogroups (previously verified by molecular markers ND4 and ITS-2) and discussed possible relationships with their capacity to disperse and colonized new habitats. The abundance of each type of sensillum (bristles, basiconics, thick- and thin-walled trichoids) on the antennae of the three haplogroups, were measured under light microscopy and compared using Kruskal-Wallis non-parametric and multivariate non-parametric analyses. Discriminant analyses indicate significant differences among the antennal phenotype of haplogroups either for adults and some nymphal stages, indicating consistency of the character to analyze intraspecific variability within the complex. The present study shows that the adult antennal pedicel of the T. dimidiata complex have abundant chemosensory sensilla, according with good capacity for dispersal and invasion of different habitats also related to their high capacity to adapt to conserved as well as modified habitats. However, the numerical differences among the haplogroups are suggesting variations in that capacity. The results here presented support the evidence of T. dimidiata as a species complex but show females and males in a different way. Given the close link between the bug's sensory system and its habitat and host-seeking behavior, AP characterization could be useful to complement genetic, neurological and ethological studies of the closely related Dimidiata Complex haplogroups for a better knowledge of their vectorial capacity and a more robust species differentiation.

Changes associated with laboratory rearing in antennal sensilla patterns of Triatoma infestans, Rhodnius prolixus, and Rhodnius pallescens (Hemiptera, Reduviidae, Triatominae)

We examined changes in the array of antennal sensilla of three species of Triatominae (Triatoma infestans, Rhodnius prolixus, and R. pallescens) following their establishment for different periods in laboratory culture. In each case, the laboratory colonies were compared with conspecific samples taken directly from the field, by quantitative analysis of the sensilla arrays on the three distal segments of the antenna in terms of the densities of three types of chemoreceptors (basiconics and thick and thin walled trichoids) and one type of mechanoreceptor (bristles). Sensilla densities were compared by ANOVA or non-parametric tests, and by multivariate discriminant analysis. Strains of the same species reared in different laboratories showed significant differences in their sensilla arrays, especially when compared to field-collected material from the same geographic origin. A Bolivian strain of T. infestans reared in the laboratory for 15 years and fed at monthly intervals, showed greatest differences from its conspecific wild forms, especially in terms of reductions in the number of chemoreceptors. By contrast, an Argentine strain of T. infestans reared for 25 years in the laboratory and fed weekly, showed a relative increase in the density of mechanoreceptors. A Colombian strain of R. prolixus reared for 20 years and fed weekly or fortnightly, showed only modest differences in the sensilla array when compared to its wild populations from the same area. However, a Colombian strain of R. pallescens reared for 12 years and fed fortnightly, did show highly significant reductions in one form of chemoreceptor compared to its conspecific wild populations. For all populations, multivariate analysis clearly discriminated between laboratory and field collected specimens, suggesting that artificial rearing can lead to modifications in the sensory array. This not only supports the idea of morphological plasticity in these species, but also suggests caution in the use of long-established laboratory material for experimental studies designed to extrapolate the natural behaviour and physiology of these species.